Steroid hormone pulsing drives cyclic gene expression

The Circadian Clock Is a Key Driver of Steroid Hormone Production in Drosophila

Biological clocks allow organisms to anticipate daily environmental changes such as temperature fluctuations, abundance of daylight, and nutrient availability. Many circadian-controlled physiological states are coordinated by the release of systemically acting hormones, including steroids and insulin [1-7]. Thus, hormones relay circadian outputs to target tissues, and disrupting these endocrine rhythms impairs human health by affecting sleep patterns, energy homeostasis, and immune functions [8-10]. It is largely unclear, however, whether circadian circuits control hormone levels indirectly via central timekeeping neurons or whether peripheral endocrine clocks can modulate hormone synthesis directly. We show here that perturbing the circadian clock, specifically in the major steroid hormone-producing gland of Drosophila, the prothoracic gland (PG), unexpectedly blocks larval development due to an inability to produce sufficient steroids. This is surprising, because classic circadian null mutants are viable and result in arrhythmic adults [4, 11-14]. We found that Timeless and Period, both core components of the insect clock [15], are required for transcriptional upregulation of steroid hormone-producing enzymes. Timeless couples the circadian machinery directly to the two canonical pathways that regulate steroid synthesis in insects, insulin and PTTH signaling [16], respectively. Activating insulin signaling directly modulates Timeless function, suggesting that the local clock in the PG is normally synced with systemic insulin cues. Because both PTTH and systemic insulin signaling are themselves under circadian control, we conclude that de-synchronization of a local endocrine clock with external circadian cues is the primary cause for steroid production to fail.

Minireview: the impact of antenatal therapeutic synthetic glucocorticoids on the developing fetal brain

The life-threatening, emotional, and economic burdens of premature birth have been greatly alleviated by antenatal glucocorticoid (GC) treatment. Antenatal GCs accelerate tissue development reducing respiratory distress syndrome and intraventricular hemorrhage in premature infants. However, they can also alter developmental processes in the brain and trigger adverse behavioral and metabolic outcomes later in life. This review summarizes animal model and clinical studies that examined the impact of antenatal GCs on the developing brain. In addition, we describe studies that assess glucocorticoid receptor (GR) action in neural stem/progenitor cells (NSPCs) in vivo and in vitro. We highlight recent work from our group on two GR pathways that impact NSPC proliferation, ie, a nongenomic GR pathway that regulates gap junction intercellular communication between coupled NSPCs through site-specific phosphorylation of connexin 43 and a genomic pathway driven by differential promoter recruitment of a specific GR phosphoisoform.

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).

Linking Inflammation, Cardiorespiratory Variability, and Neural Control in Acute Inflammation via Computational Modeling

Acute inflammation leads to organ failure by engaging catastrophic feedback loops in which stressed tissue evokes an inflammatory response and, in turn, inflammation damages tissue. Manifestations of this maladaptive inflammatory response include cardio-respiratory dysfunction that may be reflected in reduced heart rate and ventilatory pattern variabilities. We have developed signal-processing algorithms that quantify non-linear deterministic characteristics of variability in biologic signals. Now, coalescing under the aegis of the NIH Computational Biology Program and the Society for Complexity in Acute Illness, two research teams performed iterative experiments and computational modeling on inflammation and cardio-pulmonary dysfunction in sepsis as well as on neural control of respiration and ventilatory pattern variability. These teams, with additional collaborators, have recently formed a multi-institutional, interdisciplinary consortium, whose goal is to delineate the fundamental interrelationship between the inflammatory response and physiologic variability. Multi-scale mathematical modeling and complementary physiological experiments will provide insight into autonomic neural mechanisms that may modulate the inflammatory response to sepsis and simultaneously reduce heart rate and ventilatory pattern variabilities associated with sepsis. This approach integrates computational models of neural control of breathing and cardio-respiratory coupling with models that combine inflammation, cardiovascular function, and heart rate variability. The resulting integrated model will provide mechanistic explanations for the phenomena of respiratory sinus-arrhythmia and cardio-ventilatory coupling observed under normal conditions, and the loss of these properties during sepsis. This approach holds the potential of modeling cross-scale physiological interactions to improve both basic knowledge and clinical management of acute inflammatory diseases such as sepsis and trauma.

Transcriptional implications of ultradian glucocorticoid secretion in homeostasis and in the acute stress response

Endogenous glucocorticoids are secreted by the hypothalamic-pituitary-adrenal (HPA) axis in response to a wide range of stressors. Glucocorticoids exert significant downstream effects, including the regulation of many inflammatory genes. The HPA axis functions such that glucocorticoids are released in a pulsatile manner, producing ultradian rhythms in plasma glucocorticoid levels. It is becoming increasingly evident that this ultradian pulsatility is important in maintaining proper homeostatic regulation and responsiveness to stress. This is particularly interesting from a clinical perspective given that pathological dysfunctions of the HPA axis produce altered ultradian patterns. Modeling this system facilitates the understanding of how glucocorticoid pulsatility arises, how it can be lost, and the transcriptional implications of ultradian rhythms. To approach these questions, we developed a mathematical model that integrates the cyclic production of glucocorticoids by the HPA axis and their downstream effects by integrating existing models of the HPA axis and glucocorticoid pharmacodynamics. This combined model allowed us to evaluate the implications of pulsatility in homeostasis as well as in response to acute stress. The presence of ultradian rhythms allows the system to maintain a lower response to homeostatic levels of glucocorticoids, but diminished feedback within the HPA axis leads to a loss of glucocorticoid rhythmicity. Furthermore, the loss of HPA pulsatility in homeostasis correlates with a decrease in the peak output in response to an acute stressor. These results are important in understanding how cyclic glucocorticoid secretion helps maintain the responsiveness of the HPA axis.

Pulsatile glucocorticoid secretion: origins and downstream effects

Glucocorticoids are steroid hormones which, among other functions, exert an antiinflammatory effect. Endogenous glucocorticoids are normally secreted by the adrenal gland in discrete bursts. It is becoming increasingly evident that this pulsatile secretion pattern, leading to ultradian rhythms of plasma glucocorticoid levels, may have important downstream regulatory effects on glucocorticoid-responsive genes. Mathematical modeling of this system can compliment recent experimental data and quantitatively evaluate hypothesized mechanistic underpinnings of differential pulsatile signal transduction. In this paper, we describe an integrated model of pulsatile secretion of glucocorticoids by the hypothalamic-pituitary-adrenal (HPA) axis and the pharmacodynamic effect of glucocorticoids. This model is used to investigate the difference in transcriptional responses to pulsatile and constant glucocorticoid exposure. Nonlinearity in ligand-receptor kinetics leads to the differential expression of glucocorticoid-responsive genes in response to different patterns of glucocorticoid secretion, even when the total amount of glucocorticoid exposure is held constant. Understanding the implications of ultradian rhythms in glucocorticoids is important in studying the dysregulation of HPA axis function leading to altered glucocorticoid secretion patterns in disease.

Dynamic nature of transcriptional regulation of nuclear receptor target genes in the context of chromatin organization

Many members of the nuclear receptor (NR) superfamily are expressed in the skin making them a highly interesting subject of dermato-endocrine research. Natural and synthetic NR ligands are used for the treatment of various skin disorders. We discuss here the impact of the dynamic nature of chromatin organization, i.e., the spatio-temporal changes of chromatin region of NR target genes. This dynamics is triggered by environmental changes, of which for NRs the exposure with their ligands is most critical. For an understanding of skin disorders, which involve the actions of NRs, this means that the parameter time should be carefully considered in context of other factors that may influence the chromatin organization, and by this the responsiveness, of key NR target genes.

The stress of starvation: glucocorticoid restraint of beta cell development

Developmental insults during gestation, such as under-nutrition, are known to restrict the number of beta cells that form in the fetal pancreas and are maintained in adulthood, leading to increased risk of type 2 diabetes. There are now substantial data indicating that glucocorticoids mediate this effect of under-nutrition on beta cell mass and that even at physiological levels they restrain fetal beta cell development in utero. There are emerging clues that this occurs downstream of endocrine commitment by neurogenin 3 but prior to terminal beta cell differentiation. Deciphering the precise mechanism will be important as it might unveil new pathways by which to manipulate beta cell mass that could be exploited as novel therapies for patients with diabetes.

Molecular basis for gene-specific transactivation by nuclear receptors

Nuclear receptors (NRs) are key transcriptional regulators of metazoan physiology and metabolism. Different NRs bind to similar or even identical core response elements; however, they regulate transcription in a highly receptor- and gene-specific manner. These differences in gene activation can most likely be accounted for by mechanisms involving receptor-specific interactions with DNA as well as receptor-specific interactions with protein complexes binding to adjacent and distant DNA sequences. Here, we review key molecular aspects of transactivation by NRs with special emphasis on the recent advances in the molecular mechanisms responsible for receptor- and gene-specific transcriptional activation. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Cell signaling

This review explores advances in our understanding of dynamicism in cellular signaling. Areas highlighted include the role of stochasticity in producing diversity in analogous signaling circumstances; population desynchronization's effect in masking newly appreciated repetitive bursts in protein phosphorylation and messenger RNA production; double-positive feedback interactions and their ability to synchronize multiple signal transduction pathways; scaffolding proteins control over signaling feedback; and frequency-responsive transcriptional regulation as an example of dynamicism in signaling.

Activation of mineralocorticoid receptors by exogenous glucocorticoids and the development of cardiovascular inflammatory responses in adrenalectomized rats

Activation of the mineralocorticoid receptor (MR) in the context of a high salt intake produces cardiovascular inflammation plus cardiac fibrosis and failure. Inactivation of vascular 11beta-hydroxysteroid dehydrogenase type 2 activity in intact animals by carbenoxolone (CBX) produces a similar pathology, presumably reflecting coronary vascular MR activation by endogenous glucocorticoids. To test this hypothesis, we have used adrenalectomized rats, without endogenous corticosteroids, and examined the consequences of corticosterone (CORT) replacement on a series of cardiovascular disease parameters. Uninephrectomized adrenalectomized Sprague Dawley rats given 1% NaCl/0.3% KCl to drink were treated for 8 d as follows: control; 20 mg deoxycorticosterone (DOC); 2 mg/d CORT; 2.5 mg/d CBX; CORT plus CBX (CORT/CBX); and CORT/CBX plus 100 mg/kg.d eplerenone. Markers of cardiac oxidative stress (p22(phox) and NOX4 mRNA) were up-regulated in the DOC and CORT/CBX groups; in contrast, inflammatory cell infiltration was increased and endothelial nitric oxide synthase down-regulated by CORT as well as by DOC and CORT/CBX. In the kidney, connective tissue growth factor mRNA levels were increased by DOC and CORT/CBX; in contrast, DOC had no effect on mRNA levels for channel inducing factor or endothelin 3, which were elevated only by CORT/CBX. All changes noted were reversed by eplerenone. Rats given 10-fold lower CORT (0.2 mg/d) with or without CBX showed no change in any parameter. These results suggest that there exist distinct but overlapping ligand-specific MR-mediated tissue responses to a classic mineralocorticoid (DOC) and to the glucocorticoid CORT, in the presence and absence of CBX to block vascular 11beta-hydroxysteroid dehydrogenase type 2.

Mechanisms of transcription factor selectivity

The initiation of transcription is regulated by transcription factors (TFs) binding to DNA response elements (REs). How do TFs recognize specific binding sites among the many similar ones available in the genome? Recent research has illustrated that even a single nucleotide substitution can alter the selective binding of TFs to coregulators, that prior binding events can lead to selective DNA binding, and that selectivity is influenced by the availability of binding sites in the genome. Here, we combine structural insights with recent genomics screens to address the problem of TF-DNA interaction specificity. The emerging picture of selective binding site sequence recognition and TF activation involves three major factors: the cellular network, protein and DNA as dynamic conformational ensembles and the tight packing of multiple TFs and coregulators on stretches of regulatory DNA. The classification of TF recognition mechanisms based on these factors impacts our understanding of how transcription initiation is regulated.