Neurotransmitter-stimulated immediate-early gene responses are organized through differential post-synaptic receptor mechanisms

The immediate early gene product EGR1 and polycomb group proteins interact in epigenetic programming during chondrogenesis

Initiation of and progression through chondrogenesis is driven by changes in the cellular microenvironment. At the onset of chondrogenesis, resting mesenchymal stem cells are mobilized in vivo and a complex, step-wise chondrogenic differentiation program is initiated. Differentiation requires coordinated transcriptomic reprogramming and increased progenitor proliferation; both processes require chromatin remodeling. The nature of early molecular responses that relay differentiation signals to chromatin is poorly understood. We here show that immediate early genes are rapidly and transiently induced in response to differentiation stimuli in vitro. Functional ablation of the immediate early factor EGR1 severely deregulates expression of key chondrogenic control genes at the onset of differentiation. In addition, differentiating cells accumulate DNA damage, activate a DNA damage response and undergo a cell cycle arrest and prevent differentiation associated hyper-proliferation. Failed differentiation in the absence of EGR1 affects global acetylation and terminates in overall histone hypermethylation. We report novel molecular connections between EGR1 and Polycomb Group function: Polycomb associated histone H3 lysine27 trimethylation (H3K27me3) blocks chromatin access of EGR1. In addition, EGR1 ablation results in abnormal Ezh2 and Bmi1 expression. Consistent with this functional interaction, we identify a number of co-regulated targets genes in a chondrogenic gene network. We here describe an important role for EGR1 in early chondrogenic epigenetic programming to accommodate early gene-environment interactions in chondrogenesis.

Selective genomic targeting by FRA-2/FOSL2 transcription factor: regulation of the Rgs4 gene is mediated by a variant activator protein 1 (AP-1) promoter sequence/CREB-binding protein (CBP) mechanism

FRA-2/FOSL2 is a basic region-leucine zipper motif transcription factor that is widely expressed in mammalian tissues. The functional repertoire of this factor is unclear, partly due to a lack of knowledge of genomic sequences that are targeted. Here, we identified novel, functional FRA-2 targets across the genome through expression profile analysis in a knockdown transgenic rat. In this model, a nocturnal rhythm of pineal gland FRA-2 is suppressed by a genetically encoded, dominant negative mutant protein. Bioinformatic analysis of validated sets of FRA-2-regulated and -nonregulated genes revealed that the FRA-2 regulon is limited by genomic target selection rules that, in general, transcend core cis-sequence identity. However, one variant AP-1-related (AP-1R) sequence was common to a subset of regulated genes. The functional activity and protein binding partners of a candidate AP-1R sequence were determined for a novel FRA-2-repressed gene, Rgs4. FRA-2 protein preferentially associated with a proximal Rgs4 AP-1R sequence as demonstrated by ex vivo ChIP and in vitro EMSA analysis; moreover, transcriptional repression was blocked by mutation of the AP-1R sequence, whereas mutation of an upstream consensus AP-1 family sequence did not affect Rgs4 expression. Nocturnal changes in protein complexes at the Rgs4 AP-1R sequence are associated with FRA-2-dependent dismissal of the co-activator, CBP; this provides a mechanistic basis for Rgs4 gene repression. These studies have also provided functional insight into selective genomic targeting by FRA-2, highlighting discordance between predicted and actual targets. Future studies should address FRA-2-Rgs4 interactions in other systems, including the brain, where FRA-2 function is poorly understood.

Pineal gland expression of the transcription factor Egr-1 is restricted to a population of glia that are distinct from nestin-immunoreactive cells

Egr-1 is a plasticity-related transcription factor that has been implicated in circadian regulation of the pineal gland. In the present study we have investigated the cellular expression pattern of Egr-1 in the adult rat pineal. Egr-1 protein is restricted to the nucleus of a sub-population of cells. These cells were characterised using a new transgenic rat model (egr-1-d2EGFP) in which green fluorescent protein is driven by the egr-1 promoter. Cellular filling by GFP revealed that Egr-1-positive cells exhibited processes, indicating a glial cell-type morphology. This was confirmed by co-localizing the GFP-filled processes with vimentin and S-100beta. However, GFP/Egr-1 is expressed in only a tiny minority of the previously identified Id-1/vimentin-positive glial cells and therefore represents a novel sub-set of this (GFAP-negative) glial population. We have also demonstrated for the first time an extensive network of nestin-positive cells throughout the adult pineal gland, however these cells do not co-express Egr-1. Our studies have therefore broadened our understanding of the cell populations that constitute the adult pineal. Cellular localization of Egr-1 has revealed that this factor does not appear to be directly involved in pinealocyte production of melatonin but is required in a sub-set of pineal glia.

Differential expression of activator protein-1 proteins in the pineal gland of Syrian hamster and rat may explain species diversity in arylalkylamine N-acetyltransferase gene expression

Species differences have been reported for the nighttime regulation of arylalkylamine N-acetyltransferase (AA-NAT), the melatonin rhythm-generating enzyme. In particular, de novo synthesis of stimulatory transcription factors is required for Aa-nat transcription in the Syrian hamster but not in the rat pineal gland. The present work investigated the contribution of phosphorylated cAMP-responsive element-binding protein, c-FOS, c-JUN, and JUN-B in the regulation of Aa-nat transcription in Syrian hamsters compared with rats. The nighttime pattern of cAMP-responsive element-binding protein phosphorylation and regulation by norepinephrine observed in the Syrian hamster was similar to those reported in the rat. On the contrary, strong divergences in c-FOS, c-JUN, and JUN-B expression were observed between both species. In Syrian hamster, predominant expression of c-FOS and c-JUN was observed at the beginning of night, whereas a predominant expression of c-JUN and JUN-B was observed in the late night in rat. The early peak of c-FOS and c-JUN, known to form a stimulatory transcription dimer, suggests that they are involved in the nighttime stimulation of Aa-nat transcription. Indeed, early-night administration of a protein synthesis inhibitor (cycloheximide) markedly decreased AA-NAT mRNA levels in Syrian hamster. In the rat, high levels of JUN-B and c-JUN, constituting an inhibitory transcription dimer, are probably involved in the late-night inhibition of Aa-nat transcription. Early-night administration of cycloheximide actually increased AA-NAT mRNA levels toward the late night. Therefore, composition and timing of the pineal activator protein-1 complexes differ between rat and Syrian hamster and may be an activator (Syrian hamster) or an inhibitor (rat) of Aa-nat transcription.

Regulation of cAMP-induced arylalkylamine N-acetyltransferase, Period1, and MKP-1 gene expression by mitogen-activated protein kinases in the rat pineal gland

In rodent pineal glands, sympathetic innervation, which leads to norepinephrine release, is a key process in the circadian regulation of physiology and certain gene expressions. It has been shown that gene expression of the rate-limiting enzyme in the melatonin synthesis arylalkylamine N-acetyltransferase (Aa-Nat), circadian clock gene Period1, and mitogen-activated protein kinase (MAPK) phosphtase-1 (MKP-1), is controlled mainly by a norepinephrine-beta-adrenergic receptor-cAMP signaling cascade in the rat pineal gland. To further dissect the signaling cascades that regulate those gene expressions, we examined whether MAPKs are involved in cAMP-induced gene expression. Western blot and immunohistochemical analyses showed that one of the three MAPKs, c-Jun N-terminal kinase (JNK), was expressed in the pineal, and was phosphorylated by cAMP analogue stimulation with a peak 20 min after start of the stimulation, in vitro. A specific JNK inhibitor SP600125 (Anthra[1,9-cd]pyrazol-6(2H)-one1,9-pyrazoloanthrone), but not its negative control (N1-Methyl-1,9-pyrazoloanthrone), significantly reduced cAMP-stimulated Aa-Nat, Period1, and MKP-1 mRNA levels. Although another MAPK, p38(MAPK), has also been shown to be activated by cAMP stimulation, a p38(MAPK) inhibitor, SB203580 (4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole, HCl), showed no effect on cAMP-induced Aa-Nat and Period1 mRNA levels; whereas SB203580, but not its negative analogue SB202474 (4-Ethyl-2(p-methoxyphenyl)-5-(4'-pyridyl)-IH-imidazole, DiHCl), significantly reduced cAMP-induced MKP-1 mRNA levels. Taken together, our data suggest that cAMP-induced Aa-Nat and Period1 are likely to be mediated by activation of JNK, whereas MKP-1 may be mediated by both p38(MAPK) and JNK activations.

Analysis of gene expression following norepinephrine stimulation in the rat pineal gland using DNA microarray technique

Several studies have demonstrated that norepinephrine (NE) is the critical neurotransmitter for the regulation of gene expression in the pineal gland. We studied the acute effect of NE stimulation in cultured rat pineal glands using Affymetrix rat genome microarray GeneChip probe arrays. Our data demonstrate that NE stimulation affects regulation of several genes; 44 and 29 genes were up- or down-regulated more than 2.5-fold, respectively. As shown in previous studies, arylalkylamine N-acetyltransferase, cyclic AMP responsive element modulator, jun-B and c-fos mRNA levels were increased by NE stimulation. Genes that were not previously reported and increased by NE stimulation in the pineal gland were protein tyrosine phosphatase, nuclear receptors, and activity and neurotransmitter-induced early genes. Unlike up-regulated genes, most of the down-regulated genes were not reported previously. Genes encoding enzymes involved in metabolism and structural proteins were decreased following NE stimulation. Identification of genes affected by NE stimulation would provide valuable information to understanding pineal biology fully.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Norepinephrine-dependent phosphorylation of the transcription factor cyclic adenosine monophosphate responsive element-binding protein in bovine pinealocytes

Norepinephrine (NE)-dependent activation of transcription factors is of central importance for the rhythmic production of melatonin in the rodent pineal gland. At variance with rodents, NE regulates melatonin biosynthesis through post-translational mechanisms in ungulates, and it is not yet known whether transcription factors play any role in ungulate pineal functions. Here, we investigated in isolated bovine pinealocytes the NE-dependent phosphorylation of the transcription factor cyclic adenosine monophosphate (cAMP) responsive element-binding protein (CREB) and compared the effects of NE with those of vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP). Treatment with 10(-7) m NE for 30 min induced a strong nuclear phosphorylated CREB (pCREB) immunoreaction in cells that were identified as pinealocytes by immunocytochemical demonstration of serotonin, a pinealocyte-specific marker. Immunoblots showed that the NE-induced immunoreaction was due to phosphorylation of the transcription factor CREB and another protein, presumably the activating transcription factor 1 (ATF-1). 10(-7) m isoproterenol (ISO) or 10(-5) m forskolin mimicked the response to NE indicating that NE acts through the beta-adrenergic/cAMP pathway. Also 10(-7) m PACAP, but not 10(-7) m VIP-enhanced CREB phosphorylation; however, only a subpopulation of cells was responsive to PACAP. Our results suggest that, irrespective of whether or not melatonin production is controlled via transcriptional mechanisms, NE-induced CREB phosphorylation represents a very conserved element in pineal physiology of mammals because NE increases pCREB levels in all mammalian species investigated so far. However, the genes targeted by pCREB may vary from one mammalian species to the other. Our results also suggest that transcription factors other than pCREB, like ATF-1, may play a role in pineal functions of mammals.

Id-1 expression defines a subset of vimentin/S-100beta-positive, GFAP-negative astrocytes in the adult rat pineal gland

Id proteins are dominant negative members of the helix-loop-helix (HLH) transcription factor family which are involved in the differentation of many cell types, including glia. We have recently identified the adult rat pineal gland as a major site of Id-1 and Id-3 expression. In the present study, double fluorescence immunocytochemical analysis was used to examine the co-localization of Id-1 and Id-3 with both neuronal (synaptophysin, betaIII-tubulin) and astrocytic markers (GFAP, vimentin, S-100beta) in the rat pineal. In addition to localizing Id-1 and Id-3 protein to the melatonin-producing pinealocytes, we have also made the novel observation that Id-1, but not Id-3, is highly expressed in a population of vimentin-positive/S-100beta-positive/GFAP-negative astrocytes. Surprisingly, Id-1 was primarily cytoplasmic in these cells, and expression extended throughout the cellular processes. The pineal has been recognized previously as a unique region of the central nervous system in which a vimentin-positive/GFAP-negative glial phenotype is maintained in adult mammals. The exclusion of Id-1 from GFAP-positive cells, and expression in a population of vimentin-positive pineal astrocytes is evidence of a role for Id-1 in the adult stabilization of one form of astrocyte. These results identify the rat pineal gland as a model system for the functional analysis of Id-1.

cDNA array analysis of pineal gene expression reveals circadian rhythmicity of the dominant negative helix-loop-helix protein-encoding gene, Id-1

The pineal gland is a major output of the endogenous vertebrate circadian clock, with melatonin serving as the output signal. In many species, elevated nocturnal melatonin production is associated with changes in pineal gene expression. In the current study, cDNA array analysis was used in an attempt to identify additional genes that exhibit day/night differential expression in the rat pineal gland. This revealed 38 candidate genes, including Id-1 (inhibitor of DNA binding and differentiation). Id-1 encodes a helix-loop-helix (HLH) protein that lacks a basic DNA binding domain and could affect pineal physiology via a dominant negative trans-acting regulatory activity. For this reason Id-1 was selected for further analysis. Id-1 was expressed in a major population of pineal cells and the Id-1 protein was associated with a nuclear complex. The levels of Id-1 mRNA and protein exhibit approximately six-fold day/night rhythms. In contrast, the related genes Id-2 and Id-3 do not exhibit marked day/night differences in pineal expression. Rhythmic Id-1 expression is primarily limited to a C-terminally extended splice variant of Id-1, which would restrict the functional output of the rhythm to protein binding partners of this isoform of Id-1. Our findings add to the body of evidence indicating that transcriptional regulators play a role in neuroendocrine rhythms, and extend this by introducing the concept of a dominant negative HLH involvement. The rhythm in Id-1 in the pineal gland provides an experimental opportunity to identify Id-1-binding partners which may also be involved in Id-1 activity in other functional contexts.

Analysis of cell signalling in the rodent pineal gland deciphers regulators of dynamic transcription in neural/endocrine cells

In neurons, a temporally restricted expression of cAMP-inducible genes is part of many developmental and adaptive processes. To understand such dynamics, the neuroendocrine rodent pineal gland provides an excellent model system as it has a clearly defined input, the neurotransmitter norepinephrine, and a measurable output, the hormone melatonin. In this system, a regulatory scenario has been deciphered, wherein cAMP-inducible genes are rapidly activated via the transcription factor phosphoCREB to induce transcriptional events necessary for an increase in hormone synthesis. However, among the activated genes is also the inhibitory transcription factor ICER. The increasing amount in ICER protein leads ultimately to the termination of mRNA accumulation of cAMP-inducible genes, including the gene for the Aa-nat that controls melatonin production. This shift in ratio of phosphoCREB and ICER levels that depends on the duration of stimulation can be interpreted as a self-restriction of cellular responses in neurons and has also been demonstrated to interfere with cellular plasticity in many non-neuronal systems.

FosB in rat striatum: normal regional distribution and enhanced expression after 6-month haloperidol administration

Subcortical motor nuclei show differential expression of FosB immediate early gene products and specifically deltaFosB after short (8, 19, or 21 days) chronic exposure to typical and atypical neuroleptics represented by haloperidol and clozapine, respectively. We quantitatively examined whether there are light microscopic regional variations in area density of FosB or the truncated deltaFosB in several motor-related nuclei of adult rats receiving vehicle or long chronic (6 months) administration of either depot haloperidol or clozapine in their drinking water. In control animals the dorsomedial and ventromedial caudate-putamen nucleus (CPN) had a significantly higher density of FosB-immunoreactive cells than the dorsolateral and ventrolateral regions. The nucleus accumbens (NAc) core also serving motor functions had a higher basal expression than the limbic shell region in control animals. The mediolateral gradient in area density of FosB-labeled cells was maintained in animals receiving either haloperidol or clozapine. In animals receiving haloperidol, but not clozapine, however, there was a regionally selective increase in the area density of only FosB-immunoreactive neurons in the dorsolateral and ventrolateral CPN and in both the core and shell of the NAc. Only the animals receiving chronic haloperidol showed vacuous chewing movements, the animal equivalent of tardive dyskinesia in humans. Our results suggest that, whereas the medial striatal neurons are activated under basal conditions, long chronic haloperidol induced FosB expression more exclusively in the lateral CPN and NAc core, implicating these regions specifically in the motor side effects of this drug.

Differential modulatory effects of alpha- and beta-adrenoceptor agonists and antagonists on cortical immediate-early gene expression following focal cerebrocortical lesion-induced spreading depression

Unilateral, focal cerebrocortical lesion (FCL) and associated spreading depression (SD) increase immediate-early gene (IEG) expression throughout the ipsilateral hemisphere. Noradrenergic transmission is involved in the regulation of basal- and stimulation-induced expression of IEGs in cerebral cortex; and is modulated by both injury and SD. The present study further investigated the association between the noradrenergic system and cortical adaptive responses, by examining basal and FCL(SD)-induced cortical IEG expression following acute treatment with alpha(1)-, alpha(2)- and beta(1/2)-adrenoceptor (AR) agonists or antagonists. Activation of alpha(1)-ARs by NVI-085, or beta-ARs by salbutamol, increased cortical NGFI-A, c-jun and c-fos mRNA levels, whereas inhibition of alpha(1)-ARs by prazosin, or beta-ARs by propranolol, had no marked effect. The alpha(2)-AR agonists, clonidine and UK14304 also had no effect on basal IEG levels, while blockade of alpha(2)-ARs by methoxyidazoxan significantly increased NGFI-A and c-fos expression, but decreased c-jun mRNA levels. This latter effect confirms the complex and differential nature of IEG regulation in brain. In FCL(SD) rats, all AR agonists generally produced a supra-additive (synergistic) effect on expression of the examined IEGs, compared with drug-treatment or FCL alone. Prazosin reduced FCL(SD)-induced elevations of c-jun and c-fos, but not NGFI-A, mRNA. Methoxyidazoxan enhanced NGFI-A and c-fos mRNA expression after FCL(SD), but reduced c-jun. Propranolol enhanced all lesion-induced IEG levels. These results confirm that alpha(1)- and beta-ARs normally mediate a stimulatory, and alpha(2)-ARs a net inhibitory, influence on cortical cell activity (reflected by NGFI-A, c-fos expression); and demonstrate that alterations in noradrenergic tone modulate the level of cellular activation during and after SD, which is primarily elicited by K(+)/glutamate via NMDA receptors and Ca(2+)-associated mechanisms. In turn, noradrenergic transmission and interactions with excitatory systems are likely to be important in responses to brain injury, including regulation of IEGs and their downstream target genes.

Extracellular H+ stimulates the expression of c-fos/c-jun mRNA through Ca2+/calmodulin in PC12 cells

Evidence has accumulated that an increase in extracellular protons stimulates the transmembrane mechanism to induce various intracellular responses, such as the expression of c-fos and c-jun. In the present study, we aimed to obtain evidence that an increase in extracellular protons induces expression of c-fos/c-jun mRNA in PC12 pheochromocytoma cells of rats. We found that the c-fos/c-jun mRNA expression increased when extracellular pH was decreased gradually from 7.40 to 7.20 and that there was a significant correlation between extracellular pH values and the expression of c-fos/c-jun mRNA. To determine whether the Ca2+/calmodulin system subserves the H+-induced expression of c-fos/c-jun, Ca2+/calmodulin inhibitor trifluoperazine was added to PC12 cells. We found that trifluoperazine inhibited the expression of the H+-induced c-fos/c-jun mRNA by 30-35%. In contrast, trifluoperazine did not inhibit the expression of phorbol-induced c-fos/c-jun mRNA. These results indicate that an increase in extracellular protons induces the expression of c-fos/c-jun mRNA, and this expression is mediated partly by the Ca2+/calmodulin system.

Effects of intrathecal alpha1- and alpha2-noradrenergic agonists and norepinephrine on locomotion in chronic spinal cats

Noradrenergic drugs, acting on alpha adrenoceptors, have been found to play an important role in the initiation and modulation of locomotor pattern in adult cats after spinal cord transection. There are at least two subtypes of alpha adrenoceptors, alpha1 and alpha2 adrenoceptors. The aim of this study was to investigate the effects of selective alpha1 and alpha2 agonists in the initiation and modulation of locomotion in adult chronic cats in the early and late stages after complete transection at T13. Five cats, chronically implanted with an intrathecal cannula and electromyographic (EMG) electrodes were used in this study. Noradrenergic drugs including alpha2 agonists (clonidine, tizanidine, and oxymetazoline) and an antagonist, yohimbine, one alpha1 agonist (methoxamine), and a blocker, prazosin, as well as norepinephrine were injected intrathecally. EMG activity synchronized to video images of the hindlimbs were recorded before and after each drug injection. The results show differential effects of alpha1 and alpha2 agonists in the initiation of locomotion in early spinal cats (i.e., in the first week or so when there is no spontaneous locomotion) and in the modulation of locomotion and cutaneous reflexes in the late-spinal cats (i.e., when cats have recovered spontaneous locomotion). In early spinal cats, all three alpha2 agonists were found to initiate locomotion, although their action had a different time course. The alpha1 agonist methoxamine induced bouts of nice locomotor activity in three spinal cats some hours after injection but only induced sustained locomotion in one cat in which the effects were blocked by the alpha1 antagonist prazosin. In late spinal cats, although alpha2 agonists markedly increased the cycle duration and flexor muscle burst duration and decreased the weight support or extensor activity (effects blocked by an alpha2 antagonist, yohimbine), alpha1 agonist increased the weight support and primarily the extensor activity of the hindlimbs without markedly changing the timing of the step cycle. Although alpha2 agonists, especially clonidine, markedly reduced the cutaneous excitability and augmented the foot drag, the alpha1 agonist was found to increase the cutaneous reflex excitability. This is in line with previously reported differential effects of activation of the two receptors on motoneuron excitability and reflex transmission. Noradrenaline, the neurotransmitter itself, increased the cycle duration and at the same time retained the cutaneous excitability, thus exerting both alpha1 and alpha2 effects. This work therefore suggests that different subclasses of noradrenergic drugs could be used to more specifically target aspects of locomotor deficits in patients after spinal injury or diseases.

Ischemia-induced CA1 neuronal death is preceded by elevated FosB and Jun expression and reduced NGFI-A and JunB levels

Alterations in levels of the immediate-early gene (IEG) proteins Fos, FosB, DeltaFosB, Jun, JunB, JunD, and NGFI-A were investigated in rat hippocampus by immunohistochemistry 2, 12, 24, and 48 h after forebrain ischemia. Transient global ischemia of 20 min, produced by four vessel occlusion (4-VO), elicited different patterns of IEG expression in vulnerable CA1 and more resilient CA3 neurons. Cell counts revealed that except for JunD and NGFI-A, immunoreactivity for all examined IEGs was initially elevated by forebrain ischemia in both CA1 and CA3 hippocampal subfields. However, distinct patterns of IEG expression became evident in these regions at later time points. The pivotal difference was the persistence of ischemia-induced elevations of FosB and Jun expression in the CA1 region of the hippocampus. Unlike CA3 neurons, where IEG immunoreactivity had subsided to basal levels by 24-48 h, CA1 neurons continued to display increased FosB- and Jun-like immunoreactivity 48 h post-ischemia. Western blot analysis revealed that elevated expression of both FosB and DeltaFosB-like proteins were responsible for the immunohistochemical detection of enhanced FosB-like immunoreactivity in CA1 neurons at 48 h. These findings are consistent with recent in vitro studies that implicate FosB and Jun in gene signalling pathways responsible for programmed cell death. In contrast to FosB and Jun, JunB expression declined significantly below basal levels in CA1 neurons at 48 h, yet remained unaltered in CA3 neurons. Given that JunB can inhibit the transactivating properties of Jun, decreased JunB levels may contribute to the apoptotic death of CA1 neurons by enhancing the transcriptional regulating activity of Jun. Also notable at 48 h was the complete loss of constitutive NGFI-A expression from CA1 neurons of ischemic animals. These findings suggest that persistent elevations in FosB and Jun expression, concurrent with reductions in JunB and NGFI-A levels, contribute to the apoptotic death of CA1 neurons after forebrain ischemia.

Comparison of the expression of c-fos, nur77 and egr1 mRNAs in rat hypothalamic magnocellular neurons and their putative afferent projection neurons: cell- and stimulus-specific induction

Hypothalamic magnocellular neurons and their afferent inputs provide a model system in which to study the regulation of inducible transcription factors in the brain in vivo. Osmotic stimulation of rats produced by graded infusions of saline at different tonicities was found to lead to the induction of c-fos, nur77 and egr1 mRNAs in magnocellular neurons, as well as in putative afferent neurons, including those in structures of the forebrain (subfornical organ, median preoptic nucleus and organum vasculosum of the lamina terminalis). The results presented suggest that stronger levels of osmotic stimulation recruit additional afferents from the forebrain and brainstem that can act on magnocellular neurons via alternative receptors. A single systemic injection of the peptide cholecystokinin produced robust induction of c-fos and nur77 mRNAs in afferent neurons of the brainstem nucleus tractus solitarii and in magnocellular neurons. Despite the fact that these two neuronal populations are clearly electrically active, egr1 was not induced by this stimulus, providing examples of cell- and stimulus-specificity of its expression. This study re-emphasizes that the induction of transcription factors is largely dependent on the nature of the afferent input and does not correlate necessarily to the electrical activity of the neuron.

Circadian rhythms and autoregulatory transcription loops--going round in circles?

Recent advances in the molecular analysis of biological timing have appeared to bring us closer to an answer to the 'big question', namely, 'What is the timing mechanism that enables an organism to measure the circadian (around 24 h) period?'. In this minireview, we consider the validity of the fashionable concept that autoregulatory feedback loops, centered on transcription, form the basis of the clock, and we offer a fresh view of recent progress as it relates to mammalian systems.

Diurnal variation of the adenylyl cyclase type 1 in the rat pineal gland

Nocturnal melatonin production in the pineal gland is under the control of norepinephrine released from superior cervical ganglia afferents in a rhythmic manner, and of cyclic AMP. Cyclic AMP increases the expression of serotonin N-acetyltransferase and of inducible cAMP early repressor that undergo circadian oscillations crucial for the maintenance and regulation of the biological clock. In the present study, we demonstrate a circadian pattern of expression of the calcium/calmodulin activated adenylyl cyclase type 1 (AC1) mRNA in the rat pineal gland. In situ hybridization revealed that maximal AC1 mRNA expression occurred at midday (12:00-15:00), with a very low signal at night (0:00-3:00). We established that this rhythmic pattern was controlled by the noradrenergic innervation of the pineal gland and by the environmental light conditions. Finally, we observed a circadian responsiveness of the pineal AC activity to calcium/calmodulin, with a lag due to the processing of the protein. At midday, AC activity was inhibited by calcium (40%) either in the presence or absence of calmodulin, while at night the enzyme was markedly (3-fold) activated by the calcium-calmodulin complex. These findings suggest (i) the involvement of AC1 acting as the center of a gating mechanism, between cyclic AMP and calcium signals, important for the fine tuning of the pineal circadian rhythm; and (ii) a possible regulation of cyclic AMP on the expression of AC1 in the rat pineal gland.

Induction of members of the Fos/Jun family of immediate-early genes in identified hypothalamic neurons: in vivo evidence for differential regulation

In situ hybridization was used to measure the expression of members of the Fos/Jun family of immediate-early genes in hypothalamic neurons in vivo following defined stimuli that utilize different afferent pathways. Only c-jun messenger RNA was expressed in the hypothalamic supraoptic and paraventricular nuclei of control animals. Intravenous infusions of sodium chloride solutions of different tonicity produced a range of plasma osmolalities within physiological limits. While the induction of c-fos and jun B messenger RNAs followed the stimulus intensity, the expression of c-jun was repressed at low levels of stimulation. A higher level of osmotic stimulation was able to co-induce c-jun with the c-fos, jun B and fos B genes, suggesting that other signalling pathways may then be activated. Parturition or systemic administration of cholecystokinin, that activate supraoptic and paraventricular neurons via ascending afferent pathways from the brainstem, both induced c-fos, but not the other genes, in the magnocellular nuclei. Use of double in situ hybridization confirmed that, unlike with osmotic stimulation, induction of c-fos only occurred in oxytocin neurons. These two stimuli did not cause a concomitant repression of c-jun messenger RNA expression in magnocellular oxytocin neurons. These patterns of induction provide evidence for the differential regulation of members of this family of genes in a physiological context.

Trans-synaptic control of NGFI-A and jun-B expression: contrasting transcriptional and post-transcriptional mechanisms directed by common receptors

Previous studies have demonstrated that of the multiple primary response gene products which are induced in the rat pineal gland through a nocturnally activated adrenoceptor-linked mechanism. JunB is the principal component of a dark phase-specific activator protein-1 DNA binding complex. JunB is therefore implicated as a nuclear component of the mechanisms that determine nocturnal changes in pineal function. It is now shown that the marked increase in jun-B mRNA expression following norepinephrine stimulation in vitro, is mediated through a post-transcriptional mechanism that involves mRNA stabilization. This mode of regulation is contrasted with that controlling the expression of other primary response genes. In the case of NGFI-A, a co-regulated primary response gene which is controlled through a pharmacologically similar pathway, nuclear run-on transcription assays have shown that pineal mRNA levels are elevated through an increase in transcription rate that can be measured both in vitro and in vivo. These results show that multiple molecular mechanisms are engaged to effect the genomic consequences of adrenoceptor stimulation, and that rhythmic changes in gene expression may be controlled by post-transcriptional mechanisms involving mRNA stability.

Circadian expression of transcription factor Fra-2 in the rat pineal gland

Physiological changes in Fos-like immunoreactivity in the rat pineal gland are shown here to be due primarily to changes in a 42/46-kDa Fos-related antigen (Fra). Studies are presented that indicate this 42/46-kDa Fra is Fra-2, a poorly understood member of the Fos family of transcription factors. Both Fra-2 mRNA and protein are absent during the day and increase robustly at night on a circadian basis; organ culture studies indicate that regulation is mediated by an adrenergic-->cyclic AMP mechanism. AP-1 binding activity changes in parallel to changes in the level of Fra-2 protein.

Effect of administration of high dose intrathecal clonidine or morphine prior to sciatic nerve section on c-Fos expression in rat lumbar spinal cord

The effects of moderate and high intrathecal doses of clonidine, an alpha 2 adrenoceptor agonist, or a high dose of morphine on sciatic nerve section-induced expression of c-Fos-like immunoreactivity was studied in laminae I and II of the dorsal horn and laminae VIII and IX of the ventral horn of rat lumbar spinal cord. c-Fos-like immunoreactivity was examined by immunohistochemistry in normal rats (group 1), rats implanted with an intrathecal catheter with its tip on the lumbar spinal cord (group 2), injected with 10 micrograms (group 3) or 50 micrograms (group 4) clonidine intrathecally 3 h before being killed. In other groups, saline, 10 or 50 micrograms clonidine or 30 micrograms morphine was injected 1 h before unilateral nerve section, and the expression of c-Fos-like immunoreactivity was examined 2 h after axotomy. Few labeled neurons were found in normal controls. The intrathecal catheter itself caused a significant increase in bilateral c-Fos-like immunoreactivity in spinal dorsal and ventral horn compared to normals. The level of c-Fos-like immunoreactivity after 10 or 50 micrograms intrathecal clonidine was similar as in the intrathecal catheter group. Sciatic nerve section caused a significant ipsilateral increase in c-Fos-like immunoreactivity in the dorsal horn compared to the intact side in rats injected with saline. Pretreatment with 10 or 10 micrograms clonidine did not reduce sciatic nerve section-induced expression of c-Fos-like immunoreactivity, but instead caused a significant bilateral increase in c-Fos-like immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Pineal gene expression: dawn in a dark matter

The mammalian pineal gland serves as a neuroendocrine interface to convert environmental lighting conditions into a humoral message, the nocturnally elevated synthesis of melatonin. Regulation and fine tuning of the circadian melatonin production in response to external cues requires complex interactions of transsynaptic signalling. These requirements are fulfilled by a high degree of plasticity on all levels between receptor activation and cellular response. Many receptors on pinealocytic membranes and enzymes involved in melatonin synthesis are linked to the second messenger cAMP. Crosstalk between second and third messengers converges in the pineal gland--as in other tissues--eventually on a modulated activity of transcription factors. Of fundamental importance for genes involved in the transsynaptic signalling to create a circadian profile in melatonin synthesis is the cAMP-inducible promoter element, the CRE (cAMP responsive element). Indeed, the CRE is shared by many pineal genes that are of physiological importance. Recently, the deciphering of molecular determinants regulating expression of cAMP-inducible genes in the mammalian pineal gland, like NAT, c-jun, or the beta-adrenergic receptor suggests a modulation in their transcription by a dual regulatory mechanism: posttranslational activation of the early third messenger CREB (cAMP responsive element binding protein) stimulates, cis-acting cAMP-induced transcriptional upregulation of the late third messenger ICER (inducible cAMP early repressor) inhibits genes with a CRE.

Noradrenergic regulation of immediate early gene expression in rat forebrain: differential effects of alpha 1- and alpha 2-adrenoceptor drugs

Noradrenergic (NAergic) transmission in the rat cerebral cortex has recently been shown to be involved in the regulation of the basal expression of NGFI-A, an immediate early gene (IEG) which encodes a zinc-finger transcription factor. The present study further investigated the role of the NAergic system in mediating cortical IEG expression and possible topographical changes in expression of NGFI-A mRNA in rat forebrain after alpha 1- and alpha 2-adrenoceptor (AR) agonist and antagonist treatment. Expression of c-fos and c-jun, which encode leucine-zipper class transcription factors, was also studied. Male Sprague-Dawley rats were injected intraperitoneally with either an alpha 1-AR agonist (methoxamine, 5 or 10 mg/kg); an alpha 1-AR antagonist (prazosin, 5 mg/kg); an alpha 2-AR agonist (clonidine, 0.5 mg/kg); or an alpha 2-AR antagonist (methoxyidazoxan, 5 mg/kg) and killed after 1 h. IEG mRNA levels were detected by quantitative in situ hybridization histochemistry using 35S-labelled oligonucleotides. High basal levels of NGFI-A mRNA were present in cortical layers IV and VI, hippocampal CA1, piriform cortex, amygdala and caudate putamen. alpha 1-AR agonist and antagonist treatment had essentially no effect on IEG mRNA, despite producing characteristic behavioral and peripheral effects at the doses used. Methoxyidazoxan significantly increased (mean%) NGFI-A mRNA in: cerebral cortex (44); caudate putamen (82); amygdala (92); and CA1 of hippocampus (48), while clonidine significantly decreased NGFI-A mRNA in the various cortical layers to a similar extent (27-37%). Basal c-fos mRNA expression was lower than that for NGFI-A in forebrain areas including cortex, caudate putamen and hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

A daily rhythm of activator protein-1 activity in the rat pineal is dependent upon trans-synaptic induction of JunB

The daily cycle of phenotypic variation in the mammalian pineal provides a unique model for the investigation of the molecular mechanisms that regulate neurotransmitter synthesis. In the rat, a circadian adrenergic mechanism directs a change in serotonin metabolism that results in the nocturnal production of melatonin. Activity of the activator protein-1 transcriptional regulatory complex, as demonstrated by band-shift assays of rat pineal gland extracts, has now been shown to exhibit a rhythm, in vivo, which is temporally correlated with the rhythm of melatonin synthesis. Thus, nocturnal activator protein-1 activity (23.00 h) is markedly elevated, being 8-fold higher than the level of light-phase activity (P < 0.005). The nocturnal activator protein-1 protein complex is induced through a trans-synaptic, beta-adrenoceptor-linked mechanism and is characterized by the prolonged participation of JunB as demonstrated using antibodies for specific activator protein-1 proteins. Indeed, JunB appears to be a major component of nocturnal changes in activator protein-1 activity, JunD forming an additional, constitutive component which is not affected by the nocturnal adrenergic signal. The alpha 1-adrenoreceptor-linked c-Fos protein, which is coordinately induced with JunB, does not form a stable component of nocturnal activator protein-1 activity. In contrast, parallel experiments showed that c-Fos does form a major component of the hippocampal activator protein-1 complex that is induced in rats following kainic acid treatment. In the pineal, a similar, although not identical, pattern of activator protein-1 activation has also been demonstrated in cultured glands following treatment with norepinephrine. Immunoblotting has demonstrated parallel accumulation of JunB and c-Fos protein in pineal nuclear fractions following stimulation both in vivo and in vitro. The results provide evidence of posttranscriptional selection of neurotransmitter-stimulated activator protein-1 protein complexes, a mechanism which complements the differential induction of fos and jun genes in the pineal, and serves to generate a specific activator protein-1 transcription factor complex. This finding has general implications for the functional interpretation of fos and jun gene induction in neuronal systems. The stable JunB complex demonstrated here may be considered as one component of a timing mechanism which acts to perpetuate synaptic signals and thereby maintain an appropriate period of nocturnal pineal function.

A single administration of ethanol simultaneously increases c-fos mRNA and reduces c-jun mRNA in the hypothalamus and hippocampus

We have previously demonstrated that a single administration of ethanol induces the expression of c-fos mRNA in the hypothalamic paraventricular nucleus (PVN). However, Fos protein must interact with a member of the Jun family to form functional heterodimers. To determine whether ethanol may have differential effects on c-fos and c-jun expression, we injected male rats acclimated to a 25 degrees C environment with ethanol (3 g/kg b.wt.) or saline. Using in situ hybridization histochemistry with oligonucleotide probes, we found that ethanol increased c-fos mRNA in the PVN, but decreased c-jun mRNA both in the PVN and in hippocampus. Considering that ethanol produces hypothermia and that the PVN contains neurons activated during hypothermia, we evaluated the effect of cold on c-fos and c-jun mRNA. Both cold and ethanol increased c-fos mRNA, and the effects were additive. However, c-jun mRNA levels in both PVN and hippocampus were unaffected by temperature. Finally, c-jun mRNA levels in the hippocampus were significantly reduced by chronic ethanol exposure, and this trend was also observed in the PVN. These findings demonstrate that a single injection of ethanol has opposite effects on the expression of nuclear transcription factors which interact to regulate gene expression in the nervous system.

Compositional changes of AP-1 DNA-binding proteins are regulated by light in a mammalian circadian clock

Recent reports have shown that the nuclear phosphoprotein Fos is induced by light in a mammalian circadian clock, the suprachiasmatic nucleus. To learn how light and circadian phase affect the binding of Fos to DNA, we analyzed the photic and temporal regulation of immunoreactive Jun protein expression and AP-1 DNA-binding activity in the rat suprachiasmatic nucleus. Immunohistochemistry and gel mobility shift assays suggest that AP-1 activity during the night and after a light pulse consists of constant, as well as variable, protein components; JunD could be identified as a constituent of both dark- and light-activated binding complexes, whereas binding by JunB and Fos could be implicated only after photic stimulation. Since JunD or JunB could be colocalized with Fos in individual suprachiasmatic nucleus cell nuclei, light may be acting in at least some suprachiasmatic nucleus cells by altering AP-1 protein composition rather than binding site occupancy.

Noradrenergic regulation of c-jun expression in the rat pineal gland in culture: positive and negative components

One component of the nocturnal changes in cellular immediate-early gene expression in the rat pineal gland is a decrease in c-jun expression, mediated through beta-adrenoceptors. An in vitro study of the intracellular mechanisms which control c-jun expression has now shown that a norepinephrine-induced increase in c-jun mRNA levels in organ-cultured pineals is differentially modulated by protein kinase inhibitors; N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004) potentiated the response; however, in the presence of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), a significant decrease in c-jun mRNA was found. Treatment with HA-1004 alone elevated the level of c-jun mRNA, H-7 alone was without effect. Forskolin together with 3-isobutyl-1-methylxanthine suppressed c-jun, whereas phorbol 12,13-dibutyrate raised c-jun mRNA levels. The results demonstrate opposing pathways for c-jun regulation in the pineal gland, and indicate that the nocturnal attenuation of c-jun expression involves selective activation of a negative pathway which may be linked to cAMP.

Induction and suppression of immediate-early genes in the rat brain by a selective alpha-2-adrenoceptor agonist and antagonist following noxious peripheral stimulation

The effect of medetomidine, a highly selective alpha-2-adrenoceptor agonist, on noxious stimulation-induced expression of immediate-early genes was studied in the central nervous system of the rat. The expressions of c-JUN, JUN B, c-FOS FOS B and KROX-24 proteins were investigated by immunocytochemistry following the application of formalin (5%, 50 microliters) into the plantar skin of one hindpaw. Medetomidine (100 or 300 micrograms/kg i.p.) was administered 12 min or 5 min before the application of formalin. Atipamezole (1.5 mg/kg i.p.), and alpha-2-adrenoceptor antagonist, administered simultaneously with medetomidine (300 micrograms/kg), was used to reverse the alpha-2-adrenergic effects. The rats were killed and perfused 90 min after formalin injection. Formalin induced expression of all studied proteins in the ipsilateral spinal dorsal horn and the contralateral parabrachial nucleus, and in the medial thalamus bilaterally. Both medetomidine doses administered 12 min before formalin strongly suppressed the expression of c-FOS in the spinal dorsal horn; the suppression was stronger in the deep (III-VI) than in the superficial (I and II) laminae of the dorsal horn (76% and 86% for 100 micrograms/kg dose vs 97% and 99% for 300 micrograms/kg dose, respectively). However, application of medetomidine 5 min before formalin did not reduce the expression of immediate-early genes. In the parabrachial nucleus, both medetomidine doses also produced a significant suppression of c-FOS expression (68%). In contrast, medetomidine at the dose of 100 micrograms/kg was ineffective in the medical thalamus. Only the higher dose of medetomidine (300 micrograms/kg) produced a suppression by 29% and 46% in centromedian and paraventricular nuclei, respectively. Atipamezole produced a significant attenuation in spinal cord and a complete reversal in parabrachial nucleus of the medetomidine-induced suppression. However, in the medial thalamus, atipamezole produced a dramatic increase of formalin-induced c-FOS expression when compared with formalin injection alone. The expression of c-JUN, JUN B, FOS B and KROX-24 proteins paralleled that of c-FOS. It is concluded that the expression of immediate-early gene encoded proteins is more strongly suppressed by alpha-2-adrenoceptor agonists in spinal and parabrachial than in medial thalamic neurons. The increased expression of immediate-early genes in medical thalamus following atipamezole treatment may be explained by increased release of noradrenaline and the consequent activation of alpha-1- and beta-adrenoceptors. Compared with the previously reported effects of behaviorally equipotent doses of morphine, the suppression of c-FOS expression in the spinal cord was stronger following medetomidine than that following morphine.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro regulation of rat prolactin messenger ribonucleic acid poly(A) tail length: modulation by bromocriptine

Recent analysis of endocrine gene transcripts has revealed that several hormone mRNAs exhibit regulated size changes (due to alterations in length of the 3' poly(A) tail) which may function as an additional level of control in the determination of gene expression. We have now shown, through the novel application of an organ culture technique, that prolactin mRNA exhibits a similar regulated change in poly(A) tail length when rat anterior pituitary glands are explanted. The effect is observed in glands of either male or female rats and is specific with respect to growth hormone and alpha-tubulin mRNAs. Furthermore, we have also found that the size change in prolactin mRNA is attenuated in the presence of bromocriptine, indicating regulation through a dopaminergic pathway.

Differential intracellular mechanisms mediate the co-ordinate induction of c-fos and jun-B in the rat pineal gland

In the rat pineal gland the immediate early genes c-fos and jun-B are co-ordinately induced in vivo following the onset of darkness, and in vitro following treatment with adrenergic drugs. An extensive in vitro analysis of the mechanisms which regulate the induction of these two genes has revealed that, although there are common pathways of control, c-fos and jun-B exhibit differential regulation by adrenoceptor types, second messenger pathways and protein kinase C. The presence of differential intracellular mechanisms of regulation provides precise control over the expression pattern of immediate early genes, a pattern which is crucial for cellular response since the combinatorial actions of the products of these genes are a determinant of target gene activation. The pineal gland is a unique experimental model in which the molecular pharmacology of immediate early gene regulation can be investigated.