Induction of c-fos and TIS genes in cultured rat astrocytes by neurotransmitters

Activation of Gs Signaling in Cortical Astrocytes Does Not Influence Formation of a Persistent Contextual Memory Engram

Formation and retrieval of remote contextual memory depends on cortical engram neurons that are defined during learning. Manipulation of astrocytic Gq and Gi associated G-protein coupled receptor (GPCR) signaling has been shown to affect memory processing, but little is known about the role of cortical astrocytic Gs-GPCR signaling in remote memory acquisition and the functioning of cortical engram neurons. We assessed this by chemogenetic manipulation of astrocytes in the medial prefrontal cortex (mPFC) of male mice, during either encoding or consolidation of a contextual fear memory, while simultaneously labeling cortical engram neurons. We found that stimulation of astrocytic Gs signaling during memory encoding and consolidation did not alter remote memory expression. In line with this, the size of the mPFC engram population and the recall-induced reactivation of these neurons was unaffected. Hence, our data indicate that activation of Gs-GPCR signaling in cortical astrocytes is not sufficient to alter memory performance and functioning of cortical engram neurons.

Immediate Early Gene c-fos in the Brain: Focus on Glial Cells

The c-fos gene was first described as a proto-oncogene responsible for the induction of bone tumors. A few decades ago, activation of the protein product c-fos was reported in the brain after seizures and other noxious stimuli. Since then, multiple studies have used c-fos as a brain activity marker. Although it has been attributed to neurons, growing evidence demonstrates that c-fos expression in the brain may also include glial cells. In this review, we collect data showing that glial cells also express this proto-oncogene. We present evidence demonstrating that at least astrocytes, oligodendrocytes, and microglia express this immediate early gene (IEG). Unlike neurons, whose expression changes used to be associated with depolarization, glial cells seem to express the c-fos proto-oncogene under the influence of proliferation, differentiation, growth, inflammation, repair, damage, plasticity, and other conditions. The collected evidence provides a complementary view of c-fos as an activity marker and urges the introduction of the glial cell perspective into brain activity studies. This glial cell view may provide additional information related to the brain microenvironment that is difficult to obtain from the isolated neuron paradigm. Thus, it is highly recommended that detection techniques are improved in order to better differentiate the phenotypes expressing c-fos in the brain and to elucidate the specific roles of c-fos expression in glial cells.

Alcohol-associated antecedent stimuli elicit alcohol seeking in non-dependent rats and may activate the insula

Alcohol self-administration produces brain and behavior adaptations that facilitate a progressive loss of control over drinking and contribute to relapse. One possible adaptation is the ability of antecedent environmental stimuli that are consistently paired with alcohol to trigger alcohol-seeking behaviors. We previously modeled this adaptation in rats using a Pavlovian conditioning procedure in which illumination of a houselight preceded the presentation of a sipper tube that produced unsweetened alcohol when licked. However, in our previous work we did not demonstrate whether this adaptation represented a consequence of repeated exposure to alcohol or the houselight, or whether it was the consequence of associative learning and memory. Thus, in the present study, we tested the associative basis of alcohol seeking in response to houselight illumination in our task using adult male rats that were not food- or water-deprived and were not dependent on alcohol. Separate groups of rats received houselight illumination that was explicitly paired or unpaired with presentation of the retractable sipper that provided access to unsweetened alcohol. Our primary dependent variable was appetitive alcohol-directed behavior: the frequency of movement toward and interaction with the hole in the wall of the chamber through which the sipper was presented during the period of houselight illumination trial before each sipper presentation. However, we also analyzed consummatory sipper licking behavior and blood ethanol concentration in the same rats. Finally, we explored the brain basis of cue-elicited alcohol seeking using c-Fos immunohistochemistry. Our findings confirmed the associative basis of cue-elicited alcohol seeking in our paradigm and mapped these onto the insular cortex, suggesting a role for this brain region in early stages of brain and behavior adaptation to regular alcohol use.

What does the Fos say? Using Fos-based approaches to understand the contribution of stress to substance use disorders

Despite extensive research efforts, drug addiction persists as a largely unmet medical need. Perhaps the biggest challenge for treating addiction is the high rate of recidivism. While many factors can promote relapse in abstinent drug users, the contribution of stress is particularly problematic, as stress is uncontrollable and pervasive in the lives of those struggling with addiction. Thus, understanding the neurocircuitry that underlies the influence of stress on drug seeking is critical for guiding treatment. Preclinical research aimed at defining this neurocircuitry has, in part, relied upon the use of experimental approaches that allow visualization of cellular and circuit activity that corresponds to stressor-induced drug seeking in rodent relapse models. Much of what we have learned about the mechanisms that mediate stressor-induced relapse has been informed by studies that have used the expression of the immediate early gene, cfos, or its protein product, Fos, as post-mortem activity markers. In this review we provide an overview of the rodent models used to study stressor-induced relapse and briefly summarize what is known about the underlying neurocircuitry before describing the use of cfos/Fos-based approaches. In addition to reviewing findings obtained using this approach, its advantages and limitations are considered. Moreover, new techniques that leverage the expression profile of cfos to tag and manipulate cells based on their activity patterns are discussed. The intent of the review is to guide the interpretation of old and design of new studies that utilize cfos/Fos-based strategies to study the neurocircuitry that contributes to stress-related drug use.

Mapping Gene Expression in Excitatory Neurons during Hippocampal Late-Phase Long-Term Potentiation

The persistence of long-lasting changes in synaptic connectivity that underlie long-term memory require new RNA and protein synthesis. To elucidate the temporal pattern of gene expression that gives rise to long-lasting neuronal plasticity, we analyzed differentially-expressed (DE) RNAs in mouse hippocampal slices following induction of late phase long-term potentiation (L-LTP) specifically within pyramidal excitatory neurons using Translating Ribosome Affinity Purification RNA sequencing (TRAP-seq). We detected time-dependent changes in up- and down-regulated ribosome-associated mRNAs over 2 h following L-LTP induction, with minimal overlap of DE transcripts between time points. TRAP-seq revealed greater numbers of DE transcripts and magnitudes of LTP-induced changes than RNA-seq of all cell types in the hippocampus. Neuron-enriched transcripts had greater changes at the ribosome-loading level than the total RNA level, while RNA-seq identified many non-neuronal DE mRNAs. Our results highlight the importance of considering both time course and cell-type specificity in activity-dependent gene expression during memory formation.

Post-training gamma irradiation-enhanced contextual fear memory associated with reduced neuronal activation of the infralimbic cortex

The brain might be exposed to irradiation under a variety of situations, including clinical treatments, nuclear accidents, dirty bomb scenarios, and military and space missions. Correctly recalling tasks learned prior to irradiation is important but little is known about post-learning effects of irradiation. It is not clear whether exposure to X-ray irradiation during memory consolidation, a few hours following training, is associated with altered contextual fear conditioning 24h after irradiation and which brain region(s) might be involved in these effects. Brain immunoreactivity patterns of the immediately early gene c-Fos, a marker of cellular activity was used to determine which brain areas might be altered in post-training irradiation memory retention tasks. In this study, we show that post-training gamma irradiation exposure (1 Gy) enhanced contextual fear memory 24h later and is associated with reduced cellular activation in the infralimbic cortex. Reduced GABA-ergic neurotransmission in parvalbumin-positive cells in the infralimbic cortex might play a role in this post-training radiation-enhanced contextual fear memory.

Minireview: Nuclear hormone receptor 4A signaling: implications for metabolic disease

Numerous members of the nuclear hormone receptor (NR) superfamily have been demonstrated to regulate metabolic function in a cell- and tissue-specific manner. This review brings together recent studies that have associated members of the NR superfamily, the orphan NR4A subgroup, with the regulation of metabolic function and disease. The orphan NR4A subgroup includes Nur77 (NR4A1), Nurr1 (NR4A2), and Nor-1 (NR4A3). Expression of these receptors is induced in multiple tissues by a diverse range of stimuli, including stimuli associated with metabolic function, such as: β-adrenoceptor agonists, cold, fatty acids, glucose, insulin, cholesterol, and thiazolidinediones. In vitro and in vivo gain- and loss-of-function studies in major metabolic tissues (including skeletal muscle, adipose, and liver cells and tissues) have associated the NR4A subgroup with specific aspects of lipid, carbohydrate, and energy homeostasis. Most excitingly, although these orphan receptors do not have known endogenous ligands, several small molecule agonists have recently been identified. The preliminary studies reviewed in this manuscript suggest that therapeutic exploitation of the NR4A subgroup may show utility against dyslipidemia, obesity, diabetes, and cardiovascular disease.

Noradrenergic stimulation of BDNF synthesis in astrocytes: Mediation via α1- and β1/β2-adrenergic receptors

Brain-derived neurotrophic factor (BDNF) synthesis in astrocytes induced by noradrenaline (NA) is a receptor-mediated process utilizing two parallel adrenergic pathways: beta1/beta2-adrenergic/cAMP and the novel alpha1-adrenergic/PKC pathway. BDNF is produced by astrocytes, in addition to neurons, and the noradrenergic system plays a role in controlling BDNF synthesis. Since astrocytes express various subtypes of alpha- and beta-adrenergic receptors that have the potential to be activated by synaptically released NA, we focused our present study on the mediatory role of adrenergic receptors in the noradrenergic up-regulation of BDNF synthesis in cultured neonatal rat cortical astrocytes. NA (1 microM) elevates BDNF levels by four-fold after 6 h of incubation. Its stimulation was partly inhibited by either the beta1-adrenergic antagonist atenolol, the beta2-adrenergic antagonist ICI 118,551, or by the alpha1-adrenergic antagonist prazosin, while the alpha2-adrenergic antagonist yohimbine showed no effect. BDNF levels in astrocytes were increased by the specific beta1-adrenergic agonist dobutamine and the beta2-adrenergic agonist salbutamol, as well as by adenylate cyclase activation (by forskolin) and PKA activation (by dBcAMP). However, none of the tested agonists or mediators of the intracellular beta-adrenergic pathways were able to reach the level of NA's stimulatory effect. BDNF cellular levels were also elevated by the alpha1-adrenergic agonist methoxamine, but not by the alpha2-adrenergic agonist clonidine. The increase in intracellular Ca2+ by ionophore A23187 showed no effect, whereas PKC activation by phorbol 12-myristate 13-acetate (TPA) potently stimulated BDNF levels in the cells. The methoxamine-stimulated BDNF synthesis was inhibited by desensitizing pretreatment with TPA, indicating that the alpha1-stimulation was mediated via PKC activation. In conclusion, the synthesis of astrocytic BDNF stimulated by noradrenergic neuronal activity is an adaptable process using multiple types (alpha1 and beta1/beta2) of adrenergic receptor activation.

Integration of new neurons into functional neural networks

Although it is established that new granule cells can be born and can survive in the adult mammalian hippocampus, there remains some question concerning the functional integration of these neurons into behaviorally relevant neural networks. By using high-resolution confocal microscopy, we have applied a new strategy to address the question of functional integration of newborn neurons into networks that mediate spatial information processing and memory formation. Exploration-induced expression of the immediate-early gene Arc in hippocampal cells has been linked to cellular activity observed in electrophysiological recordings under the same behavioral conditions. We investigated whether mature (5-month-old), newborn granule cells express Arc in response to a discrete spatial experience by detecting the expression of Arc in combination with NeuN (neuron-specific nuclear protein)-positive and bromodeoxyuridine-positive cells. We found that mature new granule cells do indeed express Arc in response to an exploration experience, supporting the idea that these cells are well integrated into hippocampal circuits. The proportion of mature newborn neurons that expressed Arc in response to exploration, however, was significantly higher (approximately 2.8%) than the proportion of cells that expressed Arc in the already existing population of granule cells (approximately 1.6%; p < 0.01). This finding extends previous data suggesting that the cellular physiology of newborn granule neurons differs from that of the existing population by indicating that these properties are retained in mature adult-generated neurons. Thus, these data have interesting implications for network models of spatial information processing and the role of hippocampal circuits in memory, indicating that mature new neurons are selectively recruited into hippocampal cell assemblies in higher proportions than older cells.

Evidence for Nr4a1 as a cold-induced effector of brown fat thermogenesis

Acute cold exposure leads to norepinephrine release in brown adipose tissue (BAT) and activates uncoupling protein (UCP)1-mediated nonshivering thermogenesis. Chronic sympathetic stimulation is known to initiate mitochondrial biogenesis, UCP1 expression, hyperplasia of BAT, and recruitment of brown adipocytes in white adipose tissue (WAT) depots. Despite distinct functions of BAT and WAT in energy balance, only a few genes are exclusively expressed in either tissue. We identified NUR77 (Nr4a1), an orphan receptor, to be induced transiently in brown adipocytes in response to beta-adrenergic stimulation and in BAT of cold-exposed mice. Subsequent reporter gene assays demonstrated an inhibitory action of NUR77 on basal and peroxisome proliferator-activated receptor (PPAR)gamma/retinoid X receptor (RXR)alpha-mediated transactivation of the Ucp1 enhancer in heterologous cotransfection experiments. Despite this function of NUR77 in the control of Ucp1 gene expression, nonshivering thermogenesis was not affected in Nur77 knockout mice. However, we observed a superinduction of Nor1 in BAT of cold-exposed knockout mice. We conclude that NUR77 is a cold-induced negative regulator of Ucp1, but phenotypic consequences in knockout mice are compensated by functional redundancy of Nor1.

TIS7 regulation of the beta-catenin/Tcf-4 target gene osteopontin (OPN) is histone deacetylase-dependent

12-O-Tetradecanoylphorbol-13-acetate-induced sequence 7 (TIS7) acts as a transcriptional co-repressor interacting with SIN3, the histone deacetylase-containing complex. The overexpression of TIS7 down-regulates expression of a specific set of genes. Homozygous deletion of this gene in mice delays injury-induced muscle regeneration and inhibits muscle satellite cell differentiation and fusion of myoblasts in vitro. Osteopontin (OPN), a known beta-catenin/T cell factor-4 (Tcf-4) downstream target gene, is up-regulated in tumors and in cells with increased motility such as muscle cells. OPN promoter sequence contains binding sites for Sp1, glucocorticoid receptor, E-box-binding factors, octamer motif-binding protein, c-Myc, and other transcription factors. Previously we have shown that TIS7 regulates the OPN expression through the inhibition of the Sp1-activating effects. Here we show that TIS7 has the capacity to inhibit OPN expression also through Lef-1, the second identified OPN regulatory element. TIS7 has the capacity to down-regulate beta-catenin/Tcf-4 transcriptional activity. TIS7 homologous deletion in mouse embryonic fibroblasts increased not only the TOPflash reporter gene transcriptional activity but also the expression of c-Myc and OPN. Furthermore, we show that TIS7 overexpression leads to the beta-catenin interaction with enzymatically active histone deacetylases. We propose that TIS7 down-regulates the beta-catenin/Tcf-4 transcriptional activity via its interaction with histone deacetylase-containing complex thereby inhibiting the expression of beta-catenin downstream target genes such as c-Myc and OPN. We hypothesize that TIS7 as a negative regulator of transcriptional activity represses expression of OPN and beta-catenin/Tcf-4 target genes, which are involved in myogenesis, muscle maintenance, and regeneration in a histone deacetylase dependent manner.

Muscle regeneration and myogenic differentiation defects in mice lacking TIS7

The tetradecanoyl phorbol acetate-induced sequence 7 gene (tis7) is regulated during cell fate processes and functions as a transcriptional coregulator. Here, we describe the generation and analysis of mice lacking the tis7 gene. Surprisingly, TIS7 knockout mice show no gross histological abnormalities and are fertile. Disruption of the tis7 gene by homologous recombination delayed muscle regeneration and altered the isometric contractile properties of skeletal muscles after muscle crush damage in TIS7(-/-) mice. Cultured primary myogenic satellite cells (MSCs) from TIS7(-/-) mice displayed marked reductions in differentiation potential and fusion index in a strictly cell-autonomous fashion. Loss of TIS7 caused the down-regulation of muscle-specific genes, such as those for MyoD, myogenin, and laminin-alpha2. Fusion potential in TIS7(-/-) MSCs could be rescued by TIS7 expression or laminin supplementation. Therefore, TIS7 is not essential for mouse development but plays a novel regulatory role during adult muscle regeneration.

Neuron-to-glia signaling mediated by excitatory amino acid receptors regulates ErbB receptor function in astroglial cells of the neuroendocrine brain

Hypothalamic astroglial erbB tyrosine kinase receptors are required for the timely initiation of mammalian puberty. Ligand-dependent activation of these receptors sets in motion a glia-to-neuron signaling pathway that prompts the secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, from hypothalamic neuroendocrine neurons. The neuronal systems that may regulate this growth factor-mediated back signaling to neuroendocrine neurons have not been identified. Here we demonstrate that hypothalamic astrocytes contain metabotropic receptors of the metabotropic glutamate receptor 5 subtype and the AMPA receptor subunits glutamate receptor 2 (GluR2) and GluR3. As in excitatory synapses, these receptors are in physical association with their respective interacting/clustering proteins Homer and PICK1. In addition, they are associated with erbB-1 and erbB-4 receptors. Concomitant activation of astroglial metabotropic and AMPA receptors results in the recruitment of erbB tyrosine kinase receptors and their respective ligands to the glial cell membrane, transactivation of erbB receptors via a mechanism requiring metalloproteinase activity, and increased erbB receptor gene expression. By facilitating erbB-dependent signaling and promoting erbB receptor gene expression in astrocytes, a neuron-to-glia glutamatergic pathway may represent a basic cell-cell communication mechanism used by the neuroendocrine brain to coordinate the facilitatory transsynaptic and astroglial input to LHRH neurons during sexual development.

Increased c-Fos protein in the brains of scrapie-infected SAMP8, SAMR1, AKR and C57BL mice

Scrapie is a neurodegenerative disease that occurs naturally in sheep and goats. The histopathological changes include vacuolation, neuronal apoptosis and astrocytosis. The mechanisms involved in neuronal apoptosis are still unknown. Recently, we observed that activated p38 immunohistostaining was increased in scrapie-infected mice. In many neurodegenerative diseases, activation of the p38 pathway and of the immediate-early gene termed c-Fos appears to be required for the initiation of apoptosis. There are similarities in histopathological changes seen in scrapie-infected mice and in an uninfected senescence-accelerated mouse strain (SAMP8). This led us to investigate c-Fos protein levels in the brains of both uninfected and scrapie-infected SAMP8, SAMR1, AKR and C57BL mice using immunohistochemical methods. The SAMR1 strain served as a control in that it is a mouse strain that does not show accelerated ageing, but has a background that is similar to the SAMP8 strain. AKR was used because it is one of the progenitor strains of both SAM strains and, finally, C57BL is a completely unrelated strain. The results showed a low basal c-Fos expression in controls and a marked increase in c-Fos staining in scrapie-infected mice. In scrapie-positive mice, c-Fos immunoreactivity was observed in neurones in the cortex, hippocampus, thalamus, hypothalamus, medulla, midbrain, brainstem, paraterminal body, internal capsule and cerebellar Purkinje cells. Immunoreactivity of c-Fos was also observed in astrocytes in many brain areas of scrapie-infected mice, particularly in the hippocampus and cortex. Our results show that normal mouse brain (NMB)-injected AKR and SAMP8 mice had more c-Fos production than NMB-injected SAMR1 or C57BL mice; scrapie-infection induces significant increases in c-Fos immunoreactivity in all four mouse strains. Our study suggests that the increase in c-Fos levels may play a role in the neuronal apoptosis observed in scrapie-infected mice.

Immunohistochemistry of c-fos in Mouse Brain During Postnatal Development: Basal Levels and Changing Response to Metrazol and Kainate Injection

Levels and cellular distribution of FOS, the product of c-fos (onco)gene, were studied by immunohistochemistry during the development of mouse brain at rest and after the administration of convulsants. Basal FOS immunoreactivity became detectable only after postnatal day 20 (P20). Metrazol and kainate at the appropriate doses induced convulsions at all ages but, in both cases, FOS accumulated in limbic areas (particularly in the dentate gyrus) only after a certain age: P20 for kainate and P30 for Metrazol. Surprisingly, considering the different molecular targets of Metrazol and kainate, respectively, and the different type of convulsions elicited, the cell groups in the limbic areas in which FOS increased were the same in the two cases. These results suggest that both drugs produced FOS increase by finally activating the same circuit. During ontogeny, the ability to accumulate FOS, which appears after P20, could be the sign of the attained maturity of signal transduction mechanisms in the cells of the hippocampal formation; endogenous signals originating from the activity of the nervous system increase the basal FOS levels and exogenous signals (i.e. like those given, probably locally, by kainate) further increase these levels. Metrazol manifests its capability to induce FOS accumulation only at later ages. We suggest that this occurs because the Metrazol target is probably distant from the hippocampal region and thus the maturity of a nerve pathway(s) is also required for c-fos induction.

Muscarinic receptor-mediated phosphorylation of cyclic AMP response element binding protein in human neuroblastoma cells

This study describes the effect of signalling through muscarinic acetylcholine receptors on two transcription factors implicated in long-term synaptic plasticity and memory formation, EGR1 and the cyclic AMP response element binding protein (CREB). In SK-N-SH neuroblastoma cells, treatment with the cholinergic agonist carbachol led to maximal induction of EGR1 1 h after stimulation. This was preceded by the phosphorylation of CREB, which peaked as early as 5 minutes after carbachol treatment. The levels of both EGR1 and phosphorylated CREB (pCREB) slowly decayed over 4-8 h. CREB phosphorylation and EGR1 induction showed similar sensitivity to carbachol concentration, with EC(50) values in the range of 1-10 microM, and the changes in both transcription factors were blocked by the muscarinic antagonist atropine. As has been described elsewhere, EGR1 induction was dependent on activation of p42/44 MAP kinase, as it was blocked by the MEK inhibitor U0126. However, CREB phosphorylation by carbachol was largely unaffected by MAP kinase blockade. As both CREB phosphorylation and EGR1 induction have been linked to long-term potentiation and some forms of memory consolidation, these results may implicate CREB and EGR1 in independent or partially independent cholinergic signalling pathways involved in memory processes.

Astrocytic Fos expression in the rat posterior pituitary following LPS administration

Systemic lipopolysaccharide (LPS) administration has been shown to cause profound Fos expression in multiple regions of the brain. In the present experiment, Fos expression in the hypothalamic supraoptic nucleus (SON), posterior pituitary, and anterior pituitary was investigated using quantitative immunohistochemistry. In the SON and anterior pituitary, a large number of Fos-positive cells were observed by restraint stress, hyperosmotic administration (1.5, 3, and 9% NaCl), and LPS administration (5, 25, and 125 microg/kg). In the posterior pituitary, LPS administration caused a significant increase in the number of Fos-positive nuclei in a dose-dependent manner, whereas restraint stress and hyperosmotic stimulation (1.5 and 3% NaCl) did not increase the number of Fos-positive cells and 9% NaCl administration induced weak Fos immunoreactivity. Moreover, a dual-labeling study using a confocal microscope revealed that Fos-positive cells in the posterior pituitary were astrocytes using MAP2, an astrocytic marker in the posterior pituitary. Here, we demonstrated that the astrocytes of the posterior pituitary expressed Fos in response to LPS administration, which suggests that Fos expression participates in the activation of astrocytes during acute-phase responses with LPS administration.

Cholinergic stimulation of early growth response-1 DNA binding activity requires protein kinase C and mitogen-activated protein kinase kinase activation and is inhibited by sodium valproate in SH-SY5Y cells

Activation of muscarinic receptors in human neuroblastoma SH-SY5Y cells with carbachol stimulated a rapid and large increase in early growth response-1 (Egr-1, also called zif268 and NGF1-A) protein levels and DNA binding activity. Egr-1 DNA binding activity was stimulated within 15 min of treatment with carbachol and maintained a maximum 20-fold increase over basal between 1 and 2 h after treatment, and the EC50 was approximately 1 microM carbachol. Carbachol-stimulated Egr-1 DNA binding activity was dependent on protein kinase C, as it was potently inhibited by GF109203X (IC50 approximately 0.1 microM) and was reduced by 85 +/- 5% by down-regulation of protein kinase C. Inhibitors of increases in intracellular calcium levels reduced carbachol-induced Egr-1 DNA binding activity by 25-35%. Carbachol-stimulated activation of Egr-1 was reduced 35% by genistein, a tyrosine kinase inhibitor, and 60% by PD098059, an inhibitor of mitogen-activated protein kinase kinases 1/2 (MEK1/2) that activates extracellular-regulated kinases 1/2 (ERK1/2). A novel inhibitory action was caused by chronic (7-day) administration of sodium valproate but not by two other bipolar disorder therapeutic agents, lithium and carbamazepine. Valproate treatment reduced carbachol-stimulated Egr-1 DNA binding activity by 60% but did not alter carbachol-induced activation of ERK1/2 or p38 or increases in Egr-1 protein levels. These results reveal that muscarinic receptors activate Egr-1 through a signaling cascade primarily encompassing protein kinase C, MEK1/2, and ERK1/2 and that valproate substantially inhibits Egr-1 DNA binding activity stimulated by carbachol or protein kinase C.

A canonical nerve growth factor-induced gene-B response element appears not to be involved in the cyclic adenosine monophosphate-dependent expression of differentiation in thyrocytes

The expression of transcriptionally active nerve growth factor-induced gene-B (NGFI-B) is rapidly induced in thyroid follicular cells in response to cAMP stimulation. As the transcription of thyrocyte-specific genes is controlled by the cAMP cascade, we have investigated a possible involvement of NGFI-B in this control. Recombinant adenoviruses driving the expression of either the intact NGFI-B protein or a truncated form of it that lacks the capacity to transactivate a NBRE-dependent promoter, were used to infect dog thyrocytes maintained in primary culture. Northern blot analysis of total RNA from infected cells revealed that the expression of NGFI-B was not sufficient to induce a significant accumulation of specific transcripts (thyroglobulin, thyroperoxidase, sodium-iodide symporter) in unstimulated thyrocytes. The overproduction of the transcriptionally inactive form of NGFI-B in thyrocytes maintained in the presence of forskolin after infection did not impair the accumulation of the thyroid-specific transcripts. These data show that NGFI-B does not control the expression of differentiation in thyrocytes by acting through a canonical NBRE. As a consequence, we must consider that either the expression of NGFI-B in cAMP-stimulated thyrocytes is not critically linked to the expression of differentiation or that NGFI-B is implicated in a regulatory mechanism which differs from its known action at the level of a NBRE.

Neuroprotection mediated via neurotrophic factors and induction of neurotrophic factors

Neurotrophins and other neurotrophic factors have been shown to support the survival and differentiation of many neuronal populations of the central and peripheral nervous system. Therefore, administering neurotrophic factors could represent an alternative strategy for the treatment of acute and chronic brain disorders. However, the delivery of neurotrophic factors to the brain is one of the largest obstacles in the development of effective therapy for neurodegenerative disorders, because these proteins are not able to cross the blood-brain barrier. The induction of growth factor synthesis in the brain tissue by systemically administered lipophilic drugs, such as beta-adrenoceptor agonists, shown to increase endogenous nerve growth factor (NGF) synthesis in the brain, would be an elegant way to overcome these problems of application. Stimulation of beta-adrenoceptors with clenbuterol led to increased NGF synthesis in cultured central nervous system (CNS) cells and rat brain tissue. Clenbuterol-induced NGF expression was reduced to the control levels by coadministration of beta-adrenoceptor antagonist propranolol. Furthermore, clenbuterol protected rat hippocampal neurons subjected to excitotoxic damage. The neuroprotective effect of clenbuterol in vitro depended on increased NGF synthesis, since the neuroprotection was abolished by NGF antisense oligonucleotide as well as by antibodies directed against NGF itself. In vivo, clenbuterol protected rat hippocampus in a model of transient forebrain ischemia and reduced the infarct volume in a rat model of permanent middle cerebral artery occlusion (MCAo). The neuroprotective effect of clenbuterol in vivo was accompanied by enhanced NGF synthesis in brain tissue. These findings support our hypothesis that orally active NGF inducers may have a potential as therapeutic agents for the treatment of neurodegenerative disorders and stroke.

Lithium and valproate differentially regulate brain regional expression of phosphorylated CREB and c-Fos

Previous studies in our laboratory have shown that the mood stabilizers, lithium and valproate (VPA), regulate the transcription factors, cyclic AMP responsive element binding protein (CREB), c-Fos and c-Jun, differentially in cultured human neuroblastoma SH-SY5Y cells. Here, we confirm these findings in rat brain and further study the brain-regional effects of these drugs using immunohistochemistry. We found that although chronic treatment with LiCl or VPA did not change the expression of c-Fos and c-Jun, acute treatment with either drugs increased c-Fos expression but not c-Jun expression in CA1 and CA3 regions of hippocampus. Chronic treatment with LiCl, but not VPA, decreased CREB phosphorylation in rat cerebral cortex and hippocampus. These results suggest that lithium and VPA may act on different pathways to bring about their long-term prophylactic effects on bipolar disorder (BD). The regulation of CREB phosphorylation may be relevant to lithium effect. VPA, which is also effective in BD, may be linked to other pathways.

Identification of genes regulated by muscarinic acetylcholine receptors: application of an improved and statistically comprehensive mRNA differential display technique

In order to identify genes that are regulated by muscarinic acetylcholine receptors, we developed an mRNA differential display technique (DD) approach. By increasing redundancy and by evaluating optimised reagents and conditions for reverse transcription of total RNA, PCR and separation of PCR products, we generated a DD protocol that yields highly consistent results. A set of 64 distinct random primers was specifically designed in order to approach a statistically comprehensive analysis of all mRNA species in a defined cell population. This modified DD protocol was applied to total RNA of HEK293 cells stably expressing muscarinic m1 acetylcholine receptors and cells stimulated with the receptor agonist carbachol were compared to identical but non-stimulated cells. In 81 of 192 possible PCR experiments, 38 differential bands were identified. Sequence analysis followed by northern blot analyses confirmed differentially expressed genes in 19 of 23 bands analysed. These represented 10 distinct immediate-early genes that were up-regulated by m1AChR activation: Egr-1, Egr-2, Egr-3, NGFi-B, ETR101, c- jun, jun -D, Gos-3 and hcyr61, as well as the unknown gene Gig-2. These data show that this improved DD protocol can be readily applied to reliably identify differentially expressed genes.

Effects of ethanol on calcium homeostasis in the nervous system: implications for astrocytes

Ethanol is a major health concern, with neurotoxicity occurring after both in utero exposure and adult alcohol abuse. Despite a large amount of research, the mechanism(s) underlying the neurotoxicity of ethanol remain unknown. One of the cellular aspects that has been investigated in relationship to the neuroteratogenicity and neurotoxicity of ethanol is the maintenance of calcium homeostasis. Studies in neuronal cells and other cells have shown that ethanol can alter intracellular calcium levels and affect voltage and receptor-operated calcium channels, as well as G protein-mediated calcium responses. Despite increasing evidence of the important roles of glial cells in the nervous systems, few studies exist on the potential effects of ethanol on calcium homeostasis in these cells. This brief review discusses a number of reported effects of alcohol on calcium responses that may be relevant to astrocytes' functions.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Cultured astrocytes express functional receptors for galanin

The neuropeptides galanin and calcitonin gene-related peptide (CGRP) are strongly up-regulated in motoneurons following axotomy. Earlier reports have suggested that peptides might be released from injured neurons to recruit surrounding glia. In this study, the effects of galanin and CGRP on cultured rat astrocytes were investigated using the expression of immediate early genes as a model for receptor-mediated transcriptional activation. Galanin was found to induce c-fos, junB, and Tis11 mRNA in cultured astrocytes, providing evidence for the presence of functional galanin receptors on neuroglial cells. In contrast, CGRP only led to the induction of c-fos and junB mRNA. Cholecystokinin (CCK-8) and substance P, which are also up-regulated in select motoneuron populations following axotomy, fail to induce immediate early genes in astrocytes, indicating specificity of neuropeptides in their ability to stimulate glial cells. The differential induction of immediate early gene expression by galanin and CGRP in astrocytes points to differences in intracellular signal transduction mechanisms. Whereas CGRP was found to stimulate the accumulation of cyclic AMP by 10- to 20-fold, galanin had no effect on basal cyclic AMP content. The effect of CGRP on cyclic AMP accumulation was completely reversed by the CGRP receptor antagonist, CGRP(8-37). These results suggest roles for galanin and CGRP in the transcriptional activation of astrocytes.

Differential effects of mood stabilizers on Fos/Jun proteins and AP-1 DNA binding activity in human neuroblastoma SH-SY5Y cells

Lithium and sodium valproate (VPA) are effective in the treatment of bipolar disorder (BD) and may function through the regulation of signal transduction pathways and transcription factors such as c-fos and c-Jun, which in turn results to changes in gene expression. The long-term efficacy of lithium and VPA in BD suggests that the regulation of gene expression may be an important target for these drugs. Preliminary evidence suggests that c-fos levels and AP-1 binding may be regulated by lithium and VPA, but the results are inconclusive. In the present study, we report differential effects of the two most commonly prescribed mood stabilizers used to treat BD on Fos/Jun protein levels and their AP-1 binding activity in human neuroblastoma SH-SY5Y cells. At therapeutically relevant concentrations, both drugs acutely (<24 h) induced c-Fos immunoreactivity and AP-1 binding. In contrast to lithium, chronic (1 week) treatment with VPA led to continued induction of c-Fos, in addition to induction of c-Jun immunoreactivity and a 33-35 kDa band previously identified as chronic FRA. AP-1 DNA binding activity was also increased after 1 week VPA treatment. These findings suggest that both these mood stabilizers may have an effect on neuronal gene expression of target genes containing the AP-1 consensus sequence in their promoter regions after acute treatment. The present results confirm and extend previous findings on the regulation of c-fos expression and AP-1 binding after administration of mood stabilizers, and further elucidate the mechanisms through which VPA increases AP-1 DNA binding.

Stimulation of P2Y-purinoceptors on astrocytes results in immediate early gene expression and potentiation of neuropeptide action

The action of adenosine-5'-O-(2-thiodiphosphate), a non-hydrolysable purine analogue and potent P2Y1-purinoceptor agonist, was studied on immediate early gene expression in rat astrocyte cultures. A rapid and transient increase in c-fos, junB, c-jun and Tis11 messenger RNA was observed in cultured astrocytes after treatment with adenosine-5'-O-(2-thiodiphosphate). Maximal induction of immediate early gene expression was obtained within 30 min of stimulation and c-fos was the most sensitive indicator of P2Y-purinoceptor activation. Calcitonin gene-related peptide has also been shown to be a potent inducer of c-fos messenger RNA in cultured astroglial cells. The combined stimulation of astrocytes with calcitonin gene-related peptide and adenosine-5'-O-(2-thiodiphosphate) resulted in the potentiated expression of c-fos messenger RNA. The superinduction of immediate early gene expression by calcitonin gene-related peptide and extracellular ATP in cultured astrocytes might result from intracellular signal transduction cross-talk, since adenosine-5'-O-(2-thiodiphosphate) was found to increase calcitonin gene-related peptide-induced cyclic AMP accumulation by 35%. Phorbol 12-myristate 13-acetate also increased calcitonin gene-related peptide-evoked cyclic AMP accumulation and led to the induction of immediate early gene expression, suggesting that protein kinase C might be at least in part involved in purinergic cross-talk. Our results demonstrate synergistic roles for extracellular ATP and calcitonin gene-related peptide in the transcriptional activation of astroglial cells.

AP-1 and NF-kappaB stimulated by carbachol in human neuroblastoma SH-SY5Y cells are differentially sensitive to inhibition by lithium

In order to identify potential actions of lithium, the primary therapeutic agent for bipolar affective disorder, on processes regulating gene expression, its effects on two transcription factors, AP-1 and NF-kappaB, were measured in human neuroblastoma SH-SY5Y cells. The cholinergic agonist carbachol concentration-dependently stimulated AP-1 (EC50 = 2 microM) and NF-kappaB (EC50 = 14 microM). Pretreatment for 24 h with a therapeutically relevant concentration of lithium (1 mM) substantially inhibited (30-35%) carbachol-stimulation AP-1 but not NF-kappaB. Inhibition of carbachol-induced AP-1 was directly related to the concentration of lithium (1-20 mM). Besides being differentially sensitive to inhibition by lithium, activation of AP-1 and NF-kappaB demonstrated different carbachol EC50 concentrations, and carbachol-induced activation of AP-1, but not NF-kappaB, was inhibited by treating cells with Ni2+, which blocks receptor-mediated calcium influx. These findings demonstrate that one mechanism by which lithium can influence the expression of specific genes is through the selective modulation of signaling processes which emanate from cholinergic receptor stimulation.

Interferon-gamma induces the expression of immediate early genes c-fos and c-jun in astrocytes

The expression of the proto-oncogenes, c-fos and c-jun, in cultured mouse astrocytes and its induction by the potent astrocyte activator interferon-gamma (IFN-gamma), were examined by Northern blot and flow cytometry. Both proto-oncogenes were induced in a dose-dependent manner, peaking around 100 U/ml of IFN-gamma. The kinetics of expression is very transient for c-fos, reaching a maximum at 30 min and decreasing rapidly thereafter. The c-jun remained high throughout the stages analysed. Cycloheximide superinduced c-fos and c-jun induction by IFN-gamma, thus indicating that both act as immediate early genes. The products of c-fos and c-jun, proteins FOS and JUN, that act in conjunction forming the regulatory factor AP-1, were detected 1 hr after stimulation in virtually all cells, using flow cytometry. The induction in astrocytes of both proto-oncogenes could be the first stage of immunological activation of these central nervous system cells by immune interferon.

Effects of ethanol on muscarinic receptor-stimulated c-fos expression in human neuroblastoma cells

The effect of ethanol exposure on muscarinic receptor-stimulated expression of c-fos was investigated in SH-SY5Y cells. Four days of ethanol exposure enhanced carbachol-stimulated c-fos mRNA expression, analyzed with Northern blot, and Fos/AP-1 binding activity, measured with gel mobility super shift assay. Pre-incubation with muscarinic antagonists or the protein kinase C inhibitor GF109203X demonstrated that, in both control and ethanol-treated cells, carbachol-induced c-fos expression was mediated via muscarinic M1 receptors and to a large extent through protein kinase C. However, phorbol ester-induced c-fos expression was unaffected in ethanol-treated cells. Acute exposure to ethanol caused a suppression of both carbachol- and phorbol ester-stimulated c-fos expression. These results demonstrate that muscarinic receptor-stimulated gene expression is sensitive to both acute and long-term ethanol exposure.

Egr-1 activates basic fibroblast growth factor transcription. Mechanistic implications for astrocyte proliferation

The mechanisms controlling the proliferation of astrocytes are of great interest but are not well defined. We have previously shown that the endogenous neuropeptides, endothelin-3 (ET-3), and atrial natriuretic peptide (ANP), modulate the proliferation of astrocytes through positively and negatively regulating the transcription of the immediate-early gene egr-1 which transactivates basic fibroblast growth factor (bFGF) by unknown mechanisms. In these studies, we determined the involvement of MAP kinase (Erk) activation by ET-3 in the transcription of egr-1, and the molecular determinants by which Egr-1 transactivates bFGF. Transfection of astrocytes with a mitogen-activated protein (MAP) kinase (MAPK) expression vector increased the transcription of a cotransfected egr-chloramphenicol acetyltransferase (CAT) construct 3-fold. This induction was totally abolished by a dominant negative MAPK mutant. A 3-fold induction of egr-CAT expression by ET-3 was significantly reduced by treatment with ANP, or a cotransfected dominant negative MAPK plasmid. Using mobility shift assays, we showed that ET-3 induced the expression of Egr-1 protein which bound specifically to several early growth-related protein (Egr-1) binding sites on the bFGF promoter, and that this effect was significantly reversed by treatment with ANP. We also found that the Sp1 transcriptional factor was bound at these same sites, but was not stimulated by ET-3. Deletion experiments indicated that only the site at -160 bp of the bFGF promoter was significant for bFGF transactivation by Egr-1. We conclude that the astrocyte mitogen, ET-3, stimulates egr-1 transcription through a MAP kinase (Erk) related mechanism, and that Egr-1 transactivates bFGF through a specific noncanonical, Egr-1 site on the promoter. ANP inhibits each of these steps, providing a pathway for its anti-proliferative action.

A new fluorescent histological marker for ischemic neurons, EA 50: correlated with Fos and Jun/AP-1 immunoreactivity

Cerebral ischemia/hypoxia induces ischemic neuronal changes characterized by nuclear pyknosis, cytoplasmic shrinkage, and basophilia. The ischemic neurons were shown to exhibit strong and persistent c-fos proto-oncogene. The ischemic neuronal changes and c-fos expression are thought to be the consequence of release of excessive glutamate by the ischemic neurons. In the present study, we investigated with immunohisto-chemistry the subcellular distribution of Fos and Jun/AP-1, the protein products of c-fos and c-jun proto-oncogenes, and compared them with histological changes shown by hematoxylin-eosin and by EA 50 stains. The latter is a stain mixture used traditionally in the Papani-colaou procedure and has a specific affinity for ischemic neurons. The active ingredient is eosin Y, a tetrabrominated derivative of fluorescein. With EA 50, the ischemic neurons stain red and emit a yellow fluorescence, while the non-ischemic neurons are green and non-fluorescent. The subcellular site of eosin Y binding corresponds with Fos and Jun/AP-1; all are concentrated in the nuclei and spread into the perikaryon, dendrites, and axons. The eosin Y-binding appears in neurons that have shown advanced ischemic changes. The dye is thus a good histological marker for damaged neurons, but requires freshly fixed tissues and paraffin sections of less than 4 microns thick. Preincubation of tissue sections in antibodies against Fos and Jun abolishes the eosin Y binding, suggesting that the dye may interact with Fos/Jun/AP-1 protein or other protein products in the ischemic neurons.

Beta-adrenergic receptors regulate astrogliosis and cell proliferation in the central nervous system in vivo

Astrocytes express several cell surface receptors including the beta 2 -adrenergic receptor. To explore whether beta-adrenergic receptors (beta-ARs) directly regulate astrogliosis and glial scar formation, we evaluated the effects of beta-AR activation and blockade on astrocyte hypertrophy and cell proliferation in rabbit optic nerves in vivo. Artificial cerebrospinal fluid (CSF), isoproterenol (ISO; a beta-agonist), or propranolol (PROP; a beta-antagonist) were infused via osmotic minipumps into non-injured and crushed optic nerves for 14 days. Changes in nerve cell numbers and astroglial hypertrophy were monitored by ethidium bromide nuclear staining and glial fibrillary acidic protein (GFAP) immunohistochemistry, respectively. In non-injured nerves infused with CSF or PROP, there were no alterations in GFAP-immunoreactivity or cell numbers compared to normal optic nerves; however, in non-injured nerves infused with ISO, there was a significant increase in both GFAP-immunoreactivity and cell number. In crushed optic nerves, there was a significant increase in both GFAP-immunoreactivity and cell number compared to normal nerves, and this increase was not altered by infusion of either CSF or ISO. In contrast, PROP infusion significantly reduced the crush-induced increase in GFAP-immunofluorescence and cell number. These findings suggest that a) beta-AR activation, in the absence of injury, can promote astroglial hypertrophy and cell proliferation; b) after injury, beta-AR activation drives injury-induced astrogliosis and cell proliferation; c) astrocyte beta-ARs are maximally stimulated after neuronal injury; and d) neuronal regeneration may be influenced, both positively and negatively, through the pharmacological manipulation of glial receptors.

Comparative distribution of NURR1 and NUR77 nuclear receptors in the mouse central nervous system

NURR1 and NUR77 are members of the nuclear receptor superfamily of transcription factors. Both proteins can interact with common enhancer elements to regulate target gene expression. In order to establish whether both transcription factors are likely to regulate overlapping genes, we have used an in situ hybridization approach to relate the constitutive expression pattern of these mRNAs with functionally defined regions of the adult mouse brain. By Western analysis, NURR1 mRNA expressed by brain cells appeared to be translated. Here we show that both transcripts display a differential but partially overlapping pattern of expression within the central nervous system (CNS). The expression of NURR1 is more restricted than NUR77 and is localized predominantly in sensory neuronal structures associated with the limbic system and in the cerebellum. In contrast, the expression pattern of NUR77 is more widespread. Positively staining cells for NUR77 appear to overlap with NURR1-containing cells in the limbic system and cerebellum, suggesting overlapping roles for these proteins in mediating behavioral and cognitive function as well as equilibrium maintenance. However, the differential expression of NUR77 in motor areas of the cortex and basal ganglia suggest a selective role for this transcription factor in regulation of motor function at the constitutive level. Our data indicates that these nuclear receptors are likely to have both shared and independent gene regulatory roles in neuronal cells.

Calcitonin gene-related peptide and ATP induce immediate early gene expression in cultured rat microglial cells

Factors affecting gene expression in microglial cells were investigated using the induction of immediate early genes in cultured microglia as a model. In particular, the actions of calcitonin gene-related peptide (CGRP) and ATP, both of which have been proposed as signalling molecules in the activation of glial cells, were evaluated using Northern blotting and in situ hybridization methods. In the presence of CGRP, c-fos and junB mRNAs accumulated in microglial cultures, whereas no significant change in c-jun and TIS11 mRNAs occurred. A similar pattern of immediate early gene activation was obtained when adenylate cyclase was stimulated with forskolin. CGRP also stimulated cyclic AMP accumulation with a half-maximal effect in the range 2-5 nM, suggesting a possible role for cyclic AMP as a mediator of the effects of CGRP on gene expression. In contrast to the selective induction of c-fos and junB by CGRP and forskolin, ATP led to the accumulation of all four immediate early genes studied, i.e., c-fos, junB, c-jun, and TIS11. Similar results were obtained when protein kinase C was stimulated with phorbol ester indicating that the induction of immediate early gene expression by ATP and CGRP involves different intracellular mechanisms. The action of ATP was mimicked by ADP and the poorly hydrolyzable analogues, ADP beta S and 2-methylthio ATP, but not by beta, gamma-methylene ATP, AMP, or adenosine, indicating that the receptor mediating the actions of ATP on microglial gene expression is probably of the P2Y-purinoreceptor type. The results suggest roles for CGRP and ATP as transcriptional activators in microglial cells.

Expression and regulation of kainate and AMPA receptors in uncommitted and committed neural progenitors

Here we review experimental evidence of non-NMDA glutamate receptor expression in the embryonic central nervous system. AMPA- and kainate-preferring glutamate receptor subunit mRNA transcripts are detected in embryonic neurons, glia and neural progenitors. Functional assays demonstrate that in some cell subpopulations ionotropic glutamate receptors are expressed by progenitors before synapse formation and terminal differentiation, and may be present before lineage determination is specified. The activation of these receptors triggers induction of immediate early gene transcription in progenitor cells. The cloning and transcriptional analysis of upstream regulatory regions of glutamate receptor genes governing their temporal and tissue-specific expression are also discussed.

Growth conditions differentially affect the constitutive expression of primary response genes in cultured cerebellar granule cells

Cultured cerebellar granule cells underwent apoptotic degeneration when grown in medium containing 10 instead of 25 mM K+. Knowing that apoptosis is associated with changes in the expression of primary response genes, we have measured c-fos, zif/268, and c-jun mRNA levels during maturation of cultured granule cells grown in 10 or 25 mM K+. The constitutive expression of c-fos and zif/268 was differentially regulated by extracellular K+ concentration at 5 days of maturation in vitro (DIV), when cells grown under suboptimal conditions (i.e. in 10 mM K+) are committed to degenerate. At this stage, c-fos mRNA levels were detectable only in cultures grown in 25 mM K+, whereas zif/268 mRNA levels were dramatically elevated in cultures grown in 10 mM K+. This provides one of the few conditions in which c-fos and zif/268 are differentially regulated in nerve cells. Substantial changes in c-jun, or beta-actin mRNA levels were detectable only at 7 DIV, when the percentage of apoptotic cells had already reached a plateau in cultures grown in 10 mM K+. We speculate that changes in the expression of zif/268 are important in the gene program associated with the induction of apoptosis by trophic deprivation in cultured neurons.

Mechanisms of muscarinic receptor-stimulated expression of c-fos in SH-SY5Y cells

In this study, the signal cascade transducing carbachol stimulation into c-fos expression in SH-SY5Y neuroblastoma cells was investigated. 1,2-Diacylglycerol formation and c-fos expression were mediated via stimulation of muscarinic M1 receptors and the first 5 min of receptor stimulation were critical for these events. Application of 1,2-dioctanoylglycerol induced c-fos expression and this, as well as carbachol-stimulated c-fos expression, was inhibited by protein kinase C inhibitors. Increasing the intracellular Ca2+ concentration had only small effects on c-fos expression. There was a dependency on extracellular Ca2+ for maximal c-fos expression and 1,2-diacylglycerol formation. The carbachol-stimulated c-fos expression was potentiated by application of the protein phosphatase inhibitor okadaic acid. These results demonstrate the importance of 1,2-diacylglycerol formation for muscarinic receptor-stimulated, protein kinase C-mediated c-fos expression in the SH-SY5Y cells and that this cascade is counteracted by an okadaic acid-sensitive protein phosphatase.

Plasticity of astrocytes

It is becoming apparent that astrocytes carry out a large number of different functions in brain and are able to modify their characteristics throughout life, that is they exhibit a high degree of plasticity in their phenotype. For example, the morphology of astrocytes changes markedly during neuronal migration, maturation, and degeneration. It is conceivable that these cells must constantly adjust their abilities to meet changes in brain environment. Several examples of astrocytic plasticity are presented in this review. First, the ability of astrocytes to recognize neuronal signals can change qualitatively as well as quantitatively; evidence suggests that the expression of glial receptors may be developmentally regulated by both intrinsic and extrinsic signals. Second, the expression of adrenergic receptors by astrocytes in adult brain can change in response to neuronal degeneration. The up-regulation of beta-adrenergic receptors in this case suggests that these receptors play a role in function of reactive astrocytes. Finally, glial morphology can be reciprocally regulated by neurotransmitters such as norepinephrine and glutamate. This reciprocal regulation may be significant since both beta-adrenergic receptors and glutamate transporters are found predominantly in astrocytes in the brain. The change in glial morphology may also affect neuronal activity by changing the volume of the extracellular space.

Neurotransmitter-mediated signaling between axons and glial cells

Neurotransmitter-mediated signaling is not restricted to the synaptic regions of the nervous system but also takes places along fiber tracts lacking vesicular means of releasing neuroactive substances. The first demonstration for dynamic signaling of this type came in the early 1970s from studies by Villegas and co-workers in squid axons and their satellite Schwann cells. In this invertebrate system, glutamate has been identified as the mediator of this signaling in being first released from the active axons thus setting off a series of cascades, leading to a cholinergic activation of the Schwann cell membrane. Recent evidence suggests that receptor-mediated signaling also exists between glial cells and axons in vertebrates. In the frog optic nerve, axonal activity facilitated the activity of glial ion channels. In the neonatal rat optic nerve, electrical activity of axons triggered oscillations in intracellular calcium in a subset of glial cells. These observations have been postulated to reflect receptor-mediated signaling, including a mechanism in which glutamate is released from axons via the reversal of a transporter and induces intracellular calcium spiking in glial cells via metabotropic glutamate receptors. The efficacy of "axon-to-glia" transmission may, like that in "neuron-to-neuron" transmission, be modulated by co-release of multiple neuroactive substances. One possibility is that adenosine, which is known to be released from fiber tracts, can modulate glutamate signaling in white matter by modulating the periaxonal glutamate concentration through an effect on the glial glutamate uptake system.

Astrocyte growth is regulated by neuropeptides through Tis 8 and basic fibroblast growth factor

The important intracellular mechanisms of astrocyte growth are not well defined. Using an inhibitor of astrocyte proliferation, atrial natriuretic peptide (ANP), and the glial mitogen endothelin (ET-3), we sought a common pathway for growth regulation in these neural cells. In cultured fetal rat diencephalic astrocytes, ANP selectively and rapidly inhibited the Tis 8 immediate early gene and protein. After 4 h, ANP selectively inhibited the basic fibroblast growth factor (bFGF) gene and protein. ET-3 significantly stimulated both Tis 8 and bFGF mRNAs and protein, but also stimulated several other immediate early and growth factor/receptor genes. An antisense oligonucleotide to Tis 8 strongly prevented ET-stimulated thymidine incorporation, while the inhibitory action of ANP was enhanced. The Tis 8 antisense oligonucleotide also significantly reversed ET-stimulated bFGF transcription and enhanced the bFGF inhibition caused by ANP. In addition, an antisense oligonucleotide to bFGF significantly reversed the ET-stimulated thymidine incorporation and enhanced the ANP inhibition of DNA synthesis. The sequential modulation of Tis 8, followed by bFGF, provides a novel mechanism for both positive and negative regulation of astrocyte growth by endogenous neuropeptides.

Transcription factor expression is induced by axonal stimulation and glutamate in the glia of the developing optic nerve

Recent experiments have demonstrated that stimulation of the developing optic nerve affects several glial cell characteristics, such as ionic fluxes and cell proliferation. This investigation asked if transcription factor expression may be another stimulation-dependent process in the glia of the developing optic nerve. In unstimulated optic nerves, an antibody to c-fos-related antigens demonstrated positive cell body staining at postnatal days (P) 2, 7, 14, and 60. This nuclear staining was most prominent at early postnatal ages, although young adult (P60) optic nerves showed occasional positive cells. To demonstrate the inducibility of transcription factor antigens, optic nerves from P7 animals received intermittent 15-20 Hz electrical stimulation for 5-15 min. Two hours after this stimulation, an increased number of immunoreactive cells for c-fos-related antigens, c-jun, and NGFI-A was demonstrated. Additionally, optic nerves were exposed for 5-30 min to a solution of 300 microM glutamate, latter maintained in a glutamate-free solution for 2 h, and then quickly frozen. Glutamate-treated nerves showed an increased expression of c-fos-related antigens compared to control nerves. No c-fos increase was seen in the absence of calcium. Expression of c-fos or NGFI-A occurred in cells that were S-100 positive, and most likely represented type 1 astrocytes. These studies indicate that developing (P7) optic nerves show a baseline expression of c-fos-related antigens, c-jun and NGFI-A. Stimulation through electrical nerve stimulation or glutamate results in an increased expression of these transcription factors.

Long-term action of lithium: a role for transcriptional and posttranscriptional factors regulated by protein kinase C

Lithium, a simple monovalent cation, represents one of psychiatry's most important treatments and is the most effective treatment for reducing both the frequency and severity of recurrent affective episodes. Despite extensive research, the underlying biologic basis for the therapeutic efficacy this drug remains unknown, and in recent years, research has focused on signal transduction pathways to explain lithium's efficacy in treating both poles of manic-depressive illness. Critical to attributions of therapeutic relevance to any observed biochemical effect, however, is the observation that the characteristic prophylactic action of lithium in stabilizing the profound mood cycling of bipolar disorder requires a lag period for onset and is not immediately reversed upon discontinuation of treatment. Biochemical changes requiring such prolonged administration of a drug suggest alterations at the genomic level but, until recently, little has been known about the transcriptional and posttranscriptional factors regulated by chronic drug treatment, although long-term changes in neuronal synaptic function are known to be dependent upon the selective regulation of gene expression. In this paper, we will present evidence to show that chronic lithium exerts significant transcriptional and posttranscriptional effects, and that these actions of lithium may be mediated via protein kinase C (PKC)-induced alterations in nuclear transcription regulatory factors responsible for modulating the expression of proteins involved in long-term neural plasticity and cellular response. Such target sites for chronic lithium may help unravel the processes by which a simple monovalent cation can produce a long-term stabilization of mood in individuals vulnerable to bipolar illness.

Regulation of astroglial responsiveness to neuroligands in primary culture

Previous studies from this laboratory indicate that type-1 astroglia in primary culture are pharmacologically heterogeneous. Two competing hypotheses were proposed to explain the development of glial heterogeneity. First, that the heterogeneity may reflect stable subclasses of astroglia that express a set of receptor-signalling systems. Second, that astroglia can undergo qualitative changes in their expression of receptor-signalling systems with time in vitro. To distinguish between these two hypotheses, experiments were designed to examine neuroligand-evoked calcium responses within clones of type-1 astroglia. If stable and distinct subsets of astroglia were present, a clone derived from a single cell would exhibit uniform responses to a given set of neuroligands. Alternatively, if the pharmacological properties of astroglia underwent qualitative changes, astroglial clones should contain pharmacologically distinct cells. A video-based imaging system and the Ca2+ indicator dye Fura-2 were used to monitor receptor-mediated increases in Cai2+ upon receptor activation. Interestingly, only a fraction of the cells within a given clone responded to carbachol or histamine with an increase in Cai2+, whereas treatment with a P2Y purinergic receptor agonist generally increased Cai2+ in 100% of the cells within the clone. To examine the stability of the receptor signalling over time, individual astroglia within a number of clones were tested on different days for their ability to respond to neuroligands. The results of these experiments indicated that individual astroglial cells tended to lose their responsiveness to certain ligands such as carbachol and histamine as they developed responsiveness to others such as norepinephrine. Our data indicate that during development neurotransmitter receptors on astroglial cells are regulated by both internal and external mechanisms. Glial proliferation produces a variety of pharmacologically distinct astroglial cells. Exposure to neurotransmitters can qualitatively turn off some, but not all, astroglial receptor systems.

Rapid response of striatal muscarinic M1-receptor mRNA to muscarinic cholinergic agents in rat brain

The effects of a single administration of muscarinic cholinergic agents on the level of muscarinic M1-receptor messenger RNA (M1-R mRNA) in the rat striatum were studied. Carbachol increased the M1-R mRNA expression rapidly and transiently, while trihexyphenidyl decreased it. These results suggest that muscarinic cholinergic agents participate in the positive regulation of muscarinic receptor mRNA in the early stage after treatment, contrary to the negative regulation in the chronic stage.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.

Differential expression of immediate-early proteins in non-nerve cells after focal brain injury

We investigated the expression of the immediate-early proteins (IEPs, Fos, Fos B, Jun, Jun B, Jun D, Krox 20 and Krox 24) in non-nerve cells in rat brain after mechanical brain injury. Injury produced by infusion of 5 microliters of saline into the hippocampus produced a time-dependent expression of Fos, Jun and Krox 24, but not Fos B, Krox 20 or in non-nerve cells around the wound margin, in cells lining the lateral and third ventricles and in cells in the pial surfaces of the brain. Jun B and D were weakly induced in non-nerve cells 1-4 hr after brain injury. This differential expression of IEPs in non-nerve cells contrasted with neurons which expressed all IEPs measured. Thus, brain injury is associated with a differential expression of IEPs in non-nerve cells around the wound. The functional implications of this IEP expression after brain injury are presently unclear, but may be related to cellular proliferation after brain injury.