Green fluorescent protein-glucocorticoid receptor knockin mice reveal dynamic receptor modulation during thymocyte development

Tumors produce glucocorticoids by metabolite recycling, not synthesis, and activate Tregs to promote growth

Glucocorticoids are steroid hormones with potent immunosuppressive properties. Their primary source is the adrenals, where they are generated via de novo synthesis from cholesterol. In addition, many tissues have a recycling pathway in which glucocorticoids are regenerated from inactive metabolites by the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1, encoded by Hsd11b1). Here, we find that multiple tumor types express Hsd11b1 and produce active glucocorticoids. Genetic ablation of Hsd11b1 in such cells had no effect on in vitro growth, but reduced in vivo tumor progression, which corresponded with increased frequencies of CD8+ tumor-infiltrating lymphocytes (TILs) expressing activation markers and producing effector cytokines. Tumor-derived glucocorticoids were found to promote signatures of Treg activation and suppress signatures of conventional T cell activation in tumor-infiltrating Tregs. Indeed, CD8+ T cell activation was restored and tumor growth reduced in mice with Treg-specific glucocorticoid receptor deficiency. Importantly, pharmacologic inhibition of 11β-HSD1 reduced tumor growth to the same degree as gene knockout and rendered immunotherapy-resistant tumors susceptible to PD-1 blockade. Given that HSD11B1 expression is upregulated in many human tumors and that inhibition of 11β-HSD1 is well tolerated in clinical studies, these data suggest that targeting 11β-HSD1 may be a beneficial adjunct in cancer therapy.

A guide to changing paradigms of glucocorticoid receptor function-a model system for genome regulation and physiology

The glucocorticoid receptor (GR) is a bona fide ligand-regulated transcription factor. Cloned in the 80s, the GR has become one of the best-studied and clinically most relevant members of the nuclear receptor superfamily. Cooperative activity of GR with other transcription factors and a plethora of coregulators contribute to the tissue- and context-specific response toward the endogenous and pharmacological glucocorticoids (GCs). Furthermore, nontranscriptional activities in the cytoplasm are emerging as an additional function of GR. Over the past 40 years, the concepts of GR mechanisms of action had been constantly changing. Different methodologies in the pregenomic and genomic era of molecular biological research and recent cutting-edge technology in single-cell and single-molecule analysis are steadily evolving the views, how the GR in particular and transcriptional regulation in general act in physiological and pathological processes. In addition to the development of technologies for GR analysis, the use of model organisms provides insights how the GR in vivo executes GC action in tissue homeostasis, inflammation, and energy metabolism. The model organisms, namely the mouse, but also rats, zebrafish, and recently fruit flies carrying mutations of the GR became a major driving force to analyze the molecular function of GR in disease models. This guide provides an overview of the exciting research and paradigm shifts in the GR field from past to present with a focus on GR transcription factor networks, GR DNA-binding and single-cell analysis, and model systems.

Glucocorticoids in T cell development, differentiation and function

Glucocorticoids (GCs) are small lipid hormones produced by the adrenals that maintain organismal homeostasis. Circadian and stress-induced changes in systemic GC levels regulate metabolism, cardiovascular and neural function, reproduction and immune activity. Our understanding of GC effects on immunity comes largely from administration of exogenous GCs to treat immune or inflammatory disorders. However, it is increasingly clear that endogenous GCs both promote and suppress T cell immunity. Examples include selecting an appropriate repertoire of T cell receptor (TCR) self-affinities in the thymus, regulating T cell trafficking between anatomical compartments, suppressing type 1 T helper (T_H1) cell responses while permitting T_H2 cell and, especially, IL-17-producing T helper cell responses, and promoting memory T cell differentiation and maintenance. Furthermore, in addition to functioning at a distance, extra-adrenal (local) production allows GCs to act as paracrine signals, specifically targeting activated T cells in various contexts in the thymus, mucosa and tumours. These pleiotropic effects on different T cell populations during development and immune responses provide a nuanced understanding of how GCs shape immunity.

Nuclear translocation of the unliganded glucocorticoid receptor is influenced by membrane fluidity, but not A2AR agonism

Epidemiological evidence suggests that chronic consumption of caffeine, a non-selective antagonist of adenosine A_2AR receptors (A_2AR), can be neuroprotective in a number of age-related neurodegenerative disorders including Alzheimer's disease. A growing body of work shows that this neuroprotection may act via a synergistic interaction with the glucocorticoid receptor (GR) and its associated genetic response elements. Therefore, we hypothesized that A_2AR signaling may directly stimulate glucocorticoid receptor translocation via downstream signaling elements within the cell. Surprisingly, we found no effect of A_2AR agonism on GR translocation in the absence of steroid. As expected, membrane-bound dexamethasone was capable of stimulating full GR translocation, albeit at a slower rate. This non-liganded translocation was unaffected by A_2AR ligands, providing strong evidence that GR translocation occurs independently of activation of A_2ARs. To identify other potential mechanisms of translocation, membrane fluidity was increased significantly by benzyl alcohol, which also induced full nuclear translocation of the GR, but unlike the membrane-bound dexamethasone, benzyl alcohol did result in transcriptional upregulation of GR-dependent genes. Taken together, our data shows that the unliganded GR is sensitive to changes in membrane state and can be transcriptionally active.

Single-Cell Resolution and Quantitation of Targeted Glucocorticoid Delivery in the Thymus

Glucocorticoids are lipid-soluble hormones that signal via the glucocorticoid receptor (GR), a ligand-dependent transcription factor. Circulating glucocorticoids derive from the adrenals, but it is now apparent that paracrine glucocorticoid signaling occurs in multiple tissues. Effective local glucocorticoid concentrations and whether glucocorticoid delivery can be targeted to specific cell subsets are unknown. We use fluorescence detection of chromatin-associated GRs as biosensors of ligand binding and observe signals corresponding to steroid concentrations over physiological ranges in vitro and in vivo. In the thymus, where thymic epithelial cell (TEC)-synthesized glucocorticoids antagonize negative selection, we find that CD4⁺CD8⁺TCR^hi cells, a small subset responding to self-antigens and undergoing selection, are specific targets of TEC-derived glucocorticoids and are exposed to 3-fold higher levels than other cells. These results demonstrate and quantitate targeted delivery of paracrine glucocorticoids. This approach may be used to assess in situ nuclear receptor signaling in a variety of physiological and pathological contexts.

Comparative analysis of thymic subpopulations during different modes of atrophy identifies the reactive oxygen species scavenger, N-acetyl cysteine, to increase the survival of thymocytes during infection-induced and lipopolysaccharide-induced thymic atrophy

The development of immunocompetent T cells entails a complex pathway of differentiation in the thymus. Thymic atrophy occurs with ageing and during conditions such as malnutrition, infections and cancer chemotherapy. The comparative changes in thymic subsets under different modes of thymic atrophy and the mechanisms involved are not well characterized. These aspects were investigated, using mice infected with Salmonella Typhimurium, injection with lipopolysaccharide (LPS), an inflammatory but non-infectious stimulus, etoposide (Eto), a drug used to treat some cancers, and dexamethasone (Dex), a steroid used in some inflammatory diseases. The effects on the major subpopulations of thymocytes based on multicolour flow cytometry studies were, first, the CD4^-  CD8^- double-negative (DN) cells, mainly DN2-4, were reduced with infection, LPS and Eto treatment, but not with Dex. Second, the CD8⁺  CD3^lo immature single-positive cells (ISPs) were highly sensitive to infection, LPS and Eto, but not Dex. Third, treatment with LPS, Eto and Dex reduced all three subpopulations of CD4⁺  CD8⁺ double-positive (DP) thymocytes, i.e. DP1, DP2 and DP3, but the DP3 subset was relatively more resistant during infection. Fourth, both CD4⁺ and CD8⁺ single-positive (SP) thymocytes were lowered by Eto and Dex, but not during infection. Notably, LPS lowered CD4⁺ SP subsets, whereas the CD8⁺ SP subsets were relatively more resistant. Interestingly, the reactive oxygen species quencher, N-acetyl cysteine, greatly improved the survival of thymocytes, especially DNs, ISPs and DPs, during infection and LPS treatment. The implications of these observations for the development of potential thymopoietic drugs are discussed.

Soluble antigens from the neurotropic pathogen Angiostrongylus cantonensis directly induce thymus atrophy in a mouse model

The nematode Angiostrongylus cantonensis (A.C.) is a neurotropic pathogen; stage-III larva invade the human (non-permissive host) central nervous system (CNS) to cause eosinophilic meningitis or meningoencephalitis accompanied by immunosuppression. In an A.C.-infectedmouse (another non-permissive host) model, CNS damage-associated T cell immune deficiency and severe inflammation were proposed to result from activation of the hypothalamic-pituitary-adrenal (HPA) axis. However, glucocorticoids are anti-inflammatory agents. Additionally, while defects in thymic stromal/epithelial cells (TECs) are the major reason for thymic atrophy, TECs do not express the glucocorticoid receptor. Therefore, activation of the HPA axis cannot fully explain the thymic atrophy and inflammation. Using an A.C.-infected mouse model, we found that A.C.-infected mice developed severe thymic atrophy with dramatic impairments in thymocytes and TECs, particularly cortical TECs, which harbor CD4⁺CD8⁺ double-positive thymocytes. The impairments resulted from soluble antigens (sAgs) from A.C. in the thymuses of infected mice, as intrathymic injection of these sAgs into live mice and the addition of these sAgs to thymic cell culture resulted in thymic atrophy and cellular apoptosis, respectively. Therefore, in addition to an indirect effect on thymocytes through the HPA axis, our study reveals a novel mechanism by which A.C. infection in non-permissive hosts directly induces defects in both thymocytes and TECs via soluble antigens.

Differential susceptibility and maturation of thymocyte subsets during Salmonella Typhimurium infection: insights on the roles of glucocorticoids and Interferon-gamma

The thymus is known to atrophy during infections; however, a systematic study of changes in thymocyte subpopulations has not been performed. This aspect was investigated, using multi-color flow cytometry, during oral infection of mice with Salmonella Typhimurium (S. Typhimurium). The major highlights are: First, a block in the developmental pathway of CD4-CD8- double negative (DN) thymocytes is observed. Second, CD4+CD8+ double positive (DP) thymocytes, mainly in the DP1 (CD5loCD3lo) and DP2 (CD5hiCD3int), but not DP3 (CD5intCD3hi), subsets are reduced. Third, single positive (SP) thymocytes are more resistant to depletion but their maturation is delayed, leading to accumulation of CD24hiCD3hi SP. Kinetic studies during infection demonstrated differences in sensitivity of thymic subpopulations: Immature single positive (ISP) > DP1, DP2 > DN3, DN4 > DN2 > CD4+ > CD8+. Upon infection, glucocorticoids (GC), inflammatory cytokines, e.g. Ifnγ, etc are induced, which enhance thymocyte death. Treatment with RU486, the GC receptor antagonist, increases the survival of most thymic subsets during infection. Studies with Ifnγ-/- mice demonstrated that endogenous Ifnγ produced during infection enhances the depletion of DN2-DN4 subsets, promotes the accumulation of DP3 and delays the maturation of SP thymocytes. The implications of these observations on host cellular responses during infections are discussed.

Rapid Nongenomic Glucocorticoid Actions in Male Mouse Hypothalamic Neuroendocrine Cells Are Dependent on the Nuclear Glucocorticoid Receptor

Corticosteroids act classically via cognate nuclear receptors to regulate gene transcription; however, increasing evidence supports rapid, nontranscriptional corticosteroid actions via activation of membrane receptors. Using whole-cell patch clamp recordings in hypothalamic slices from male mouse genetic models, we tested for nongenomic glucocorticoid actions at glutamate and gamma aminobutyric acid (GABA) synapses in hypothalamic neuroendocrine cells, and for their dependence on the nuclear glucocorticoid receptor (GR). In enhanced green fluorescent protein-expressing CRH neurons of the paraventricular nucleus (PVN) and in magnocellular neurons of the PVN and supraoptic nucleus (SON), dexamethasone activated postsynaptic membrane-associated receptors and G protein signaling to elicit a rapid suppression of excitatory postsynaptic inputs, which was blocked by genetic deletion of type I cannabinoid receptors and a type I cannabinoid receptor antagonist. In magnocellular neurons, dexamethasone also elicited a rapid nitric oxide-dependent increase in inhibitory postsynaptic inputs. These data indicate a rapid, synapse-specific glucocorticoid-induced retrograde endocannabinoid signaling at glutamate synapses and nitric oxide signaling at GABA synapses. Unexpectedly, the rapid glucocorticoid effects on both excitatory and inhibitory synaptic transmission were lost with conditional deletion of GR in the PVN and SON in slices from a single minded-1-cre-directed conditional GR knockout mouse. Thus, the nongenomic glucocorticoid actions at glutamate and GABA synapses on PVN and SON neuroendocrine cells are dependent on the nuclear GR. The nuclear GR, therefore, is responsible for transducing the rapid steroid response at the membrane, or is either a critical component in the signaling cascade or regulates a critical component of the signaling cascade of a distinct membrane GR.

The circadian clock and glucocorticoids--interactions across many time scales

Glucocorticoids are steroid hormones of the adrenal gland that are an integral component of the stress response and regulate many physiological processes, including metabolism and immune response. Their release into the blood is highly dynamic and occurs in about hourly pulses, the amplitude of which is modulated in a daytime dependent fashion. In addition, in many species seasonal changes in basal glucocorticoid levels have been reported. In their target tissues, glucocorticoids bind to cytoplasmic receptors of the nuclear receptor superfamily. Upon binding, these receptors regulate transcription in a highly dynamic fashion, which involves stochastic binding to regulatory DNA elements on a time scale of seconds and heat shock protein mediated receptor-ligand complex recycling within minutes. The glucocorticoid hormone system interacts with another highly dynamic system, the circadian clock. The circadian clock is an endogenous biological timing mechanism that allows organisms to anticipate regular daily changes in their environment. It regulates daily rhythms of glucocorticoid release by a variety of mechanisms, modulates glucocorticoid signaling and is itself influenced by glucocorticoids. Here, we discuss mechanisms, functions and interactions of the circadian and glucocorticoid systems across time scales ranging from seconds (DNA binding by transcriptional regulators) to years (seasonal rhythms).

11β-hydroxysteroid dehydrogenases: intracellular gate-keepers of tissue glucocorticoid action

Glucocorticoid action on target tissues is determined by the density of "nuclear" receptors and intracellular metabolism by the two isozymes of 11β-hydroxysteroid dehydrogenase (11β-HSD) which catalyze interconversion of active cortisol and corticosterone with inert cortisone and 11-dehydrocorticosterone. 11β-HSD type 1, a predominant reductase in most intact cells, catalyzes the regeneration of active glucocorticoids, thus amplifying cellular action. 11β-HSD1 is widely expressed in liver, adipose tissue, muscle, pancreatic islets, adult brain, inflammatory cells, and gonads. 11β-HSD1 is selectively elevated in adipose tissue in obesity where it contributes to metabolic complications. Similarly, 11β-HSD1 is elevated in the ageing brain where it exacerbates glucocorticoid-associated cognitive decline. Deficiency or selective inhibition of 11β-HSD1 improves multiple metabolic syndrome parameters in rodent models and human clinical trials and similarly improves cognitive function with ageing. The efficacy of inhibitors in human therapy remains unclear. 11β-HSD2 is a high-affinity dehydrogenase that inactivates glucocorticoids. In the distal nephron, 11β-HSD2 ensures that only aldosterone is an agonist at mineralocorticoid receptors (MR). 11β-HSD2 inhibition or genetic deficiency causes apparent mineralocorticoid excess and hypertension due to inappropriate glucocorticoid activation of renal MR. The placenta and fetus also highly express 11β-HSD2 which, by inactivating glucocorticoids, prevents premature maturation of fetal tissues and consequent developmental "programming." The role of 11β-HSD2 as a marker of programming is being explored. The 11β-HSDs thus illuminate the emerging biology of intracrine control, afford important insights into human pathogenesis, and offer new tissue-restricted therapeutic avenues.

Development of a novel fluorescent protein construct by genetically fusing green fluorescent protein to the N-terminal of aspartate dehydrogenase

We developed a fluorescent protein construct by genetically fusing green fluorescent protein (GFP) to aspartate dehydrogenase from Thermotoga maritima. The fusion protein was cloned, heterologously expressed in Escherichia coli cells, and purified by Ni-chelate affinity chromatography. It was then introduced into a measurement cuvette to monitor its fluorescence signal. Aspartate dehydrogenase functioned as the biorecognition element, and aspartate-induced conformational change was converted to a fluorescence signal by GFP. The recombinant protein responded to l-aspartate (l-Asp) linearly within the concentration range of 1-50 mM, and it was capable of giving a fluorescence signal in 1 Min. Although a linear response was also observed for l-Glu, the fluorescence signal was 2.7 times lower than that observed for l-Asp. In the present study, we describe two novelties: development of a genetically encoded fluorescent protein construct for monitoring of l-Asp in vitro, and employment of aspartate dehydrogenase scaffold as a biorecognition element. A few genetically encoded amino-acid biosensors have been described in the literature, but to our knowledge, a protein has not been constructed solely for determination of l-Asp. Periplasmic ligand binding proteins offer high binding affinity in the micromolar range, and they are frequently used as biorecognition elements. Instead of choosing a periplasmic l-Asp binding protein, we attempted to use the substrate specificity of aspartate dehydrogenase enzyme.

CD8 lineage-specific regulation of interleukin-7 receptor expression by the transcriptional repressor Gfi1

Interleukin-7 receptor α (IL-7Rα) is essential for T cell survival and differentiation. Glucocorticoids are potent enhancers of IL-7Rα expression with diverse roles in T cell biology. Here we identify the transcriptional repressor, growth factor independent-1 (Gfi1), as a novel intermediary in glucocorticoid-induced IL-7Rα up-regulation. We found Gfi1 to be a major inhibitory target of dexamethasone by microarray expression profiling of 3B4.15 T-hybridoma cells. Concordantly, retroviral transduction of Gfi1 significantly blunted IL-7Rα up-regulation by dexamethasone. To further assess the role of Gfi1 in vivo, we generated bacterial artificial chromosome (BAC) transgenic mice, in which a modified Il7r locus expresses GFP to report Il7r gene transcription. By introducing this BAC reporter transgene into either Gfi1-deficient or Gfi1-transgenic mice, we document in vivo that IL-7Rα transcription is up-regulated in the absence of Gfi1 and down-regulated when Gfi1 is overexpressed. Strikingly, the in vivo regulatory role of Gfi1 was specific for CD8(+), and not CD4(+) T cells or immature thymocytes. These results identify Gfi1 as a specific transcriptional repressor of the Il7r gene in CD8 T lymphocytes in vivo.

FRET analysis of androgen receptor structure and biochemistry in living cells

The androgen receptor (AR) is the central component of a dynamic conformational and interaction cascade initiated by androgenic hormones. AR function can be modified by cellular inputs not examined in test tube studies of AR action. Thus, there is a need to measure AR conformation and biochemistry directly within the cell where the intracellular locations, levels and availability of the hormone, AR, AR-interacting factors, DNA-binding sites, enzymes that modify those components of AR action, and factors that compete for the formation of functional AR-cofactor complexes may affect AR action. The dynamic nature of the AR functional cycle itself may introduce temporal fluctuations in factor status and location to affect AR output in the intact cell. This chapter focuses on the method of Förster resonance energy transfer which uniquely has the resolving power and ability to directly measure the conformation and biochemistry of AR signaling in living cells.

Expression of 11beta-hydroxysteroid dehydrogenase 2 contributes to glucocorticoid resistance in lymphoblastic leukemia cells

Synthetic glucocorticoids (GCs) form a crucial first-line treatment for childhood acute lymphoblastic leukemia (ALL). However prolonged GC therapy frequently leads to GC-resistance with an unclear molecular mechanism. 11β-hydroxysteroid dehydrogenase (11β-HSD) 2 inactivates GCs within cells. Here, we show the association between GC sensitivity and 11β-HSD2 expression in human T-cell leukemic cell lines. 11β-HSD2 mRNA and protein levels were considerably higher in GC-resistant MOLT4F cells than in GC-sensitive CCRF-CEM cells. The 11β-HSD inhibitor, carbenoxolone pre-treatment resulted in greater cell death with prednisolone assessed by methyl-thiazol-tetrazolium assay and caspase-3/7 assay, suggesting that 11β-HSD2 is a cause of GC-resistance in ALL.

Specificity of glucocorticoid receptor primary antibodies for analysis of receptor localization patterns in cultured cells and rat hippocampus

After glucocorticoid stimulation, glucocorticoid receptors (GRs) are translocated to the nucleus to modulate transcription of glucocorticoid target genes. The subcellular distribution and trafficking of GR in cultured cells has been studied quite intensively using several techniques. However, the intracellular localization of nuclear receptors in ligand-free and stimulated conditions in vivo is still controversial, in part because of inconsistent results with different antibodies. Knowledge of trafficking of GR in vivo could greatly contribute to understanding nuclear receptor signaling. Therefore, in this study we systematically compared a panel of different primary GR antibodies using immunohistochemistry and confocal imaging. Nuclear translocation patterns at different time points after glucocorticoid stimulation were compared in cultured AtT20 cells and rat hippocampal CA1 and dentate gyrus cells. The BuGR2 antibody consistently detected GR nuclear translocation patterns between in vivo and in vitro settings, but the other GR primary antibodies provided contradictory results. While GR H300 and P20 strongly detected nuclear GR immunoreactivity after glucocorticoid stimulation in both CA1 and dentate gyrus cells, the same antibodies provided poor results in cultured cells. The opposite was found for the primary GR M20 antibody. These data indicate that with a particular glucocorticoid receptor antibody the findings in cell culture studies cannot always be extrapolated to in vivo situations. Moreover, different antibodies disclose different features of the glucocorticoid receptor translocation process.

Maternal and early life stress effects on immune function: relevance to immunotoxicology

Stress is triggered by a variety of unexpected environmental stimuli, such as aggressive behavior, fear, forced physical activity, sudden environmental changes, social isolation or pathological conditions. Stressful experiences during very early life (particularly, maternal stress during fetal ontogeny) can permanently alter the responsiveness of the nervous system, an effect called programming or imprinting. Programming affects the hypothalamic-pituitary-adrenocortical (HPA) axis, brain neurotransmitter systems, sympathetic nervous system (SNS), and the cognitive abilities of the offspring, which can alter neural regulation of immune function. Prenatal or early life stress may contribute to the maladaptive immune responses to stress that occur later in life. This review focuses on the effect of maternal and early life stress on immune function in the offspring across life span. It highlights potential mechanisms by which prenatal stress impacts immune functions over life span. The literature discussed in this review suggests that psychosocial stress during pre- and early postnatal life may increase the vulnerability of infants to the effects of immunotoxicants or immune-mediated diseases, with long-term consequences. Neural-immune interactions may provide an indirect route through which immunotoxicants affect the developing immune system. A developmental approach to understanding how immunotoxicants interact with maternal and early life stress-induced changes in immunity is needed, because as the body changes physiologically across life span so do the effects of stress and immunotoxicants. In early and late life, the immune system is more vulnerable to the effects of stress. Stress can mimic the effects of aging and exacerbate age-related changes in immune function. This is important because immune dysregulation in the elderly is more frequently and seriously associated with clinical impairment and death. Aging, exposure to teratogens, and psychological stress interact to increase vulnerability and put the elderly at the greatest risk for disease.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Direct visualization of glucocorticoid receptor positive cells in the hippocampal regions using green fluorescent protein transgenic mice

The hippocampal formation is a plastic brain structure important for certain types of learning and memory, and also vulnerable to the effects of stress and trauma. Since hippocampal neurons express high levels of corticosteroid receptor, the morphological changes, including alterations in the size of soma, and the length and number of neurites and spines, in response to glucocorticoids released as a result of stress are intriguing. In order to highlight the morphology of neurons that express glucocorticoid receptor (GR), we have generated a transgenic mouse line expressing green fluorescent protein (GFP) under the control of the GR promoter. We found strong green fluorescence in the pyramidal cell layer of the CA1 and CA2 regions and the granule cell layer of the dentate gyrus of the hippocampus in brain sections of the transgenic mice. GFP fluorescence was observed not only in somas, but also in neurites including both dendrites and axons. In dissociated culture, we also observed GFP fluorescence in the soma, neurites including both dendrites and axons, and dendritic spines. Microtubule-associated protein 2 immunopositive pyramidal-shaped neurons clearly showed two different populations, GFP positive and GFP negative neurons. These results indicate that this transgenic mouse line should be useful for live imaging of neuronal structure in animals as well as GR-positive cultured cells using GFP as a specific indicator.

Normal immune development and glucocorticoid-induced thymocyte apoptosis in mice deficient for the T-cell death-associated gene 8 receptor

T-cell death-associated gene 8 (TDAG8) is a G-protein-coupled receptor transcriptionally upregulated by glucocorticoids (GCs) and implicated by overexpression studies in psychosine-mediated inhibition of cytokinesis and in GC-induced apoptosis. To examine the physiological function of TDAG8, we generated knockout (KO) mice by homologous recombination. An enhanced green fluorescent protein reporter was knocked into the disrupted tdag8 locus to allow the analysis of TDAG8 expression in living cells. Interestingly, we found that during thymocyte development, TDAG8 expression resembled the dynamic regulation described for known modulators of GC-induced apoptosis, including Bcl-2, Notch1, and GC receptor. TDAG8 was expressed in double-negative cells, was downregulated at the double-positive transition, and was upregulated in single-positive thymocytes. However, despite this striking expression pattern, maturation and selection of thymocytes, as well as major immune functions, were not affected in TDAG8 KO mice. In contrast to previous overexpression results, TDAG8 was dispensable for psychosine-induced formation of multinucleated cells. Furthermore, TDAG8 KO thymocytes showed normal apoptosis following in vivo and in vitro GC treatment. These results, while establishing a useful reporter strain to study T-lymphocyte maturation, argue against a critical role for TDAG8 in immune development, psychosine-mediated inhibition of cytokinesis, and GC-induced cell death.

Different roles for glucocorticoids in thymocyte homeostasis?

Glucocorticoids (GCs) have important immunoregulatory effects on thymocytes and T cells. Ectopic production of GCs has been demonstrated in thymic epithelial cells (TECs) but the role of GCs in thymocyte homeostasis is controversial. Studies in several different mouse models, genetically modified for the GC receptor (GR) expression or function, have demonstrated conflicting results in terms of the effect of the hormone on thymocytes. Here, we summarize these data and suggest that GCs can mediate both positive and negative effects in the organ depending on the local hormonal concentration. Basal GC levels might promote growth of early thymocytes in young mice, and increased levels, generated through a stress reaction, apoptosis in these cells. A gradual loss of GC synthesis in TECs during aging might contribute to thymic involution, a process so far unexplained.

The Expression of 11β-Hydroxysteroid Dehydrogenase Type I by Lymphocytes Provides a Novel Means for Intracrine Regulation of Glucocorticoid Activities

The 11beta-hydroxysteroid dehydrogenase (11beta-HSD) enzymes control the interconversion of active glucocorticoids (GCS) and their inactive 11-keto metabolites, a process commonly referred to as the cortisone/cortisol shuttle. Although the prereceptor metabolism of GCS by 11beta-HSD is well documented in a variety of cells and tissues, it has not yet been carefully investigated in the major cell types of the immune system. In this study, we demonstrate that 11beta-HSD1 transcripts, protein, and enzyme activities are actively expressed in murine CD4(+), CD8(+), and B220(+) lymphocytes, as well as CD11c(+) dendritic cells. Only reductase activity was observed in living cells, evidenced by the restricted conversion of cortisone to cortisol. Activation of CD4(+) T cells increased their 11beta-HSD1 activity, as did their polarization into Th1 or Th2 cells. CD4(+) T cells isolated from aged donors (>16 mo) had increased 11beta-HSD1 protein and an elevated capacity to convert cortisone to cortisol. The GCS generated in murine CD4(+) T cells from their inactive 11-keto metabolites could activate the GCS receptor, demonstrated by an up-regulation of IL-7Ralpha and GCS-induced leucine zipper gene expression. The presence of a functional 11beta-HSD1 provides lymphocytes with a novel intracrine regulatory mechanism that could influence such processes as lymphocyte development, effector function, and susceptibility to apoptosis. Thus, the presence of 11beta-HSD1 provides an additional means to facilitate GCS influences over lymphocyte activities, uncoupled from the plasma concentration of GCS.

Visualization of glucocorticoid receptor in the brain of green fluorescent protein–glucocorticoid receptor knockin mice

Glucocorticoids exert various neuroendocrinological effects, including stress response, in the central nervous system via glucocorticoid receptor (GR). GRs are transported from the cytoplasm to the nucleus upon ligand binding, and then exert the transcriptional activity. Although it is important for unraveling the actual property of the GR in vivo, subcellular dynamics of the GR are still unclear within the brain tissue in which the neuronal circuitry is maintained. To address this issue, we generated green fluorescent protein (GFP)-GR knockin mice, whose GR has been replaced by a GFP-GR fusion protein that is functionally indistinguishable from endogenous GR. In fixed brain sections of the GFP-GR knockin mice, the distribution of the green fluorescence was similar to that of GR immunoreactivity. By subtracting autofluorescence using fluorescent emission fingerprinting method with confocal laser scanning microscope, nuclear localization of GFP-GR was identifiable in the hippocampal CA3 subregion, where subcellular localization of the GR has been unsolved compared with other areas. To examine the subcellular trafficking of GFP-GR in vivo, we performed adrenalectomy on the GFP-GR knockin mice. GFP-GR was translocated from the nucleus to the cytoplasm and neurites two days after adrenalectomy. Furthermore, laser scanning cytometry by which fluorescence intensity in situ can be quantitatively measured revealed the entire GFP-GR expression level was increased. We then examined the dynamic changes in the subcellular localization of GFP-GR in living hippocampal neurons both in dissociated culture and in tissue slices. GFP-GR was localized in not only the perikarya but also neurites in the absence of ligand, and nuclear translocation following ligand treatment was observed. This is the first report visualizing subcellular trafficking of the GR in the mouse brain in more physiological condition. The present results propose new avenues for the research of the GR dynamics both in vitro and in vivo.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

Thymus-derived glucocorticoids are insufficient for normal thymus homeostasis in the adult mouse

BACKGROUND

It is unclear if thymus-derived glucocorticoids reach sufficient local concentrations to support normal thymus homeostasis, or if adrenal-derived glucocorticoids from the circulation are required. Modern approaches to this issue (transgenic mice that under or over express glucocorticoid receptor in the thymus) have yielded irreconcilably contradictory results, suggesting fundamental problems with one or more the transgenic mouse strains used. In the present study, a more direct approach was used, in which mice were adrenalectomized with or without restoration of circulating corticosterone using timed release pellets. Reversal of the increased number of thymocytes caused by adrenalectomy following restoration of physiological corticosterone concentrations would indicate that corticosterone is the major adrenal product involved in thymic homeostasis.

RESULTS

A clear relationship was observed between systemic corticosterone concentration, thymus cell number, and percentage of apoptotic thymocytes. Physiological concentrations of corticosterone in adrenalectomized mice restored thymus cell number to normal values and revealed differential sensitivity of thymocyte subpopulations to physiological and stress-inducible corticosterone concentrations.

CONCLUSION

This indicates that thymus-derived glucocorticoids are not sufficient to maintain normal levels of death by neglect in the thymus, but that apoptosis and possibly other mechanisms induced by physiological, non stress-induced levels of adrenal-derived corticosterone are responsible for keeping the total number of thymocytes within the normal range.

Expression of the glucocorticoid receptor from the 1A promoter correlates with T lymphocyte sensitivity to glucocorticoid-induced cell death

Glucocorticoid (GC) hormones cause pronounced T cell apoptosis, particularly in immature thymic T cells. This is possibly due to tissue-specific regulation of the glucocorticoid receptor (GR) gene. In mice the GR gene is transcribed from five separate promoters designated: 1A, 1B, 1C, 1D, and 1E. Nearly all cells express GR from promoters 1B-1E, but the activity of the 1A promoter has only been reported in the whole thymus or lymphocyte cell lines. To directly assess the role of GR promoter use in sensitivity to glucocorticoid-induced cell death, we have compared the activity of the GR 1A promoter with GC sensitivity in different mouse lymphocyte populations. We report that GR 1A promoter activity is restricted to thymocyte and peripheral lymphocyte populations and the cortex of the brain. The relative level of expression of the 1A promoter to the 1B-1E promoters within a lymphocyte population was found to directly correlate with susceptibility to GC-induced cell death, with the extremely GC-sensitive CD4+CD8+ thymocytes having the highest levels of GR 1A promoter activity, and the relatively GC-resistant alphabetaTCR+CD24(int/low) thymocytes and peripheral T cells having the lowest levels. DNA sequencing of the mouse GR 1A promoter revealed a putative glucocorticoid-response element. Furthermore, GR 1A promoter use and GR protein levels were increased by GC treatment in thymocytes, but not in splenocytes. These data suggest that tissue-specific differences in GR promoter use determine T cell sensitivity to glucocorticoid-induced cell death.

Expression System for Enhanced Green Fluorescence Protein Conjugated Recombinant Antibody Fragment

Recent development of recombinant antibody technology has enabled fusion of recombinant antibody fragment with fluorescent proteins for various applications such as flow cytometry, fluorescence immunoassay, and fluorescent microscopy. In this study, we generated various forms of green fluorescence protein (EGFP)-fused anti-c-Met antibody fragment. Among these fusion proteins, EGFP fusion to the light chain showed high expression in a soluble form of protein in E. coli, and high binding activity to c-Met. A feasibility of the constructs was further examined by replacing the Fab gene by a Fab library of catalytic subunit of protein kinase A (PKA) to construct the Fab library in EGFP fused form. We also constructed the conventional Fab library. After a series of biopanning, we found that the binding capability of EGFP-anti-PKA Fab was comparable with anti-PKA Fab. Sequence analysis of the selected clones showed > or =99% identity in amino acid sequence and shared the same CDR sequence. These results demonstrate that EGFP fusion to the light chain using our vector system does not influence the selection of reactive Fab and that this vector system is useful for EGFP fusion to Fab to develop a one-step detection system.

Fas ligand is enriched in the caveolae membrane domains of thymic epithelial cells

Both Fas and Fas ligand (FasL) are expressed in the thymus. Although reports suggest that they are important throughout the thymocyte maturation process their precise role remains elusive. The present paper characterizes the expression of FasL in the thymus and in the TEA3A1 and BT1B functional thymic epithelial cell (TEC) lines. FasL expression by thymus fractions, TEA3A1, and BT1B cells was detected by Northern blot analysis. In TEA3A1 cells, we discovered that FasL protein expression was localized to caveolae membrane domains. This restricted subcellular localization of FasL, together with reports describing the localization of the major histocompatibility complex proteins, the T cell receptor and Fas to caveolae membrane domains, may provide a mechanism for the deletion of thymocytes during negative selection. Finally, using semi-quantitative RT-PCR we found that FasL expression by TECs is regulated by glucocorticoids.

Effects of glucocorticoid on IL-13-induced Muc5ac expression in airways of mice

To study the effects of glucocorticoid on the IL-13-induced Muc5ac expression in airways of mice, and investigate its role in mucus secretion of airways, 24 pathogen-free BALB/c mice were randomly divided into 3 groups. IL-13 group received an nasal instillation of 100 microg of recombinant murine IL-13 solution on days 1, 3 and 5. In dexamethasone group, dexamethasone (0.5 mg/kg) was administered intraperitoneally 24 h before and 1 h before the first instillation of IL-13 and on 4 consecutive days (day 0 to day 5, 6 consecutive days in total), while control group was not treated with IL-13 or dexamethasone. Bronchoalveolar lavage fluid (BALF) was collected and eosinophils were counted, and expression of Muc5ac mRNA and protein in lungs were detected by reverse transcription-polymerase chain reaction (RT-PCR) technology and immunohistochemical assay respectively. Our results showed that the number of mice, with positve Muc5ac protein expression, expression of Muc5ac mRNA and eosinophils in BALF after IL-13 treatment were all significantly higher than that of control group (all P<0.01). Despite eosinophils reduced (P<0.01), the number of mice with positive Muc5ac protein expression, expression of Muc5ac mRNA afterdexamethasone treatment didn't decreas significantly as compared with that of IL-13 group. It is concluded that IL-13 can up-regulate the expression of Muc5ac mRNA and protein, which may play a pivotal role in the mucus overproduction of airways. Dexamethasone can suppress IL-13-induced eosinophilic infiltration in lung but can't inhibit the mucus overproduction.

The glucocorticoid receptor mediates the thymic epithelial cell-induced apoptosis of CD4+8+ thymic lymphoma cells

"Negative selection" and "death by neglect" are governed by apoptotic processes occurring in the thymus that shape the repertoire of maturing T cells. We have previously developed an in vitro model that recapitulates "death by neglect": Co-cultivation of double positive (DP) thymocytes or thymic lymphoma cells (PD1.6) with thymic epithelial cells (TEC) caused TcR-independent apoptosis of the former. We further demonstrated that this apoptosis could be attenuated by aminoglutethimide, an inhibitor of steroid synthesis, suggesting a role of TEC-derived glucocorticoids (GC) in this death process. We have now substantiated the role of the GC-glucocorticoid receptor (GR) axis by using a GC-resistant subline (PD1.6Dex(-)) obtained from the GC-sensitive PD1.6 cells by repeated exposures to increasing doses of dexamethasone (Dex). The PD1.6Dex(-) cells barely express GR and are much less sensitive to TEC-induced apoptosis. Re-expression of GR in PD1.6Dex(-) cells restored their sensitivity to both Dex and TEC, highlighting the central role of GR in these apoptotic processes. Likewise, repeated exposures of PD1.6 cells to TEC led to the selection of TEC-resistant cells (PD1.6TEC(-)) that are insensitive to corticosterone and less sensitive to Dex, though their GR level was only moderately reduced. This is in line with the low levels of corticosterone secreted by TEC. Altogether, our data show that TEC eliminates DP thymic lymphoma cells in a GR-dependent manner and modulates the GC sensitivity of the surviving cells.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.

Dissecting adrenal and behavioral responses to stress by targeted gene inactivation in mice

To define the molecular pathways modulating adrenal and behavioral responses to stress, we have generated mice with inactivation of hypothalamic neuropeptides and signaling pathways. Studies in mice deficient in corticotropin-releasing hormone (CRH) have revealed the essential role for CRH in adrenal glucocorticoid production in response to many physiological and psychological stressors. Immune system activation in CRH-deficient mice provides a unique exception to the necessity for CRH in stimulating adrenal glucocorticoid production. By analyzing mice deficient in interleukin-6 (IL-6) and CRH, we find that restoration of glucocorticoid output with inflammation is largely mediated by dysregulated IL-6 production. Current studies focus on identifying cellular and gene targets by which glucocorticoids regulate immune system function. In contrast to impaired adrenocortical responses to stress, CRH-deficient mice exhibit normal behavioral responses to stress. To determine signaling pathways that may contribute to the behavioral responses to stress, we have generated and analyzed mice deficient in adenylyl cyclase type 8 (AC8). AC8 deficient mice have intact adrenocortical responses to stress, but an inability to undergo stress-induced alterations in behavior.

Stress and the immune system

Stressors can positively or adversely affect immune and inflammatory responses. However, the current understanding of these effects at the cellular and molecular levels is not sufficient to allow prediction of the effects of a particular stressor on a particular immune or inflammatory function. Three complementary conceptual frameworks are presented that may prove useful in developing such an understanding. In addition, specific examples of the action of particular stress mediators on particular immune or inflammatory end points are discussed, and the relationship of these observations to the conceptual frameworks is indicated. Several of the effects discussed are relevant clinically, and the prospects for pharmacological intervention to prevent adverse effects of stressors on the immune system are discussed. Finally, some of the factors that can (sometimes unexpectedly) influence the outcome of stress-immunology studies and some of the pitfalls that continue to make this area of research controversial in some circles are discussed.

Differential regulation by glucocorticoid of interleukin-13-induced eosinophilia, hyperresponsiveness, and goblet cell hyperplasia in mouse airways

Interleukin (IL)-13 induces important features of bronchial asthma such as eosinophilic infiltration, airway hyperresponsiveness (AHR), and mucus hypersecretion. Although glucocorticoids suppress airway inflammation and remain the most effective therapy for asthma, the effects of glucocorticoids on the IL-13-dependent features are unknown. We studied the effects of dexamethasone on eotaxin production, eosinophil accumulation, goblet cell hyperplasia, and AHR after IL-13 administration into the airways of mice in vivo. MUC5AC gene expression, a marker of goblet cell hyperplasia, was also analyzed. IL-13 alone dose dependently induced AHR. Treatment with dexamethasone inhibited eotaxin expression and completely abolished eosinophil accumulation, but it did not affect AHR, MUC5AC overexpression, or goblet cell hyperplasia induced by IL-13. The effects of tumor necrosis factor-alpha on IL-13-induced AHR were also examined. Tumor necrosis factor-alpha did not affect AHR despite marked enhancement of eosinophil infiltration in IL-13-treated mice. These findings suggest that glucocorticoid is not sufficient to suppress IL-13-induced AHR or goblet cell hyperplasia and that eotaxin expression and eosinophilic inflammation do not have a causal relationship to the induction of AHR or goblet cell hyperplasia by IL-13. Control of steroid-resistant features induced by IL-13, including AHR and mucus production, may provide new therapeutic modalities for asthma.

Glucocorticoid receptor deficient thymic and peripheral T cells develop normally in adult mice

The involvement of glucocorticoid receptor (GR) signaling in T cell development is highly controversial, with several studies for and against. We have previously demonstrated that GR(-/-) mice, which usually die at birth because of impaired lung development, exhibit normal T cell development, at least in embryonic mice and in fetal thymus organ cultures. To directly investigate the role of GR signaling in adult T cell development, we analyzed the few GR(-/-) mice that occasionally survive birth, and irradiated mice reconstituted with GR(-/-) fetal liver precursors. All thymic and peripheral T cells, as well as other leukocyte lineages, developed and were maintained at normal levels. Anti-CD3-induced cell death of thymocytes in vitro, T cell repertoire heterogeneity and T cell proliferation in response to anti-CD3 stimulation were normal in the absence of GR signaling. Finally, we show that metyrapone, an inhibitor of glucocorticoid synthesis (commonly used to demonstrate a role for glucocorticoids in T cell development), impaired thymocyte development regardless of GR genotype indicating that this reagent inhibits thymocyte development in a glucocorticoid-independent fashion. These data demonstrate that GR signaling is not required for either normal T cell development or peripheral maintenance in embryonic or adult mice.