Sex-Dependent Effects of Chronic Circadian Disruption in AβPP/PS1 Mice

BACKGROUND

Chronic disruption of the circadian timing system, often reflected as a loss of restful sleep, also includes myriad other pathophysiological effects.

OBJECTIVE

The current study examined how chronic circadian disruption (CD) could contribute to pathology and rate of progression in the AβPP/PS1 mouse model of Alzheimer's disease (AD).

METHODS

A chronic CD was imposed until animals reached 6 or 12 months of age in AβPP/PS1 and C57BL/6J control mice. Home cage activity was monitored for a period of 3-4 weeks prior to the endpoint along with a single timepoint measure of glucose sensitivity. To assess long term effects of CD on the AD phenotype, animals were re-entrained to a no disruption (ND) schedule just prior to the endpoint, after which a Morris water maze (MWM) was used to assess spatial learning and memory.

RESULTS

Dampening of nighttime activity levels occurred in disrupted animals, and female animals demonstrated a greater adaptability to CD. Diminished arginine vasopressin (AVP) and vasoactive intestinal peptide (VIP) levels in the suprachiasmatic nucleus (SCN) of 12-month male AβPP/PS1 exposed to the CD paradigm were observed, potentially accounting for the diminished re-entrainment response. Similarly, CD worsened performance in the MWM in 12-month male AβPP/PS1 animals, whereas no effect was seen in females.

CONCLUSIONS

Collectively, these findings show that exposure to chronic CD impairs circadian behavioral patterns and cognitive phenotypes of AβPP/PS1 mouse model in a sex-dependent manner.

Deficiency of Adipose Aryl Hydrocarbon Receptor Protects against Diet-Induced Metabolic Dysfunction through Sexually Dimorphic Mechanisms

The molecular mechanisms underlying diet-induced obesity are complex and remain unclear. The activation of the aryl hydrocarbon receptor (AhR), a xenobiotic sensor, by obesogens may contribute to diet-induced obesity through influences on lipid metabolism and insulin resistance acting at various sites, including adipose tissue. Thus, our hypothesis was that conditional AhR depletion, specifically from mature adipose tissue (CadKO), would improve high-fat diet (HFD)-induced metabolic dysfunction. CadKO protects mice from HFD-induced weight gain. CadKO females eat fewer calories, leading to increased energy expenditure (EE) and improved glucose tolerance on HFD. Our exploration of adipose tissue biology suggests that the depletion of AhR from adipocytes provides female mice with an increased capacity for adipogenesis and lipolysis, allowing for the maintenance of a healthy adipocyte phenotype. The HFD-induced leptin rise was reduced in CadKO females, but the hypothalamic leptin receptor (LepR) was increased in the energy regulatory regions of the hypothalamus, suggesting an increased sensitivity to leptin. The estrogen receptor α (ERα) was higher in CadKO female adipose tissue and the hypothalamus. CadKO males displayed a delayed progression of obesity and insulin resistance. In males, CadKO ameliorated proinflammatory adipocytokine secretion (such as TNFα, IL1β, IL6) and displayed reduced inflammatory macrophage infiltration into adipose depots. Overall, CadKO improves weight control and systemic glucose homeostasis under HFD challenge but to a more profound extent in females. CadKO facilitates a lean phenotype in females and mediates healthy adipose-hypothalamic crosstalk. In males, adipose-specific AhR depletion delays the development of obesity and insulin resistance through the maintenance of healthy crosstalk between adipocytes and immune cells.

Aryl hydrocarbon receptor affects circadian-regulated lipolysis through an E-Box-dependent mechanism

An internal circadian clock regulates timing of systemic energy homeostasis. The central clock in the hypothalamic suprachiasmatic nucleus (SCN) directs local clocks in peripheral tissues such as liver, muscle, and adipose tissue to synchronize metabolism with food intake and rest/activity cycles. Aryl hydrocarbon receptor (AhR) interacts with the molecular circadian clockworks. Activation of AhR dampens rhythmic expression of core clock genes, which may lead to metabolic dysfunction. Given the importance of appropriately-timed adipose tissue function to regulation of energy homeostasis, this study focused on mechanisms by which AhR may influence clock-controlled adipose tissue activity. We hypothesized that AhR activation in adipose tissue would impair lipolysis by dampening adipose rhythms, leading to a decreased lipolysis rate during fasting, and subsequently, altered serum glucose concentrations. Levels of clock gene and lipolysis gene transcripts in mouse mesenchymal stem cells (BMSCs) differentiated into mature adipocytes were suppressed by the AhR agonist β-napthoflavone (BNF), in an AhR dependent manner. BNF altered rhythms of core clock gene and lipolysis gene transcripts in C57bl6/J mice. BNF reduced serum free fatty acids, glycerol and liver glycogen. Chromatin immunoprecipitation indicated that BNF increased binding of AhR to E-Box elements in clock gene and lipolysis gene promoters. These data establish a link between AhR activation and impaired lipolysis, specifically by altering adipose tissue rhythmicity. In response to the decreased available energy from impaired lipolysis, the body increases glycogenolysis, thereby degrading more glycogen to provide necessary energy.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Assessing Sex-Specific Circadian, Metabolic, and Cognitive Phenotypes in the AβPP/PS1 and APPNL-F/NL-F Models of Alzheimer's Disease

BACKGROUND

Circadian disruption has long been recognized as a symptom of Alzheimer's disease (AD); however, emerging data suggests that circadian dysfunction occurs early on in disease development, potentially preceding any noticeable cognitive deficits.

OBJECTIVE

This study compares the onset of AD in male and female wild type (C57BL6/J), transgenic (AβPP/PS1), and knock-in (APPNL-F/NL-F) AD mouse models from the period of plaque initiation (6 months) through 12 months.

METHODS

Rhythmic daily activity patterns, glucose sensitivity, cognitive function (Morris water maze, MWM), and AD pathology (plaques formation) were assessed. A comparison was made across sexes.

RESULTS

Sex-dependent hyperactivity in AβPP/PS1 mice was observed. In comparison to C57BL/6J animals, 6-month-old male AβPP/PS1 demonstrated nighttime hyperactivity, as did 12-month-old females. Female AβPP/PS1 animals performed significantly worse on a MWM task than AβPP/PS1 males at 12 months and trended toward increased plaque pathology. APPNL-F/NL-F 12-month-old males performed significantly worse on the MWM task compared to 12-month-old females. Significantly greater plaque pathology occurred in AβPP/PS1 animals as compared to APPNL-F/NL-F animals. Female AβPP/PS1 animals performed significantly worse than APPNL-F/NL-F animals in spatial learning and memory tasks, though this was reversed in males.

CONCLUSION

Taken together, this study provides novel insights into baseline sex differences, as well as characterizes baseline diurnal activity variations, in the AβPP/PS1 and APPNL-F/NL-F AD mouse models.

Aryl Hydrocarbon Receptor Deficiency Alters Circadian and Metabolic Rhythmicity

PAS domain-containing proteins can act as environmental sensors that capture external stimuli to allow coordination of organismal physiology with the outside world. These proteins permit diverse ligand binding and heterodimeric partnership, allowing for varied combinations of PAS-dependent protein-protein interactions and promoting crosstalk among signaling pathways. Previous studies report crosstalk between circadian clock proteins and the aryl hydrocarbon receptor (AhR). Activated AhR forms a heterodimer with the circadian clock protein Bmal1 and thereby functionally inhibits CLOCK/Bmal1 activity. If physiological activation of AhR through naturally occurring, endogenous ligands inhibits clock function, it seems plausible to hypothesize that decreased AhR expression releases AhR-induced inhibition of circadian rhythms. Because both AhR and the clock are important regulators of glucose metabolism, it follows that decreased AhR will also alter metabolic function. To test this hypothesis, rhythms of behavior, metabolic outputs, and circadian and metabolic gene expression were measured in AhR-deficient mice. Genetic depletion of AhR enhanced behavioral responses to changes in the light-dark cycle, increased rhythmic amplitude of circadian clock genes in the liver, and altered rhythms of glucose and insulin. This study provides evidence of AhR-induced inhibition that influences circadian rhythm amplitude.

Aryl hydrocarbon receptor-deficient mice are protected from high fat diet-induced changes in metabolic rhythms

High fat diet (HFD) consumption alters the synchronized circadian timing system resulting in harmful loss, gain or shift of transcriptional oscillations. The aryl hydrocarbon receptor (AhR) shares structural homology to clock genes, containing both PAS domains and basic helix-loop helix structural motifs, allowing for interaction with components of the primary circadian feedback loop. Activation of AhR alters circadian rhythmicity, primarily through inhibition of Clock/Bmal1-mediated regulation of Per1. AhR-deficient mice are protected from diet-induced metabolic dysfunction, exhibiting enhanced insulin sensitivity and glucose tolerance. This study examined whether AhR haploinsufficiency can also protect against diet-induced alterations in rhythm. After feeding AhR+/+ and AhR+/- mice an HFD (60% fat) for 15 weeks, samples were collected every 4 hours over a 24-hour period. HFD altered the rhythm of serum glucose and the metabolic transcriptome, including hepatic nuclear receptors Rev-erbα and PPARγ in wild-type c57bl6/j mice. AhR reduction provided protection against diet-induced transcriptional oscillation changes; serum glucose and metabolic gene rhythms were protected from the disruption caused by HFD feeding. These data highlight the critical role of AhR signaling in the regulation of metabolism and provide a potential therapeutic target for diseases characterized by rhythmic desynchrony.

Circadian Disruption Reveals a Correlation of an Oxidative GSH/GSSG Redox Shift with Learning and Impaired Memory in an Alzheimer's Disease Mouse Model

It is unclear whether pre-symptomatic Alzheimer's disease (AD) causes circadian disruption or whether circadian disruption accelerates AD pathogenesis. In order to examine the sensitivity of learning and memory to circadian disruption, we altered normal lighting phases by an 8 h shortening of the dark period every 3 days (jet lag) in the APPSwDI NOS2-/- model of AD (AD-Tg) at a young age (4-5 months), when memory is not yet affected compared to non-transgenic (non-Tg) mice. Analysis of activity in 12-12 h lighting or constant darkness showed only minor differences between AD-Tg and non-Tg mice. Jet lag greatly reduced activity in both genotypes during the normal dark time. Learning on the Morris water maze was significantly impaired only in the AD-Tg mice exposed to jet lag. However, memory 3 days after training was impaired in both genotypes. Jet lag caused a decrease of glutathione (GSH) levels that tended to be more pronounced in AD-Tg than in non-Tg brains and an associated increase in NADH levels in both genotypes. Lower brain GSH levels after jet lag correlated with poor performance on the maze. These data indicate that the combination of the environmental stress of circadian disruption together with latent stress of the mutant amyloid and NOS2 knockout contributes to cognitive deficits that correlate with lower GSH levels.

Role of Aryl Hydrocarbon Receptor in Circadian Clock Disruption and Metabolic Dysfunction

The prevalence of metabolic syndrome, a clustering of three or more risk factors that include abdominal obesity, increased blood pressure, and high levels of glucose, triglycerides, and high-density lipoproteins, has reached dangerous and costly levels worldwide. Increases in morbidity and mortality result from a combination of factors that promote altered glucose metabolism, insulin resistance, and metabolic dysfunction. Although diet and exercise are commonly touted as important determinants in the development of metabolic dysfunction, other environmental factors, including circadian clock disruption and activation of the aryl hydrocarbon receptor (AhR) by dietary or other environmental sources, must also be considered. AhR binds a range of ligands, which prompts protein-protein interactions with other Per-Arnt-Sim (PAS)-domain-containing proteins and subsequent transcriptional activity. This review focuses on the reciprocal crosstalk between the activated AhR and the molecular circadian clock. AhR exhibits a rhythmic expression and time-dependent sensitivity to activation by AhR agonists. Conversely, AhR activation influences the amplitude and phase of expression of circadian clock genes, hormones, and the behavioral responses of the clock system to changes in environmental illumination. Both the clock and AhR status and activation play significant and underappreciated roles in metabolic homeostasis. This review highlights the state of knowledge regarding how AhR may act together with the circadian clock to influence energy metabolism. Understanding the variety of AhR-dependent mechanisms, including its interactions with the circadian timing system that promote metabolic dysfunction, reveals new targets of interest for maintenance of healthy metabolism.

Influences of the circadian clock on neuronal susceptibility to excitotoxicity

Stroke is the third leading cause of death and the primary cause of morbidity in the United States, thus posing an enormous burden on the healthcare system. The factors that determine the risk of an individual toward precipitation of an ischemic event possess a strong circadian component as does the ischemic event itself. This predictability provided a window of opportunity toward the development of chronopharmaceuticals which provided much better clinical outcomes. Experiments from our lab showed for the first time that neuronal susceptibility to ischemic events follows a circadian pattern; hippocampal neurons being most susceptible to an ischemic insult occurring during peak activity in a rodent model of global cerebral ischemia. We also demonstrated that the SCN2.2 cells (like their in vivo counterpart) are resistant to excitotoxicity by glutamate and that this was dependent on activation of ERK signaling. We are currently working on elucidating the complete neuroprotective pathway that provides a barricade against glutamate toxicity in the SCN2.2 cells. Our future experiments will be engaged in hijacking the neuroprotective mechanism in the SCN2.2 cells and applying it to glutamate-susceptible entities in an effort to prevent their death in the presence of excitotoxicity. Despite the advancement in chronopharmaceuticals, optimal clinical outcome with minimal adverse events are difficult to come by at an affordable price. Superior treatment options require a better understanding of molecular mechanisms that define the disease, including the role of the circadian clock.

Beta-naphthoflavone (DB06732) mediates estrogen receptor-positive breast cancer cell cycle arrest through AhR-dependent regulation of PI3K/AKT and MAPK/ERK signaling

Beta-naphthoflavone (BNF, DB06732) is an agonist of aryl hydrocarbon receptor (AhR) and a putative chemotherapeutic agent that has antitumor activity against mammary carcinomas in vivo. However, the mechanism by which BNF exerts this antitumor effect remains unclear. Thus, we explored mechanisms of BNF's antitumor effects in human breast cancer cells. This study showed that BNF suppressed cell proliferation and induced cell cycle arrest in the G0/G1 phase with downregulation of cyclin D1/D3 and CDK4 and upregulation of p21(Cip1/Waf1), leading to a senescence-like phenotype in estrogen receptor (ER)-positive MCF-7 cells, but not in ER-negative MDA-MB-231 cells. In addition, BNF inhibited PI3K/AKT signaling, and the PI3K inhibitor, LY294,002, exhibited the same inhibitory effects on cyclinD1/D3, CDK4 and the cell cycle as BNF. Interestingly, BNF activated mitogen-activated protein kinase-extracellular signal-regulated kinase (MAPK-ERK) signaling, and more notably, MEK inhibitor PD98059 significantly blocked the BNF-induced cell cycle arrest and upregulation of p21(Cip1/Waf1). Furthermore, specific ERα and AhR siRNA studies indicate that ERα is required in BNF-induced p21(Cip1/Waf1) expression, and BNF-mediated cell cycle arrest and modulation of AKT and ERK signaling is AhR-dependent. Taken together, AhR-dependent inhibition of the PI3K/AKT pathway, activation of MAPK/ERK and modulation of ERα is a novel mechanism underlying BNF-mediated antitumor effects in breast cancer, which may represent a promising strategy to be exploited in future clinical trials.

Aryl hydrocarbon receptor activation attenuates Per1 gene induction and influences circadian clock resetting

Light-stimulated adjustment of the circadian clock is an important adaptive physiological response that allows maintenance of behavioral synchrony with solar time. Our previous studies indicate that the aryl hydrocarbon receptor (AhR) agonist 2,3,7,8- tetrachlorodibenzo-p-dioxin attenuates light-induced phase resetting in early night. However, the mechanism of inhibition remains unclear. In this study, we showed that another potent AhR agonist-β-naphthoflavone (BNF)-significantly decreased light-induced phase shifts in wild-type (WT) mice, whereas AhR knockout mice had an enhanced response to light that was unaffected by BNF. Mechanistically, BNF blocked light induction of the Per1 transcript in suprachiasmatic nucleus and liver in WT mice, and BNF blocked forskolin (FSK)-induced Per1 transcripts in Hepa-1c1c7 (c7) cells. An E-box decoy did not affect BNF inhibition of FSK-induced Per1 transcripts in c7 cells. cAMP-response element (CRE)-dependent induction of Per1 promoter activity in response to FSK in combination with phorbol 12-tetradecanoate 13-acetate was suppressed in cells that expressed high levels of AhR (c7) compared with cells lacking functional AhR activity (c12). In addition, the inhibitory effect of BNF on FSK-induced Per1 was dependent on phosphorylation of JNK. Together, these results suggest that AhR activation inhibits light-induced phase resetting through the activation of JNK, negative regulation of CREs in the Per1 promoter, and suppression of Per1.

Aryl hydrocarbon receptor deficiency enhances insulin sensitivity and reduces PPAR-α pathway activity in mice

BACKGROUND

Numerous man-made pollutants activate the aryl hydrocarbon receptor (AhR) and are risk factors for type 2 diabetes. AhR signaling also affects molecular clock genes to influence glucose metabolism.

OBJECTIVE

We investigated mechanisms by which AhR activation affects glucose metabolism.

METHODS

Glucose tolerance, insulin resistance, and expression of peroxisome proliferator-activated receptor-α (PPAR-α) and genes affecting glucose metabolism or fatty acid oxidation and clock gene rhythms were investigated in wild-type (WT) and AhR-deficient [knockout (KO)] mice. AhR agonists and small interfering RNA (siRNA) were used to examine the effect of AhR on PPAR-α expression and glycolysis in the liver cell line Hepa-1c1c7 (c7) and its c12 and c4 derivatives. Brain, muscle ARNT-like protein 1 (Bmal1) siRNA and Ahr or Bmal1 expression plasmids were used to analyze the effect of BMAL1 on PPAR-α expression in c7 cells.

RESULTS

KO mice displayed enhanced insulin sensitivity and improved glucose tolerance, accompanied by decreased PPAR-α and key gluconeogenic and fatty acid oxidation enzymes. AhR agonists increased PPAR-α expression in c7 cells. Both Ahr and Bmal1 siRNA reduced PPAR-α and metabolism genes. Moreover, rhythms of BMAL1 and blood glucose were altered in KO mice.

CONCLUSIONS

These results indicate a link between AhR signaling, circadian rhythms, and glucose metabolism. Furthermore, hepatic activation of the PPAR-α pathway provides a mechanism underlying AhR-mediated insulin resistance.

ERK/MAPK is essential for endogenous neuroprotection in SCN2.2 cells

Background

Glutamate (Glu) is essential to central nervous system function; however excessive Glu release leads to neurodegenerative disease. Strategies to protect neurons are underdeveloped, in part due to a limited understanding of natural neuroprotective mechanisms, such as those present in the suprachiasmatic nucleus (SCN). This study tests the hypothesis that activation of ERK/MAPK provides essential protection to the SCN after exposure to excessive Glu using the SCN2.2 cells as a model.

Methodology

Immortalized SCN2.2 cells (derived from SCN) and GT1-7 cells (neurons from the neighboring hypothalamus) were treated with 10 mM Glu in the presence or absence of the ERK/MAPK inhibitor PD98059. Cell death was assessed by Live/Dead assay, MTS assay and TUNEL. Caspase 3 activity was also measured. Activation of MAPK family members was determined by immunoblot. Bcl2, neuritin and Bid mRNA (by quantitative-PCR) and protein levels (by immunoblot) were also measured.

Principal Findings

As expected Glu treatment increased caspase 3 activity and cell death in the GT1-7 cells, but Glu alone did not induce cell death or affect caspase 3 activity in the SCN2.2 cells. However, pretreatment with PD98059 increased caspase 3 activity and resulted in cell death after Glu treatment in SCN2.2 cells. This effect was dependent on NMDA receptor activation. Glu treatment in the SCN2.2 cells resulted in sustained activation of the anti-apoptotic pERK/MAPK, without affecting the pro-apoptotic p-p38/MAPK. In contrast, Glu exposure in GT1-7 cells caused an increase in p-p38/MAPK and a decrease in pERK/MAPK. Bcl2-protein increased in SCN2.2 cells following Glu treatment, but not in GT1-7 cells; bid mRNA and cleaved-Bid protein increased in GT1-7, but not SCN2.2 cells.

Conclusions

Facilitation of ERK activation and inhibition of caspase activation promotes resistance to Glu excitotoxicity in SCN2.2 cells.

Significance

Further research will explore ERK/MAPK as a key molecule in the prevention of neurodegenerative processes.

Circadian clock disruption in the mouse ovary in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Activation of the aryl hydrocarbon receptor (AhR) by the highly toxic, prototypical ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or other dioxin-like compounds compromises ovarian function by altering follicle maturation and steroid synthesis. Although alteration of transcription after nuclear translocation and heterodimerization of AhR with its binding partner, aryl hydrocarbon nuclear transporter (ARNT), is often cited as a primary mechanism for mediating the toxic effects of dioxins, recent evidence indicates that crosstalk between AhR and several other signaling pathways also occurs. Like the circadian clock genes, AhR is a member of the basic helix-loop-helix, Per-ARNT-SIM (bHLH-PAS) domain family of proteins. Thus, these studies tested the hypothesis that TCDD can act to alter circadian clock regulation in the ovary. Adult female c57bl6/J mice entrained to a typical 12h light/12h dark cycle were exposed to a single 1 μg/kg dose of TCDD by gavage. Six days after exposure, animals were released into constant darkness and ovaries were collected every 4h over a 24h period. Quantitative real-time PCR and immunoblot analysis demonstrated that TCDD exposure alters expression of the canonical clock genes, Bmal1 and Per2 in the ovary. AhR transcript and protein, which displayed a circadian pattern of expression in the ovaries of control mice, were also altered after TCDD treatment. Immunohistochemistry studies revealed co-localization of AhR with BMAL1 in various ovarian cell types. Furthermore, co-immunoprecipitation demonstrated time-of-day dependent interactions of AhR with BMAL1 that were enhanced after TCDD treatment. Collectively these studies suggest that crosstalk between classical AhR signaling and the molecular circadian clockworks may be responsible for altered ovarian function after TCDD exposure.

Considerations for the use of anesthetics in neurotoxicity studies

Anesthetics are widely used in experiments investigating neurotoxicity and neuroprotection; however, these agents are known to interfere with the outcome of these experiments. The purpose of this overview is to review these effects and suggest methods for minimizing unintended consequences on experimental outcomes. Information on the neuroprotective and neurotoxic effects of isoflurane, dexmedetomidine, propofol, ketamine, barbiturates, halothane, xenon, carbon dioxide, and nitrous oxide is summarized. The pertinent cell signaling pathways of these agents are discussed. Methods of humane animal euthanasia without anesthetics are considered. Most anesthetics alter the processes of neuronal survival and death. When designing survival surgeries, sham controls subjected to anesthesia but not the surgical intervention should be compared with controls subjected to neither anesthesia nor surgery. Additional controls could include using an anesthetic with a different mechanism of action from the primary anesthetic used. Because the effects of anesthetics lessen with time after surgery, survival surgeries should include later time points until at least 7 d after the procedure. Humane methods of animal euthanasia that do not require anesthetics exist and should be used whenever appropriate.

Suprachiasmatic nucleus neurons display endogenous resistance to excitotoxicity

A comprehensive understanding of neuroprotective pathways is essential to progress in the battle against numerous neurodegenerative conditions. The hypothalamic suprachiasmatic nucleus (SCN) is endogenously resistant to glutamate (Glu) excitotoxicity in vivo. This study was designed to determine whether immortalized SCN neurons (SCN2.2 cells) retain this characteristic. We first established that SCN2.2 cells retained the ability to respond to Glu. SCN2.2 cells expressed N-methyl-d-aspartate (NMDA) receptor subtypes NR1 and NR2A/2B, suggesting the presence of functional receptors. mRNA for the NMDA receptor subunits NR2A and NR2B were higher in the SCN2.2 than in the control hypothalamic neurons (GT1-7). Specific NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate and d-(-)-2-amino-5-phosphonovaleric acid blocked Glu-induced activation of gene expression. SCN2.2 cells were resistant to Glu excitotoxicity compared with GT1-7 neurons as assessed with a mitochondrial function assay, cell death by trypan blue exclusion and apoptosis by terminal deoxynucleotidyl transferase dUTP nick end labeling. SCN2.2 resistance to Glu excitoxicity was retained in the presence of the broad spectrum Glu transport inhibitor, l-trans-pyrrolidine-2,4 dicarboxylate, excluding glial Glu uptake as a major neuroprotective mechanism. Collectively, these observations demonstrate endogenous neuroprotection in SCN2.2 cells; this cell line is resistant to excitotoxicity under conditions that are toxic to other immortalized cell lines. Thus, the SCN2.2 cell line may provide insights into the molecular mechanisms that confer endogenous neuroprotection in the SCN.

Disruption of CLOCK-BMAL1 transcriptional activity is responsible for aryl hydrocarbon receptor-mediated regulation of Period1 gene

The aryl hydrocarbon receptor (AhR) is a period-aryl hydrocarbon receptor nuclear transporter-simple minded domain transcription factor that shares structural similarity with circadian clock genes and readily interacts with components of the molecular clock. Activation of AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters behavioral circadian rhythms and represses the Period1 (Per1) gene in murine hematopoietic stem and progenitor cells. Per1 expression is driven by circadian locomotor activity cycles kaput-brain muscle ARNT-like (CLOCK-BMAL1)-dependent activation of Eboxes in the Per1 promoter. We hypothesized that the effects of AhR activation on the circadian clock are mediated by disruption of CLOCK-BMAL1 function and subsequent Per1 gene suppression. Effects of AhR activation on rhythmic Per1 transcripts were examined in livers of mice after treatment with the AhR agonist, TCDD; the molecular mechanisms of Per1 repression by AhR were determined in hepatoma cells using TCDD and beta-napthoflavone as AhR activators. This study reports, for the first time, that AhR activation by TCDD alters the Per1 rhythm in the mouse liver and that Per1 gene suppression depends upon the presence of AhR. Furthermore, AhR interaction with BMAL1 attenuates CLOCK-BMAL1 activity and decreases CLOCK binding at Ebox1 and Ebox3 in the Per1 promoter. Taken together, these data suggest that AhR activation represses Per1 through disrupting CLOCK-BMAL1 activity, producing dysregulation of rhythmic Per1 gene expression. These data define alteration of the Per1 rhythm as novel signaling events downstream of AhR activation. Downregulation of Per1 could contribute to metabolic disease, cancer, and other detrimental effects resulting from exposure to certain environmental pollutants.

In vivo circadian rhythms in gonadotropin-releasing hormone neurons

Although it is generally accepted that the circadian clock provides a timing signal for the luteinizing hormone (LH) surge, mechanistic explanations of this phenomenon remain underexplored. It is known, for example, that circadian locomotor output cycles kaput (clock) mutant mice have severely dampened LH surges, but whether this phenotype derives from a loss of circadian rhythmicity in the suprachiasmatic nucleus (SCN) or altered circadian function in gonadotropin-releasing hormone (GnRH) neurons has not been resolved. GnRH neurons can be stimulated to cycle with a circadian period in vitro and disruption of that cycle disturbs secretion of the GnRH decapeptide. We show that both period-2 (PER2) and brain muscle Arnt-like-1 (BMAL1) proteins cycle with a circadian period in the GnRH population in vivo. PER2 and BMAL1 expression both oscillate with a 24-hour period, with PER2 peaking during the night and BMAL1 peaking during the day. The population, however, is not as homogeneous as other oscillatory tissues with only about 50% of the population sharing peak expression levels of BMAL1 at zeitgeber time 4 (ZT4) and PER2 at ZT16. Further, a light pulse that induced a phase delay in the activity rhythm of the GnRH-eGFP mice caused a similar delay in peak expression levels of BMAL1 and PER2. These studies provide direct evidence for a functional circadian clock in native GnRH neurons with a phase that closely follows that of the SCN.

Activation of aryl hydrocarbon receptor signaling by cotton balls used for environmental enrichment

Dioxins are nearly ubiquitous environmental contaminants that are produced as byproducts during industrial processes, including the bleaching of paper and textiles. Contamination of animal bedding material with dioxins has been a concern for both laboratory and farm animals. The objective of this study was to determine whether the presence of cotton balls, provided to mice for enrichment, caused induction of the cytochrome P450 1A1 gene (Cyp1A1), which typically is stimulated through activation of the aryl hydrocarbon receptor (AhR) by dioxins and dioxin-like compounds. Cyp1A1 transcripts and protein in the liver were increased significantly by either exposure to cotton balls or treatment with a single dose of 2,3,7,8-tetrachlorodibenzo-para-dioxin. Unexposed controls displayed low levels of Cyp1A1 transcript and undetectable levels of CYP1A1 protein. These results suggest that cotton balls are potentially contaminated with dioxins and/or dioxin-like compounds that act as potent inducers of Cyp1a1 in laboratory animals if used as nesting material. This study underscores the necessity of considering dioxin content in products used for enrichment in animal facilities.

Behavioral rhythmicity of mice lacking AhR and attenuation of light-induced phase shift by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Transcription factors belonging to the Per/Arnt/Sim (PAS) domain family are highly conserved and many are involved in circadian rhythm regulation. One member of this family, aryl hydrocarbon receptor (AhR), is an orphan receptor whose physiological role is unknown. Recent findings have led to the hypothesis that AhR has a role in circadian rhythm, which is the focus of the present investigation. First, time-of-day-dependent mRNA expression of AhR and its signaling target, cytochrome p4501A1 (Cyp1a1), was determined in C57BL/6J mice by quantitative RT-PCR. Circadian expression of AhR and Cyp1a1 was observed both in the suprachiasmatic nucleus (SCN) and liver. Next, the circadian phenotype of mice lacking AhR (AhRKO) was investigated using behavioral monitoring. Intact AhRKO mice had robust circadian rhythmicity with a similar tau under constant conditions compared to wild-type mice, but a significant difference in tau was observed between genotypes in ovariectomized female mice. Time to reentrainment following 6-h advances or delays of the light/dark cycle was not significantly different between genotypes. However, mice exposed to the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 1 microg/kg of body weight) displayed decreased phase shifts in response to light and had altered expression of Per1 and Bmal1. These results suggest that chronic activation of AhR may affect the ability of the circadian timekeeping system to adjust to alterations in environmental lighting by affecting canonical clock genes. Further studies are necessary to decipher the mechanism of how AhR agonists could disrupt light-induced phase shifts. If AhR does have a role in circadian rhythm, it may share redundant roles with other PAS domain proteins and/or the role of AhR may not be exhibited in the behavioral activity rhythm, but could be important elsewhere in the peripheral circadian system.

Effects of tryptophan photoproducts in the circadian timing system: searching for a physiological role for aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates adverse effects of dioxins, but its physiological role remains ambiguous. The similarity between AhR and canonical circadian clock genes suggests potential involvement of AhR in regulation of circadian timing. Photoproducts of tryptophan (TRP), including 6-formylindolo[3,2-b]carbazole (FICZ), have high affinity for AhR and are postulated as endogenous ligands. Although TRP photoproducts activate AhR signaling in vitro, their effects in vivo have not been investigated in mammals. Because TRP photoproducts may act as transducers of light, we examined their effects on the circadian clock. Intraperitoneal injection of TRP photoproducts or FICZ to C57BL/6J mice dose dependently induced AhR downstream targets, cytochrome P4501A1 (CYP1A1) and cytochrome P4501B1 mRNA expression, in liver. c-fos mRNA, a commonly used marker for light responses, was also induced with FICZ, and all responses were AhR dependent. A rat-immortalized suprachiasmatic nucleus (SCN) cell line, SCN 2.2, was used to examine the direct effect of TRP photoproducts on the molecular clock. Both TRP photoproducts and FICZ-increased CYP1A1 expression and prolonged FICZ incubation altered the circadian expression of clock genes (Per1, Cry1, and Cry2) in SCN 2.2 cells. Furthermore, FICZ inhibited glutamate-induced phase shifting of the mouse SCN electrical activity rhythm. Circadian light entrainment is critical for adjustment of the endogenous rhythm to environmental light cycle. Our results reveal a potential for TRP photoproducts to modulate light-dependent regulation of circadian rhythm through triggering of AhR signaling. This may lead to further understanding of toxicity of dioxins and the role of AhR in circadian rhythmicity.

Circadian Clock Gene Expression in the Ovary: Effects of Luteinizing Hormone1

A molecular device that measures time on a daily, or circadian, scale is a nearly ubiquitous feature of eukaryotic organisms. A core group of clock genes, whose coordinated function is required for this timekeeping, is expressed both in the central clock and within numerous peripheral organs. We examined expression of clock genes in the rat ovary. Transcripts for core oscillator elements (Arntl, Clock, Per1, Per2, and Cry1) were present in the ovary as indicated by quantitative real-time RT-PCR. Rhythmic expression patterns of Arntl and Per2 transcripts and protein products were out of phase with respect to the central oscillator and in complete antiphase to each other. Expression of Arntl was significantly elevated after the LH surge on the day of proestrus. Finally, hCG treatment induced cyclic expression of both Arntl and Per2 gene products in hypophysectomized, immature rats primed with eCG. Collectively, these data suggest that the core underpinnings of the transcriptional/translational feedback loop that drives circadian rhythmicity is present in the rat ovary. Furthermore, the study identifies LH as a potential regulator of circadian clock gene rhythms in the ovary.

Oligodeoxynucleotide methods for analyzing the circadian clock in the suprachiasmatic nucleus

The recent identification of specific genes responsible for the generation of endogenous circadian rhythmicity in the suprachiasmatic nucleus presents a new level of investigation into endogenous rhythmicity and mechanisms of synchronization of this circadian clock with the environmental light?dark cycle. This article describes techniques that employ antisense and decoy oligodeoxynucleotides (ODN) to determine the roles of specific molecular substrates both in endogenous rhythmicity and in regulating the effects of light on the mammalian circadian clock. Application of antisense ODN technology has revealed a role for timeless (Tim) in the core clock mechanism and established that induction of period1 (Per1) is required for light responsiveness. Likewise, a decoy ODN designed to sequester activated CREB protein definitively demonstrated a requirement for CRE-mediated transcription in light signaling. Experiments designed with these molecular tools offer new insights on the interaction of cellular processes and signaling with the molecular clockworks.

Protein kinase G type II is required for night-to-day progression of the mammalian circadian clock

Circadian clocks comprise a cyclic series of dynamic cellular states, characterized by the changing availability of substrates that alter clock time when activated. To determine whether circadian clocks, like the cell cycle, exhibit regulation by key phosphorylation events, we examined endogenous kinase regulation of timekeeping in the mammalian suprachiasmatic nucleus (SCN). Short-term inhibition of PKG-II but not PKG-Ibeta using antisense oligodeoxynucleotides delayed rhythms of electrical activity and Bmal1 mRNA. Phase resetting was rapid and dynamic; inhibition of PKG-II forced repetition of the last 3.5 hr of the cycle. Chronic inhibition of PKG-II disrupted electrical activity rhythms and tonically increased Bmal1 mRNA. PKG-II-like immunoreactivity was detected after coimmunoprecipitation with CLOCK, and CLOCK was phosphorylated in the presence of active PKG-II. PKG-II activation may define a critical control point for temporal progression into the daytime domain by acting on the positive arm of the transcriptional/translational feedback loop.

Requirement of mammalian Timeless for circadian rhythmicity

Despite a central circadian role in Drosophila for the transcriptional regulator Timeless (dTim), the relevance of mammalian Timeless (mTim) remains equivocal. Conditional knockdown of mTim protein expression in the rat suprachiasmatic nucleus (SCN) disrupted SCN neuronal activity rhythms, and altered levels of known core clock elements. Full-length mTim protein (mTIM-fl) exhibited a 24-hour oscillation, where as a truncated isoform (mTIM-s) was constitutively expressed. mTIM-fl associated with the mammalian clock Period proteins (mPERs) in oscillating SCN cells. These data suggest that mTim is required for rhythmicity and is a functional homolog of dTim on the negative-feedback arm of the mammalian molecular clockwork.

Circadian clock-controlled regulation of cGMP-protein kinase G in the nocturnal domain

The suprachiasmatic nucleus (SCN) circadian clock exhibits a recurrent series of dynamic cellular states, characterized by the ability of exogenous signals to activate defined kinases that alter clock time. To explore potential relationships between kinase activation by exogenous signals and endogenous control mechanisms, we examined clock-controlled protein kinase G (PKG) regulation in the mammalian SCN. Signaling via the cGMP-PKG pathway is required for light- or glutamate (GLU)-induced phase advance in late night. Spontaneous cGMP-PKG activation occurred at the end of subjective night in free-running SCN in vitro. Phasing of the SCN rhythm in vitro was delayed by approximately 3 hr after treatment with guanylyl cyclase (GC) inhibitors, PKG inhibition, or antisense oligodeoxynucleotide (alphaODN) specific for PKG, but not PKA inhibitor or mismatched ODN. This sensitivity to GC-PKG inhibition was limited to the same 2 hr time window demarcated by clock-controlled activation of cGMP-PKG. Inhibition of the cGMP-PKG pathway at this time caused delays in the phasing of four endogenous rhythms: wheel-running activity, neuronal activity, cGMP, and Per1. Timing of the cGMP-PKG-necessary window in both rat and mouse depended on clock phase, established by the antecedent light/dark cycle rather than solar time. Because behavioral, neurophysiological, biochemical, and molecular rhythms showed the same temporal sensitivities and qualitative responses, we predict that clock-regulated GC-cGMP-PKG activation may provide a necessary cue as to clock state at the end of the nocturnal domain. Because sensitivity to phase advance by light-GLU-activated GC-cGMP-PKG occurs in juxtaposition, these signals may induce a premature shift to this PKG-necessary clock state.

Ca2+/cAMP response element-binding protein (CREB)-dependent activation of Per1 is required for light-induced signaling in the suprachiasmatic nucleus circadian clock

Light is a prominent stimulus that synchronizes endogenous circadian rhythmicity to environmental light/dark cycles. Nocturnal light elevates mRNA of the Period1 (Per1) gene and induces long term state changes, expressed as phase shifts of circadian rhythms. The cellular mechanism for Per1 elevation and light-induced phase advance in the suprachiasmatic nucleus (SCN), a process initiated primarily by glutamatergic neurotransmission from the retinohypothalamic tract, was examined. Glutamate (GLU)-induced phase advances in the rat SCN were blocked by antisense oligodeoxynucleotide (ODN) against Per1 and Ca(2+)/cAMP response element (CRE)-decoy ODN. CRE-decoy ODN also blocked light-induced phase advances in vivo. Furthermore, the CRE-decoy blocked GLU-induced accumulation of Per1 mRNA. Thus, Ca(2+)/cAMP response element-binding protein (CREB) and Per1 are integral components of the pathway transducing light-stimulated GLU neurotransmission into phase advance of the circadian clock.

Differential cAMP gating of glutamatergic signaling regulates long-term state changes in the suprachiasmatic circadian clock

We investigated a role for cAMP/protein kinase A (PKA) in light/glutamate (GLU)-stimulated state changes of the mammalian circadian clock in the suprachiasmatic nucleus (SCN). Nocturnal GLU treatment elevated [cAMP]; however, agonists of cAMP/PKA did not mimic the effects of light/GLU. Coincident activation of cAMP/PKA enhanced GLU-stimulated state changes in early night but blocked light/GLU-induced state changes in the late night, whereas inhibition of cAMP/PKA reversed these effects. These responses are distinct from those mediated by mitogen-activated protein kinase (MAPK). MAPK inhibitors attenuated both GLU-induced state changes. Although GLU induced mPer1 mRNA in both early and late night, inhibition of PKA blocked this event only in early night, suggesting that cellular mechanisms regulating mPer1 are gated by the suprachiasmatic circadian clock. These data support a diametric gating role for cAMP/PKA in light/GLU-induced SCN state changes: cAMP/PKA promotes the effects of light/GLU in early night, but opposes them in late night.

Suprachiasmatic nucleus: the brain's circadian clock

The tiny suprachiasmatic nucleus (SCN) of the hypothalamus plays a central role in the daily programming of organismic functions by regulating day-to-day oscillations of the internal milieu and synchronizing them to the changing cycles of day and night and of body state. This biological clock drives the daily expression of vital homeostatic functions as diverse as feeding, drinking, body temperature, and neurohormone secretion. It adaptively organizes these body functions into near-24-hour oscillations termed circadian rhythms. The SCN imposes temporal order 1) through generating output signals that relay time-of-day information, and 2) through gating its own sensitivity to incoming signals that adjust clock timing. Each of these properties, derived from the timebase of the SCN's endogenous near-24-hour pacemaker, persists when the SCN is maintained in a hypothalamic brain slice in vitro. Single-unit recording experiments demonstrate a spontaneous peak in the electrical activity of the ensemble of SCN neurons near midday. By utilizing this time of peak as a "pulse" of the clock, we have characterized a series of time domains, or windows of sensitivity, in which the SCN restricts its own sensitivity to stimuli that are capable of adjusting clock phase. Pituitary adenylyl cyclase-activating peptide (PACAP) and cAMP comprise agents that reset clock phase during the day time domain; both PACAP and membrane-permeable cAMP analogs cause phase advances only when applied during the day. In direct contrast to PACAP and cAMP, acetylcholine and cGMP analogs phase advance the clock only when applied during the night. Sensitivity to light and glutamate arises concomitant with sensitivity to acetylcholine and cGMP. Light and glutamate cause phase delays in the early night, by acting through elevation of intracellular Ca2+, mediated by activation of a neuronal ryanodine receptor. In late night, light and glutamate utilize a cGMP-mediated mechanism to induce phase advances. Finally, crepuscular domains, or dusk and dawn, are characterized by sensitivity to phase resetting by the pineal hormone, melatonin, acting through protein kinase C. Our findings indicate that the gates to both daytime and nighttime phase resetting lie beyond the level of membrane receptors; they point to critical gating within the cell, downstream from second messengers. The changing patterns of sensitivities in vitro demonstrate that the circadian clock controls multiple molecular gates at the intracellular level, to assure that they are selectively opened in a permissive fashion only at specific points in the circadian cycle. Discerning the molecular mechanisms that generate these changes is fundamental to understanding the integrative and regulatory role of the SCN in hypothalamic control of organismic rhythms.

Oscillation and light induction of timeless mRNA in the mammalian circadian clock

Circadian rhythms in Drosophila melanogaster depend on a molecular feedback loop generated by oscillating products of the period (per) and timeless (tim) genes. In mammals, three per homologs are cyclically expressed in the suprachiasmatic nucleus (SCN), site of the circadian clock, and two of these, mPer1 and mPer2, are induced in response to light. Although this light response distinguishes the mammalian clock from its Drosophila counterpart, overall regulation, including homologous transcriptional activators, appears to be similar. Thus, the basic mechanisms used to generate circadian timing have been conserved. However, contrary to expectations, the recently isolated mammalian tim homolog was reported not to cycle. In this study, we examined mRNA levels of the same tim homolog using a different probe. We observed a significant (approximately threefold) diurnal variation in mTim expression within mouse SCN using two independent methods. Peak levels were evident at the day-to-night transition in light-entrained animals, and the oscillation persisted on the second day in constant conditions. Furthermore, light pulses known to induce phase delays caused significant elevation in mTim mRNA. In contrast, phase-advancing light pulses did not affect mTim levels. The mTim expression profile and the response to nocturnal light are similar to mPer2 and are delayed compared with mPer1. We conclude that temporal ordering of mTim and mPer2 parallels that of their fly homologs. We predict that mTIM may be the preferred functional partner for mPER2 and that expression of mTim and mPer2 may, in fact, be driven by mPER1.

A neuronal ryanodine receptor mediates light-induced phase delays of the circadian clock

Circadian clocks are complex biochemical systems that cycle with a period of approximately 24 hours. They integrate temporal information regarding phasing of the solar cycle, and adjust their phase so as to synchronize an organism's internal state to the local environmental day and night. Nocturnal light is the dominant regulator of this entrainment. In mammals, information about nocturnal light is transmitted by glutamate released from retinal projections to the circadian clock in the suprachiasmatic nucleus of the hypothalamus. Clock resetting requires the activation of ionotropic glutamate receptors, which mediate Ca2+ influx. The response induced by such activation depends on the clock's temporal state: during early night it delays the clock phase, whereas in late night the clock phase is advanced. To investigate this differential response, we sought signalling elements that contribute solely to phase delay. We analysed intracellular calcium-channel ryanodine receptors, which mediate coupled Ca2+ signalling. Depletion of intracellular Ca2+ stores during early night blocked the effects of glutamate. Activators of ryanodine receptors induced phase resetting only in early night; inhibitors selectively blocked delays induced by light and glutamate. These findings implicate the release of intracellular Ca2+ through ryanodine receptors in the light-induced phase delay of the circadian clock restricted to the early night.

Characterization of the growth center of the avian preovulatory follicle

Anatomical studies have suggested that the germinal disc (GD) region (GDR; GD plus overlying granulosa cells) is the growth center of the avian preovulatory follicle. The objective of this study was to characterize the physiology of the GDR by comparing the functions of two morphologically distinct populations of granulosa cells. The three markers of the physiology of individual granulosa cells examined were 1) proliferation, 2) production of plasminogen activator (PA), and 3) production of progesterone. The effect of LH on each of these functions was also evaluated. Sections 8 mm in diameter were obtained from granulosa cells associated with the GD (GD granulosa cells) and from granulosa cells on the layer distal to the GD (nonGD granulosa cells) from the five largest preovulatory follicles (F5-F1, F1 designated the largest) 12-14 h (before the LH surge) or 2 h (after the LH surge) before ovulation. Proliferation was measured using [3H]thymidine incorporation. PA activity was measured using the chromogenic substrate S-2251. Progesterone was measured by RIA. Incorporation of [3H]thymidine was very high in GD and nonGD granulosa cells from F5 and F4 follicles and decreased dramatically as the follicle progressed through the hierarchy, but remained significantly higher in GD granulosa cells compared to nonGD granulosa cells at all stages of development examined (F5-F1). Exposure of follicles to LH in vivo inhibited [3H]thymidine incorporation by GD granulosa cells in all follicles except the F5. In contrast, in vivo exposure to LH had no effect on [3H]thymidine incorporation by nonGD granulosa cells. PA production by GD granulosa cells was high throughout the stages of maturation studied (F5-F1), whereas PA production by nonGD granulosa cells decreased as follicles matured from F5 to F1. Interestingly, LH stimulated PA production by F5 GD granulosa cells, had no effect on PA production by F3 GD granulosa cells, and inhibited PA production by F1 GD granulosa cells. In contrast, LH inhibited PA production by nonGD granulosa cells in F3 and F1 follicles. Progesterone production by GD granulosa cells was low in F3 and F1 follicles. Progesterone production by nonGD granulosa cells increased as the follicle matured from the F3 to F1 stage and was stimulated significantly by LH. These data indicate that physiological differences in granulosa cell function are dependent upon the location of granulosa cells relative to the GD. GD granulosa cells are less mature, proliferate more rapidly, and produce more PA than nonGD granulosa cells, which produce more progesterone and less PA. Differences in granulosa cell function may be due to the influence of the GD, providing physiological evidence that the GDR may be the growth center of the follicle.

Granulosa layer: primary site of regulation of plasminogen activator messenger ribonucleic acid by luteinizing hormone in the avian ovary

Plasminogen activator (PA) is hypothesized to be important in the remodeling of the extracellular matrix during follicular growth. The granulosa layer produces high amounts of PA in response to a stimulatory factor, produced by the theca layer, that is inhibited by LH. To determine the site and mechanism by which LH inhibits PA production, we asked 1) whether LH acts on the granulosa layer and/or the theca layer to inhibit PA production by the largest preovulatory follicle (F1), and 2) whether LH affects PA production by acting at the mRNA or protein level. Sections (10 mm in diameter) of granulosa layers obtained from the F1 follicle before (14 h before ovulation) or after (2 h before ovulation) the LH surge were incubated (24 h at 37 degrees C) in theca-conditioned medium; this medium had been prepared by incubation of 10-mm-diameter sections of theca layers, obtained before (14 h before ovulation) or after (2 h before ovulation) the LH surge, in Dulbecco's Modified Eagle's Medium for 24 h at 37 degrees C. PA production in culture medium was measured with use of the chromogenic substrate S-2251. PA production was high when granulosa layers obtained before the LH surge were incubated in theca-conditioned medium obtained before the LH surge; it was also high when granulosa layers obtained before the LH surge were incubated in theca-conditioned medium obtained after the LH surge. PA production was low when granulosa layers obtained after the LH surge were incubated in theca-conditioned medium obtained before the LH surge, and was also low when granulosa layers obtained after the LH surge were incubated in theca-conditioned medium obtained after the LH surge. Northern and Western blots and activity assays performed on granulosa layer homogenates indicated that PA mRNA, protein, and activity were high before the LH surge and low after the LH surge. Production of the stimulatory factor by the theca layer is apparently unaffected by LH. After exposure to LH, the granulosa layer is no longer capable of producing PA, even in the presence of the theca-derived stimulatory factor. We conclude that the granulosa layer is the site of mRNA and/or protein regulation of PA production by LH.

Avian germinal disc region secretes factors that stimulate proliferation and inhibit progesterone production by granulosa cells

Microscopic analysis of ovarian follicles in the domestic hen has revealed differences in the cellular structure of granulosa cells that are dependent upon the location of granulosa cells relative to the germinal disc, which contains the female gamete. These differences appear as a morphological gradient, which implies variations in granulosa cell function. This observation prompted us to hypothesize that the germinal disc region (GDR) of the avian preovulatory follicle participates in the process of follicular growth by producing factors that act in a paracrine manner to stimulate proliferation of and inhibit steroidogenesis in the granulosa layer, establishing a gradient in the morphology and physiology of the granulosa layer. To test our hypothesis, we asked two questions: 1) Are physiological gradients of proliferation and steroidogenesis present within the granulosa layer of a preovulatory follicle? 2) Does the GDR secrete factors that affect granulosa cell proliferation and/or steroidogenesis? Incorporation of 3H-thymidine was used as a measure of proliferation, and production of progesterone was used as a measure of steroidogenesis. In the first experiment, 8-mm-diameter sections were obtained from three morphologically distinct regions of the granulosa monolayer: 1) the GDR, 2) granulosa cells distal to the GDR (distal granulosa) and 3) granulosa cells midway between the GDR and distal granulosa cells (proximal granulosa cells). The GDR incorporated the most 3H-thymidine and produced the least progesterone. Distal granulosa cells incorporated the least 3H-thymidine and produced the most progesterone. Proximal granulosa cells incorporated an intermediate amount of 3H-thymidine and produced an intermediate amount of progesterone. To answer the second question, conditioned medium was prepared from GDRs and distal granulosa cells (control) obtained from the F1 (largest preovulatory follicle) and F3 (the third-largest preovulatory follicle) follicles. Sections (8-mm in diameter) of the distal granulosa layer (F3 for 3H-thymidine incorporation, F1 for progesterone production) were incubated in GDR-conditioned medium or granulosa cell-conditioned medium to determine whether factors secreted into the medium by the GDR and distal granulosa cells affect granulosa cell proliferation and/or steroidogenesis. Certain samples of GDR-conditioned medium and granulosa cell-conditioned medium were boiled, protease-treated or charcoal-stripped. F3 and F1 GDRs produced heat- and protease-sensitive factors that promoted proliferation and inhibited progesterone production by granulosa cells. These data indicate that diametrically opposed gradients of proliferation and steroidogenesis are present within the granulosa layer of an individual preovulatory follicle. Furthermore, the GDR produces proliferation-stimulating and steroidogenesis-inhibiting factors that may act in an autocrine or paracrine manner to influence proliferation and steroidogenesis in granulosa cells.

Destruction of the germinal disc region of an immature preovulatory follicle suppresses follicular maturation and ovulation

Previous work has suggested that the germinal disc and the overlying layer of granulosa cells located near the germinal disc, collectively referred to as the germinal disc region (GDR), is a primary "growth center," regulating granulosa cell proliferation within follicles in the rapid growth phase. This study was designed to determine whether the presence of a viable GDR is required for the completion of follicular maturation and ovulation of the avian follicle. Twelve or 24 h before the expected time of ovulation, the GDR of the largest preovulatory follicle (F1) was destroyed by applying solid CO2 (2 x 2 mm) for 20 sec. A region of the follicle wall equal in size to but opposite the GDR was destroyed in control birds. Blood samples were taken 24 h and 6 h before and at the time of ovulation. The effects of GDR destruction on plasma progesterone (P4), LH, and ovulation were determined. Destruction of the F1 GDR 24 h before ovulation resulted in an absence of the preovulatory rise in plasma P4, attenuation of the LH surge, blocked ovulation, and atresia of the F1 follicle. Controls displayed typical preovulatory profiles of plasma P4 and LH and ovulated at the expected time. In contrast to the data collected after destruction of the GDR 24 h before ovulation, destruction of the GDR 12 h before ovulation did not disrupt ovulation. Furthermore, destruction of the GDR 24 h before ovulation had no effect on basal or LH-stimulated P4 production by granulosa cell cultures prepared 12 h after destruction of the GDR.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasminogen activator production by the granulosa layer is stimulated by factor(s) produced by the theca layer and inhibited by the luteinizing hormone surge in the chicken

The stages of follicular maturation of a preovulatory follicle in the hen can be divided into an extended proliferative phase (prior to LH surge) and a brief ovulatory phase (after LH surge). Previous studies suggest involvement of plasminogen activator (PA) in both the proliferative and ovulatory phases. The goals of the present study were 1) to determine whether PA production by granulosa and theca is dependent upon interaction of the two cell layers; 2) to investigate whether the structural difference of the stigma (site of follicular rupture) and nonstigma regions of the theca layer affect PA production; 3) to determine whether there is a change in the ability of the granulosa layer and stigma or nonstigma regions of the theca layer to produce PA as the follicle makes the transition from the proliferative to the ovulatory phase; and 4) to characterize the type(s) of PA produced by the hen follicle. Equal proportions of the granulosa layer (10-mm diameter) and stigma or nonstigma regions of the theca layer (10 mg) obtained from the F1 preovulatory follicle 8 h before ovulation (before LH surge) or 2 h before ovulation (after LH surge) were incubated alone or in combination for 24 h. PA was measured in tissue homogenates and medium by use of the chromogenic substrate S-2251. The granulosa layer or stigma or nonstigma regions of the theca layer incubated alone and obtained either 8 h or 2 h before ovulation had very low amounts of PA activity in the medium and tissue homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

A specific membrane binding protein for progesterone in rat brain: sex differences and induction by estrogen

Progesterone conjugated to bovine serum albumin (BSA) was used as a probe to study sex differences and the effects of hormonal status on binding of progesterone to crude synaptosomal membrane preparations (P2) derived from the mediobasal hypothalamic-anterior hypothalamic-preoptic area or the corpus striatum. Binding of 125I-labeled BSA linked to progesterone at the 11 position of the steroid (P-11-BSA) was decreased by competition with unlabeled P-11-BSA or P-3-BSA (in which progesterone is bound to BSA at the 3 position). P-3-BSA displayed higher affinity than P-11-BSA. Hypothalamic and striatal preparations from adult females show high specific binding (60-80%) to the progesterone-BSA conjugate. Specific binding was reduced more than 80% 14 days after ovariectomy. Estrogen treatment (10 micrograms per rat for 4 days) of 14-day ovariectomized rats restored specific binding to levels equivalent to intact females. In contrast, adult males displayed drastically reduced or no specific binding in either tissue. No specific binding was detected after orchidectomy. Estrogen treatment of orchidectomized animals induced specific binding sites similar to those in intact females. Additionally, an affinity probe was developed by linking primary amines on the P-3-BSA conjugate to agarose activated aldehydes in an AminoLink column. A digitoxin-solubilized fraction from female rat P2 cerebellum preparations yielded a single major band after affinity purification with an estimated molecular mass of 40-50 kDa in an SDS/PAGE system after silver stain. These results show a reversible sex difference in the specific binding of progesterone to synaptosomal membrane sites in the central nervous system of male and female rats which is dependent on estrogen.