NMDA receptor-dependent CREB activation in survival of cerebellar granule cells during in vivo and in vitro development

NMDA receptor-mediated Ca2+ signaling: Impact on cell cycle regulation and the development of neurodegenerative diseases and cancer

NMDA receptors are Ca2+-permeable ligand-gated ion channels that mediate fast excitatory transmission in the central nervous system. NMDA receptors regulate the proliferation and differentiation of neural progenitor cells and also play critical roles in neural plasticity, memory, and learning. In addition to their physiological role, NMDA receptors are also involved in glutamate-mediated excitotoxicity, which results from excessive glutamate stimulation, leading to Ca2+ overload, and ultimately to neuronal death. Thus, NMDA receptor-mediated excitotoxicity has been linked to several neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's, dementia, and stroke. Interestingly, in addition to its effects on cell death, aberrant expression or activation of NMDA receptors is also involved in pathological cellular proliferation, and is implicated in the invasion and proliferation of various types of cancer. These disorders are thought to be related to the contribution of NMDA receptors to cell proliferation and cell death through cell cycle modulation. This review aims to discuss the evidence implicating NMDA receptor activity in cell cycle regulation and the link between aberrant NMDA receptor activity and the development of neurodegenerative diseases and cancer due to cell cycle dysregulation. The information presented here will provide insights into the signaling pathways and the contribution of NMDA receptors to these diseases, and suggests that NMDA receptors are promising targets for the prevention and treatment of these diseases, which are leading causes of death and disability worldwide.

Gut neurotoxin p-cresol induces differential expression of GLUN2B and GLUN2A subunits of the NMDA receptor in the hippocampus and nucleus accumbens in healthy and audiogenic seizure-prone rats

Mislocalization and abnormal expression of N-methyl-D-aspartate glutamate receptor (NMDAR) subunits is observed in several brain disorders and pathological conditions. Recently, we have shown that intraperitoneal injection of the gut neurotoxin p-cresol induces autism-like behavior and accelerates seizure reactions in healthy and epilepsy-prone rats, respectively. In this study, we evaluated the expression of GLUN2B and GLUN2A NMDAR subunits, and assessed the activity of cAMP-response element binding protein (CREB) and Rac1 in the hippocampi and nucleus accumbens of healthy and epilepsy-prone rats following p-cresol administration. We have found that subchronic intraperitoneal injection of p-cresol induced differential expression of GLUN2B and GLUN2A between the two brain regions, and altered the GLUN2B/GLUN2A ratio, in rats in both groups. Moreover, p-cresol impaired CREB phosphorylation in both brain structures and stimulated Rac activity in the hippocampus. These data indicate that p-cresol differently modulates the expression of NMDAR subunits in the nucleus accumbens and hippocampi of healthy and epilepsy-prone rats. We propose that these differences are due to the specificity of interactions between dopaminergic and glutamatergic pathways in these structures.

PPARα serves as a new receptor of aspirin for neuroprotection

Acetyl salicylic acid, commonly known as aspirin, has been being widely used as an anti-inflammatory drug for almost 100 years. However, there was no receptor known for this popular drug. Recently, we have established that peroxisome proliferator-activated receptor alpha (PPARα) acts as a novel receptor of aspirin. Activation of PPARα by aspirin stimulated a series of downstream signaling pathways that could potentially ameliorate different Alzheimer's disease (AD)-related pathologies. In this mini-review, we have discussed how aspirin-PPARα interaction plays a pivotal role in the amelioration of AD pathology via the stimulation of neurotrophic factors, upregulation of plasticity-associated genes, and removal of plaque burden in hippocampal neurons.

rTMS Ameliorates Prenatal Stress-Induced Cognitive Deficits in Male-Offspring Rats Associated With BDNF/TrkB Signaling Pathway

BACKGROUND

Growing evidences suggest that brain-derived neurotrophic factor/tropomyosin receptor kinase B (BDNF/TrkB) plays a key role in the regulation of hippocampal synaptic plasticity in a prenatal stress (PNS) rat model. Repetitive transcranial magnetic stimulation (rTMS) is currently being acknowledged to affect attention and memory in both preclinical and clinical studies, although the mechanism is still unclear.

OBJECTIVE

The current study aimed to explore whether a whole brain rTMS (5 Hz, 14 days) could ameliorate cognitive dysfunction-induced PNS in male offspring, and examine if the positive effect of rTMS was associated with the BDNF/TrkB signaling in the hippocampus.

METHODS

The rats were randomly divided into 5 groups: CON, PNS, PNS + rTMS, PNS + rTMS + DMSO (dimethyl sulfoxide), and PNS + rTMS + K252a. Spatial cognition was evaluated by using Morris water maze test. Following behavioral assessment, both paired-pulse facilitation and long-term potentiation were recorded from Schaffer collaterals to CA1 region in the hippocampus. Synaptic, apoptotic, and BDNF/TrkB signaling proteins were measured by Western blot.

RESULTS

PNS-exposed offspring exhibited cognitive deficits, long-term potentiation inhibition in the hippocampus, the decrease of synaptic and BDNF/TrkB signaling proteins expression, apoptosis, and reduced number of cells in the CA1 region. Five-hertz rTMS significantly alleviated the PNS-induced abnormalities. However, the effect of rTMS was antagonized by intracerebroventricular infusion of K252a (a TrkB inhibitor).

CONCLUSIONS

The findings suggest that 5-Hz rTMS significantly improves the impairment of spatial cognition and hippocampal synaptic plasticity, which is possibly associated with the activation of BDNF/TrkB signaling.

Cyclic nucleotide phosphodiesterases: potential therapeutic targets for alcohol use disorder

Alcohol use disorder (AUD), which combines the criteria of both alcohol abuse and dependence, contributes as an important causal factor to multiple health and social problems. Given the limitation of current treatments, novel medications for AUD are needed to better control alcohol consumption and maintain abstinence. It has been well established that the intracellular signal transduction mediated by the second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) crucially underlies the genetic predisposition, rewarding properties, relapsing features, and systemic toxicity of compulsive alcohol consumption. On this basis, the upstream modulators phosphodiesterases (PDEs), which critically control intracellular levels of cyclic nucleotides by catalyzing their degradation, are proposed to play a role in modulating alcohol abuse and dependent process. Here, we highlight existing evidence that correlates cAMP and cGMP signal cascades with the regulation of alcohol-drinking behavior and discuss the possibility that PDEs may become a novel class of therapeutic targets for AUD.

Activation of the CREB/c-Fos Pathway during Long-Term Synaptic Plasticity in the Cerebellum Granular Layer

The induction of long-term potentiation and depression (LTP and LTD) is thought to trigger gene expression and protein synthesis, leading to consolidation of synaptic and neuronal changes. However, while LTP and LTD have been proposed to play important roles for sensori-motor learning in the cerebellum granular layer, their association with these mechanisms remained unclear. Here, we have investigated phosphorylation of the cAMP-responsive element binding protein (CREB) and activation of the immediate early gene c-Fos pathway following the induction of synaptic plasticity by theta-burst stimulation (TBS) in acute cerebellar slices. LTP and LTD were localized using voltage-sensitive dye imaging (VSDi). At two time points following TBS (15 min and 120 min), corresponding to the early and late phases of plasticity, slices were fixed and processed to evaluate CREB phosphorylation (P-CREB) and c-FOS protein levels, as well as Creb and c-Fos mRNA expression. High levels of P-CREB and Creb/c-Fos were detected before those of c-FOS, as expected if CREB phosphorylation triggered gene expression followed by protein synthesis. No differences between control slices and slices stimulated with TBS were observed in the presence of an N-methyl-D-aspartate receptor (NMDAR) antagonist. Interestingly, activation of the CREB/c-Fos system showed a relevant degree of colocalization with long-term synaptic plasticity. These results show that NMDAR-dependent plasticity at the cerebellum input stage bears about transcriptional and post-transcriptional processes potentially contributing to cerebellar learning and memory consolidation.

Spontaneous Glutamatergic Synaptic Activity Regulates Constitutive COX-2 Expression in Neurons: OPPOSING ROLES FOR THE TRANSCRIPTION FACTORS CREB (cAMP RESPONSE ELEMENT BINDING) PROTEIN AND Sp1 (STIMULATORY PROTEIN-1)

Burgeoning evidence supports a role for cyclooxygenase metabolites in regulating membrane excitability in various forms of synaptic plasticity. Two cyclooxygenases, COX-1 and COX-2, catalyze the initial step in the metabolism of arachidonic acid to prostaglandins. COX-2 is generally considered inducible, but in glutamatergic neurons in some brain regions, including the cerebral cortex, it is constitutively expressed. However, the transcriptional mechanisms by which this occurs have not been elucidated. Here, we used quantitative PCR and also analyzed reporter gene expression in a mouse line carrying a construct consisting of a portion of the proximal promoter region of the mouse COX-2 gene upstream of luciferase cDNA to characterize COX-2 basal transcriptional regulation in cortical neurons. Extracts from the whole brain and from the cerebral cortex, hippocampus, and olfactory bulbs exhibited high luciferase activity. Moreover, constitutive COX-2 expression and luciferase activity were detected in cortical neurons, but not in cortical astrocytes, cultured from wild-type and transgenic mice, respectively. Constitutive COX-2 expression depended on spontaneous but not evoked excitatory synaptic activity and was shown to be N-methyl-d-aspartate receptor-dependent. Constitutive promoter activity was reduced in neurons transfected with a dominant-negative cAMP response element binding protein (CREB) and was eliminated by mutating the CRE-binding site on the COX-2 promoter. However, mutation of the stimulatory protein-1 (Sp1)-binding site resulted in an N-methyl-d-aspartate receptor-dependent enhancement of COX-2 promoter activity. Basal binding of the transcription factors CREB and Sp1 to the native neuronal COX-2 promoter was confirmed. In toto, our data suggest that spontaneous glutamatergic synaptic activity regulates constitutive neuronal COX-2 expression via Sp1 and CREB protein-dependent transcriptional mechanisms.

Defects in the CAPN1 Gene Result in Alterations in Cerebellar Development and Cerebellar Ataxia in Mice and Humans

A CAPN1 missense mutation in Parson Russell Terrier dogs is associated with spinocerebellar ataxia. We now report that homozygous or heterozygous CAPN1-null mutations in humans result in cerebellar ataxia and limb spasticity in four independent pedigrees. Calpain-1 knockout (KO) mice also exhibit a mild form of ataxia due to abnormal cerebellar development, including enhanced neuronal apoptosis, decreased number of cerebellar granule cells, and altered synaptic transmission. Enhanced apoptosis is due to absence of calpain-1-mediated cleavage of PH domain and leucine-rich repeat protein phosphatase 1 (PHLPP1), which results in inhibition of the Akt pro-survival pathway in developing granule cells. Injection of neonatal mice with the indirect Akt activator, bisperoxovanadium, or crossing calpain-1 KO mice with PHLPP1 KO mice prevented increased postnatal cerebellar granule cell apoptosis and restored granule cell density and motor coordination in adult mice. Thus, mutations in CAPN1 are an additional cause of ataxia in mammals, including humans.

Delineating the factors and cellular mechanisms involved in the survival of cerebellar granule neurons

Cerebellar granule neurons (CGNs) constitute the most abundant neuronal population in the mammalian brain. Their postnatal generation and the feasibility to induce their apoptotic death in vitro make them an excellent model to study the effect of several neurotransmitters and neurotrophins. Here, we first review which factors are involved in the generation and proliferation of CGNs in the external granule layer (EGL) and in the regulation of their differentiation and migration to internal granule layer (IGL). Special attention was given to the role of several neurotrophins and the NMDA subtype of glutamate receptor. Then, using the paradigm of potassium deprivation in cultured CGNs, we address several extracellular factors that promote the survival of CGNs, with particular emphasis on the cellular mechanisms. The role of specific protein kinases leading to the regulation of transcription factors and recent data involving the small G protein family is also discussed. Finally, the participation of some members of Bcl-2 family and the inhibition of mitochondria-related apoptotic pathway is also considered. Altogether, these studies evidence that CGNs are a key model to understand the development and the survival of neuronal populations.

Nucleotides in neuroregeneration and neuroprotection

Brain injury generates the release of a multitude of factors including extracellular nucleotides, which exhibit bi-functional properties and contribute to both detrimental actions in the acute phase and also protective and reparative actions in the later recovery phase to allow neuroregeneration. A promising strategy toward restoration of neuronal function is based on activation of endogenous adult neural stem/progenitor cells. The implication of purinergic signaling in stem cell biology, including regulation of proliferation, differentiation, and cell death has become evident in the last decade. In this regard, current strategies of acute transplantation of ependymal stem/progenitor cells after spinal cord injury restore altered expression of P2X4 and P2X7 receptors and improve functional locomotor recovery. The expression of both receptors is transcriptionally regulated by Sp1 factor, which plays a key role in the startup of the transcription machinery to induce regeneration-associated genes expression. Finally, general signaling pathways triggered by nucleotide receptors in neuronal populations converge on several intracellular kinases, such as PI3K/Akt, GSK3 and ERK1,2, as well as the Nrf-2/heme oxigenase-1 axis, which specifically link them to neuroprotection. In this regard, regulation of dual specificity protein phosphatases can become novel mechanism of actions for nucleotide receptors that associate them to cell homeostasis regulation. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Microglial displacement of inhibitory synapses provides neuroprotection in the adult brain

Microglia actively survey the brain microenvironment and play essential roles in sculpting synaptic connections during brain development. While microglial functions in the adult brain are less clear, activated microglia can closely appose neuronal cell bodies and displace axosomatic presynaptic terminals. Microglia-mediated stripping of presynaptic terminals is considered neuroprotective, but the cellular and molecular mechanisms are poorly defined. Using 3D electron microscopy, we demonstrate that activated microglia displace inhibitory presynaptic terminals from cortical neurons in adult mice. Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band. Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury. These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.

Microglia play essential roles in sculpting synaptic connections during brain development but their role in the adult brain is less clear. Here the authors show that activated microglia can prophylactically protect the adult rodent brain from injury by migrating to and displacing inhibitory synapses from cortical neurons.

Ras protein activation is a key event in activity-dependent survival of cerebellar granule neurons

Neuronal activity promotes the survival of cerebellar granule neurons (CGNs) during the postnatal development of cerebellum. CGNs that fail to receive excitatory inputs will die by apoptosis. This process could be mimicked in culture by exposing CGNs to either a physiological concentration of KCl (5 mm or K5) plus N-methyl-d-aspartate (NMDA) or to 25 mm KCl (K25). We have previously described that a 24-h exposure to NMDA (100 μm) or K25 at 2 days in vitro induced long term survival of CGNs in K5 conditions. Here we have studied the molecular mechanisms activated at 2 days in vitro in these conditions. First we showed that NMDA or K25 addition promoted a rapid stimulation of PI3K and a biphasic phosphorylation on Ser-473 of Akt, a PI3K substrate. Interestingly, we demonstrated that only the first wave of Akt phosphorylation is necessary for the NMDA- and K25-mediated survival. Additionally, we detected that both NMDA and K25 increased ERK activity with a similar time-course. Moreover, our results showed that NMDA-mediated activation of the small G-protein Ras is necessary for PI3K/Akt pathway activation, whereas Rap1 was involved in NMDA phosphorylation of ERK. On the other hand, Ras, but not Rap1, mediates K25 activation of PI3K/Akt and MEK/ERK pathways. Because neuroprotection by NMDA or K25 is mediated by Ras (and not by Rap1) activation, we propose that Ras stimulation is a crucial event in NMDA- and K25-mediated survival of CGNs through the activation of PI3K/Akt and MEK/ERK pathways.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

Involvement of decreased glutamate receptor subunit GluR2 expression in lead-induced neuronal cell death

Lead is known to induce neurotoxicity, particularly in young children, and GluR2, an AMPA-type glutamate receptor subunit, plays an important role in neuronal cell survival. Therefore, we hypothesized that altered GluR2 expression plays a role in lead-induced neuronal cell death. To test this idea, we investigated the effect of exposure to 5 and 20 µM lead for 1-9 days on the viability and GluR2 expression of primary-cultured rat cortical neurons. The number of trypan-blue stained cells was increased by exposure to 5 µM lead for 9 days or 20 µM lead for 7-9 days, and LDH release was increased after exposure to 20 µM lead for 9 days. GluR2 expression was reduced by exposure to 5-100 µM lead, but not 0.1-1 µM lead, for 9 days. Immunocytochemistry also confirmed that GluR2 expression was decreased in the presence of lead. Application of 50 ng/ml brain-derived neurotrophic factor (BDNF) led to a recovery of lead-induced neuronal cell death, accompanied with increased GluR2 expression. Our results suggest that long-term exposure to lead induces neuronal cell death, in association with a decrease of GluR2 expression.

PCSK9 regulates neuronal apoptosis by adjusting ApoER2 levels and signaling

The secreted protease proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to low-density lipid (LDL) receptor family members LDLR, very low density lipoprotein receptor (VLDLR) and apolipoprotein receptor 2 (ApoER2), and promotes their degradation in intracellular acidic compartments. In the liver, LDLR is a major controller of blood LDL levels, whereas VLDLR and ApoER2 in the brain mediate Reelin signaling, a critical pathway for proper development of the nervous system. Expression level of PCSK9 in the brain is highest in the cerebellum during perinatal development, but is also increased in the adult brain after ischemia. The mechanism of PCSK9 function and its involvement in neuronal apoptosis is poorly understood. We show here that RNAi-mediated knockdown of PCSK9 significantly reduced the death of potassium-deprived cerebellar granule neurons (CGN), as shown by reduced levels of nuclear phosphorylated c-Jun and activated caspase-3, as well as condensed apoptotic nuclei. ApoER2 protein levels were increased in PCSK9 RNAi cells. Knockdown of ApoER2 but not of VLDLR was sufficient to reverse the protection provided by PCSK9 RNAi, suggesting that proapoptotic signaling of PCSK9 is mediated by altered ApoER2 function. Pharmacological inhibition of signaling pathways associated with lipoprotein receptors suggested that PCSK9 regulates neuronal apoptosis independently of NMDA receptor function but in concert with ERK and JNK signaling pathways. PCSK9 RNAi also reduced staurosporine-induced CGN apoptosis and axonal degeneration in the nerve growth factor-deprived dorsal root ganglion neurons. We conclude that PCSK9 potentiates neuronal apoptosis via modulation of ApoER2 levels and related anti-apoptotic signaling pathways.

Nurr1 protein is required for N-methyl-D-aspartic acid (NMDA) receptor-mediated neuronal survival

NMDA receptor (NMDAR) stimulation promotes neuronal survival during brain development. Cerebellar granule cells (CGCs) need NMDAR stimulation to survive and develop. These neurons differentiate and mature during its migration from the external granular layer to the internal granular layer, and lack of excitatory inputs triggers their apoptotic death. It is possible to mimic this process in vitro by culturing CGCs in low KCl concentrations (5 mm) in the presence or absence of NMDA. Using this experimental approach, we have obtained whole genome expression profiles after 3 and 8 h of NMDA addition to identify genes involved in NMDA-mediated survival of CGCs. One of the identified genes was Nurr1, a member of the orphan nuclear receptor subfamily Nr4a. Our results report a direct regulation of Nurr1 by CREB after NMDAR stimulation. ChIP assay confirmed CREB binding to Nurr1 promoter, whereas CREB shRNA blocked NMDA-mediated increase in Nurr1 expression. Moreover, we show that Nurr1 is important for NMDAR survival effect. We show that Nurr1 binds to Bdnf promoter IV and that silencing Nurr1 by shRNA leads to a decrease in brain-derived neurotrophic factor (BDNF) protein levels and a reduction of NMDA neuroprotective effect. Also, we report that Nurr1 and BDNF show a similar expression pattern during postnatal cerebellar development. Thus, we conclude that Nurr1 is a downstream target of CREB and that it is responsible for the NMDA-mediated increase in BDNF, which is necessary for the NMDA-mediated prosurvival effect on neurons.

Purinergic receptor antagonists inhibit odorant-mediated CREB phosphorylation in sustentacular cells of mouse olfactory epithelium

Background

Extracellular nucleotides have long been known to play neuromodulatory roles and to be involved in intercellular signalling. In the olfactory system, ATP is released by olfactory neurons, and exogenous ATP can evoke an increase in intracellular calcium concentration in sustentacular cells, the nonneuronal supporting cells of the olfactory epithelium. Here we investigate the hypothesis that olfactory neurons communicate with sustentacular cells via extracellular ATP and purinergic receptor activation.

Results

Here we show that exposure of mice to a mixture of odorants induced a significant increase in the levels of the transcription factor CREB phosphorylated at Ser-133 in the nuclei of both olfactory sensory neurons and sustentacular cells. This activation was dependent on adenylyl cyclase III-mediated olfactory signaling and on activation of P2Y purinergic receptors on sustentacular cells. Purinergic receptor antagonists inhibited odorant-dependent CREB phosphorylation specifically in the nuclei of the sustentacular cells.

Conclusion

Our results point to a possible role for extracellular nucleotides in mediating intercellular communication between the neurons and sustentacular cells of the olfactory epithelium in response to odorant exposure. Maintenance of extracellular ionic gradients and metabolism of noxious chemicals by sustentacular cells may therefore be regulated in an odorant-dependent manner by olfactory sensory neurons.

ERK1/2 activation is involved in the neuroprotective action of P2Y13 and P2X7 receptors against glutamate excitotoxicity in cerebellar granule neurons

Cerebellar granule neurons express several types of nucleotide receptors, with the metabotropic P2Y(13) and the ionotropic P2X7 being the most relevant in this model. In the present study we investigated the role of P2Y(13) and P2X7 nucleotide receptors in ERK1/2 signalling. The nucleotidic agonists 2MeSADP (2-methylthioadenosine-5'-diphosphate) for P2Y(13) and BzATP (2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate) for P2X7 receptors were coupled to ERK1/2 activation in granule neurons, being able to increase around two-fold the levels of ERK1/2 phosphorylation. These effects were sensitive to the inhibitory action of the antagonists MRS-2211 and A-438079, specific for P2Y(13) and P2X7 receptors, respectively. Although both receptor subtypes shared the same pattern of transient ERK1/2 phosphorylation, they differed in the intracellular cascades they triggered, being PI3K-dependent for P2Y(13) and calcium/calmodulin kinase II (CaMKII)-dependent for P2X7. These two different ERK-mediated pathways were involved in the neuroprotective effects displayed by both P2Y(13) and P2X7 receptors against apoptosis induced by an excitotoxic concentration of glutamate, in a similar manner to the neurotrophin, BDNF. In addition, P2Y(13) and P2X7 receptor agonists were also able to phosphorylate and activate the ERK-dependent target CREB, which could be involved in their neuroprotective effect. These results indicate that nucleotide receptors share with trophic factors the same survival routes in neurons, such as the ERK signalling route, and therefore, can contribute to the maintenance of granule neurons in conditions in which survival is being compromised.

Reduced blockade by extracellular Mg(2+) is permissive to NMDA receptor activation in cerebellar granule neurons that model a migratory phenotype

NMDA receptors (NMDAR) contribute to neuronal development throughout the CNS. However, their mode(s) of activation preceding synaptic maturation is unclear, as they are not co-localized with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) which normally provide sufficient depolarization to relieve voltage-dependent blockade by Mg(2+). We used cerebellar granule neurons (CGNs) cultured at a near-physiological KCl concentration to examine maturation-dependent changes in NMDAR responses. In contrast, most studies use KCl-supplemented medium to promote survival. At 2-4 days in vitro CGNs: (i) express developmental markers resembling the in vivo migratory phenotype; (ii) maintain a basal amount of calcium responsive element-binding protein phosphorylation that requires NMDARs and calcium/calmodulin-dependent kinases, but not AMPARs; (iii) exhibit NMDA-mediated Ca(2+) influx not effectively blocked by ambient Mg(2+) (0.75 mM) or AMPARs; (iv) maintain a more depolarized resting membrane potential and increased resistance compared to synaptically-connected CGNs. Moreover, migrating CGNs in explant cultures demonstrate NMDA-mediated Ca(2+) influx not effectively blocked by 0.75 mM Mg(2+), and NMDAR but not AMPAR antagonists slow migration. These data suggest the biophysical properties of immature CGNs render NMDARs less sensitive to Mg(2+) blockade, enhancing the likelihood of activation in the absence of AMPAR depolarization.

Bone morphogenetic protein-6 promotes cerebellar granule neurons survival by activation of the MEK/ERK/CREB pathway

Bone morphogenetic proteins (BMPs) have been implicated in the generation and postnatal differentiation of cerebellar granule cells (CGCs). Here, we examined the eventual role of BMPs on the survival of these neurons. Lack of depolarization causes CGC death by apoptosis in vivo, a phenomenon that is mimicked in vitro by deprivation of high potassium in cultured CGCs. We have found that BMP-6, but not BMP-7, is able to block low potassium-mediated apoptosis in CGCs. The neuroprotective effect of BMP-6 is not accompanied by an increase of Smad translocation to the nucleus, suggesting that the canonical pathway is not involved. By contrast, activation of the MEK/ERK/CREB pathway by BMP-6 is necessary for its neuroprotective effect, which involves inhibition of caspase activity and an increase in Bcl-2 protein levels. Other pathways involved in the regulation of CGC survival, such as the c-Jun terminal kinase and the phosphatidylinositol 3-kinase (PI3K)-Akt/PKB, were not affected by BMP-6. Moreover, failure of BMP-7 to activate the MEK/ERK/CREB pathway could explain its inability to protect CGCs from low potassium-mediated apoptosis. Thus, this study demonstrates that BMP-6 acting through the noncanonical MEK/ERK/CREB pathway plays a crucial role on CGC survival.

Coupling of the NMDA receptor to neuroprotective and neurodestructive events

NMDA (N-methyl-D-aspartate) receptors are a subtype of ionotropic glutamate receptor with an important role in the physiology and pathophysiology of central neurons. Inappropriate levels of Ca(2+) influx through the NMDA receptor can contribute to neuronal loss in acute trauma such as ischaemia and traumatic brain injury, as well as certain neurodegenerative diseases such as Huntington's disease. However, normal physiological patterns of NMDA receptor activity can promote neuroprotection against both apoptotic and excitotoxic insults. As a result, NMDA receptor blockade can promote neuronal death outright or render neurons vulnerable to secondary trauma. Thus responses to NMDA receptor activity follow a classical hormetic dose-response curve: both too much and too little can be harmful. There is a growing knowledge of the molecular mechanisms underlying both the neuroprotective and neurodestructive effects of NMDA receptor activity, as well as the factors that determine whether an episode of NMDA receptor activity is harmful or beneficial. It is becoming apparent that oxidative stress plays a role in promoting neuronal death in response to both hyper- and hypo-activity of the NMDA receptor. Increased understanding in this field is leading to the discovery of new therapeutic targets and strategies for excitotoxic disorders, as well as a growing appreciation of the harmful consequences of NMDA receptor blockade.

Crucial role for Ser133-phosphorylated form of cyclic AMP-responsive element binding protein signaling in the differentiation and survival of neural progenitors under chronic cerebral hypoperfusion

Various stimuli, such as ischemia/hypoxia enhance newborn cell survival in the subventricular zone and their migration tangentially in chains toward the olfactory bulb. The present study assessed the fate of newborn neurons from subventricular zone to olfactory bulb under conditions of chronic cerebral hypoperfusion, and examined the role of cAMP-responsive element binding protein signaling on the survival of these neurons by using cilostazol, a potent inhibitor of type III phosphodiesterase. Rats underwent bilateral common carotid artery ligation. They were divided into sham-operated (n=70), vehicle- (n=70), and type III phosphodiesterase inhibitor-treated (n=70) groups. Immunohistochemically-stained section for 5-bromodeoxyuridine and a series of neuronal and glial markers were analyzed at days 7, 14, 21 and 28 after hypoperfusion. The reduction of olfactory bulb size gradually progressed in the vehicle group (P<0.05), but not in the sham-operated and type III phosphodiesterase inhibitor-treated group. The subventricular zone of the vehicle-treated rats contained significantly larger numbers of newborn neuroblasts after hypoperfusion, compared with sham-operated rats (P<0.05), but significantly lower numbers in the rostral migratory stream and olfactory bulb (P<0.05). Treatment of rats with type III phosphodiesterase inhibitor increased the number of neuroblasts and enhanced the survival and differentiation of cells (P<0.05). Phosphorylated cAMP-responsive element binding protein within neuroblasts was markedly decreased in the subventricular zone, rostral migratory stream, and olfactory bulb of vehicle-treated rats (P<0.05), but treatment with type III phosphodiesterase inhibitor resulted in recovery of this expression throughout hypoperfusion, leading to enhanced neurogenesis (P<0.05). These effects were abrogated by protein kinase A and C inhibitor. Our results indicated that cAMP-responsive element binding protein signaling is a key mediator of neurogenesis after prolonged hypoperfusion and provide the basis for new regenerative therapies for ischemic brain injury.

A single exposure to alcohol during brain development induces microencephaly and neuronal losses in genetically susceptible mice, but not in wild type mice

Maternal alcohol abuse during pregnancy can damage the fetal brain and lead to fetal alcohol syndrome (FAS). Despite public warnings discouraging alcohol use during pregnancy, many pregnant women continue to drink intermittently because they do not believe that occasional exposures to alcohol can be harmful to a fetus. However, because of genetic differences, some fetuses are much more susceptible than others to alcohol-induced brain injury. Thus, a relatively low quantity of alcohol that may be innocuous to most fetuses could damage a genetically susceptible fetus. Neuronal nitric oxide synthase (nNOS) can protect developing mouse neurons against alcohol toxicity by synthesizing neuroprotective nitric oxide. This study examined whether a single exposure to alcohol, which causes no evident injury in wild type mice, can damage the brains of mice genetically deficient for nNOS (nNOS-/- mice). Wild type and nNOS-/- mice received intraperitoneal injections of alcohol (0.0, 2.2, or 4.4mg/g body weight) either as a single dose on postnatal day (PD) 4 or as repeated daily doses over PD4-9. Brain volumes and neuronal numbers within the hippocampus and cerebral cortex were determined on PD10. Alcohol exposure on PD4-9 restricted brain growth and caused neuronal death in both strains of mice, but the severity of microencephaly and neuronal loss were more severe in the nNOS-/- mice than in wild type. The 4.4 mg/g alcohol dose administered on PD4 alone caused significant neuronal loss and microencephaly in the nNOS-/- mice, while this same dose caused no evident injury in the wild type mice. Thus, during development, a single exposure to alcohol can injure a genetically vulnerable brain, while it leaves a wild type brain unaffected. Since the genes that confer alcohol resistance and vulnerability in developing humans are unknown, any particular human fetus is potentially vulnerable. Thus, women should be counseled to consume no alcohol during pregnancy.

Maturation-dependent alcohol resistance in the developing mouse: cerebellar neuronal loss and gene expression during alcohol-vulnerable and -resistant periods

BACKGROUND

Alcohol abuse during pregnancy injures the fetal brain. One of alcohol's most important neuroteratogenic effects is neuronal loss. Rat models have shown that the cerebellum becomes less vulnerable to alcohol-induced neuronal death as it matures. We determined if maturation-dependent alcohol resistance occurs in mice and compared patterns of gene expression during the alcohol resistant and sensitive periods.

METHODS

Neonatal mice received alcohol daily over postnatal day (PD) 2 to 4 or PD8 to 10. Purkinje cells and granule cells were quantified on PD25. The temporal expression patterns of 4 neuro-developmental genes and 3 neuro-protective genes in the cerebellum were determined daily over PD0 to 15 to determine how gene expression changes as the cerebellum transitions from alcohol-vulnerable to alcohol-resistant. The effect of alcohol on expression of these genes was determined when the cerebellum is alcohol sensitive (PD4) and resistant (PD10).

RESULTS

Purkinje and granule cells were vulnerable to alcohol-induced death at PD2 to 4, but not at PD8 to 10. Acquisition of maturation-dependent alcohol resistance coincided with changes in the expression of neurodevelopmental genes. The vulnerability of cerebellar neurons to alcohol toxicity declined in parallel with decreasing levels of Math1 and Cyclin D2, markers of immature granule cells. Likewise, the rising resistance to alcohol toxicity paralleled increasing levels of GABA alpha-6 and Wnt-7a, markers of mature granule neurons. Expression of growth factors and genes with survival promoting function (IGF-1, BDNF, and cyclic AMP response element binding protein) did not rise as the cerebellum transitioned from alcohol-vulnerable to alcohol-resistant. All 3 were expressed at substantial levels during the vulnerable period and were not expressed at higher levels later. Acute alcohol exposure altered the expression of neurodevelopmental genes and growth factor genes when administered either during the alcohol vulnerable period or resistant period. However, the patterns in which gene expression changed varied among the genes and depended on timing of alcohol administration.

CONCLUSIONS

Mice have a temporal window of vulnerability in the first week of life, during which cerebellar neurons are more sensitive to alcohol toxicity than during the second week. Expression of genes governing neuronal maturation changes in synchrony with the acquisition of alcohol resistance. Growth factors do not rise as the cerebellum transitions from alcohol-vulnerable to alcohol-resistant. Thus, a process intrinsic to neuronal maturation, rather than rising levels of growth factors, likely underlies maturation-dependent alcohol resistance.

Calcium/calmodulin-dependent protein kinase II regulates the phosphorylation of CREB in NMDA-induced retinal neurotoxicity

We examined the role of the phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) and cyclic AMP-response element binding protein (CREB) in N-methyl-d-aspartate (NMDA)-induced neurotoxicity in the rat retina. Western blot analysis showed early elevation of phosphorylated CaMKII (p-CaMKII) protein levels and subsequential elevation of phosphorylated CREB (p-CREB) protein after NMDA injection. Immunohistochemistry showed that p-CaMKII was colocalized with Thy-1-positive retinal ganglion cells (RGCs) after NMDA injection. The increase in the p-CaMKII protein level was significantly inhibited by the preinjection of CaMKII small interfering RNA (siRNA), whereas negative control siRNA did not affect. Moreover, the increase in the p-CREB protein level after NMDA injection was also prevented by preinjection of CaMKII siRNA. In addition, our morphometric study of neurotracer retrograde labeling and Thy-1-positive cells showed that CaMKII siRNA significantly accelerated NMDA-induced RGC loss. Furthermore, the prevention of CREB binding by CRE decoy oligonucleotide also exacerbated RGC loss. These results suggest that the activation of CaMKII may regulate CREB phosphorylation and that the transient phosphorylation of CaMKII and CREB may be a neuroprotective response against NMDA-induced neurotoxicity.

Interleukin-1 beta impairs brain derived neurotrophic factor-induced signal transduction

The expression of IL-1 is elevated in the CNS in diverse neurodegenerative disorders, including Alzheimer's disease. The hypothesis was tested that IL-1 beta renders neurons vulnerable to degeneration by interfering with BDNF-induced neuroprotection. In trophic support-deprived neurons, IL-1 beta compromised the PI3-K/Akt pathway-mediated protection by BDNF and suppressed Akt activation. The effect was specific as in addition to Akt, the activation of MAPK/ERK, but not PLC gamma, was decreased. Activation of CREB, a target of these signaling pathways, was severely depressed by IL-1 beta. As the cytokine did not influence TrkB receptor and PLC gamma activation, IL-1 beta might have interfered with BDNF signaling at the docking step conveying activation to the PI3-K/Akt and Ras/MAPK pathways. Indeed, IL-1 beta suppressed the activation of the respective scaffolding proteins IRS-1 and Shc; this effect might involve ceramide generation. IL-1-induced interference with BDNF neuroprotection and signal transduction was corrected, in part, by ceramide production inhibitors and mimicked by the cell-permeable C2-ceramide. These results suggest that IL-1 beta places neurons at risk by interfering with BDNF signaling involving a ceramide-associated mechanism.

Stimulation of the cAMP pathway protects cultured cerebellar granule neurons against alcohol-induced cell death by activating the neuronal nitric oxide synthase (nNOS) gene

Neuronal loss is a key component of fetal alcohol syndrome pathophysiology. Therefore, identification of molecules and signaling pathways that ameliorate alcohol-induced neuronal death is important. We have previously reported that neuronal nitric oxide synthase (nNOS) can protect developing cerebellar granule neurons (CGN) against alcohol-induced death both in vitro and in vivo. However, the upstream signal controlling nNOS expression in CGN is unknown. Activated cAMP response element binding protein (CREB) has been strongly linked to the survival of multiple cell types, including CGN. Furthermore, the promoter of the nNOS gene contains two cAMP response elements (CRE). Using cultures of CGN, we tested the hypothesis that cAMP mediates nNOS activation and the protective effect of nNOS against alcohol-induced cell death. Forskolin, an activator of the cAMP pathway, stimulated expression of a reporter gene under the control of the nNOS promoter, and this stimulation was substantially reduced when the two CREs were mutated. Forskolin increased nNOS mRNA levels several fold, increased production of nitric oxide, and abolished alcohol's toxic effect in wild type CGN. Furthermore, forskolin's protective effect was substantially reduced in CGN cultures genetically deficient for nNOS (from nNOS-/- mice). Delivery of nNOS cDNA using a replication-deficient adenoviral vector into nNOS-/- CGN abolished alcohol-induced neuronal death. In addition, overexpression of nNOS in wild type CGN ameliorated alcohol-induced cell death. These results indicate that the neuroprotective effect of the cAMP pathway is mediated, in part, by the pathway's downstream target, the nNOS gene.

Kainic acid triggers oligodendrocyte precursor cell proliferation and neuronal differentiation from striatal neural stem cells

Glutamate is an excitatory amino acid that serves important functions in mammalian brain development through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/ kainate receptor stimulation. Neural stem cells with self-renewal and multilineage potential are a useful tool to study the signals involved in the regulation of brain development. We have investigated the role played by AMPA/kainate receptors during the differentiation of neural stem cells derived from fetal rat striatum. The application of 1 and 10 microM kainic acid increased significantly the phosphorylation of the cyclic AMP response element binding protein (CREB), raised bromodeoxyuridine incorporation in O4-positive oligodendrocyte precursors, and increased the number of O1-positive cells in the cultures. Increased CREB phosphorylation and proliferation were prevented by the AMPA receptor antagonist 4-4(4-aminophenyl)-1,2-dihydro-1-methyl-2-propylcarbamoyl-6,7-methylenedioxyphthalazine (SYM 2206) and by protein kinase A and protein kinase C inhibitors. Cultures treated with 100 microM kainic acid showed decreased proliferation, a lower proportion of O1-positive cells, and apoptosis of O4-positive cells. None of these effects were prevented by SYM 2206, suggesting that kainate receptors take part in these events. We conclude that AMPA receptor stimulation by kainic acid promotes the proliferation of oligodendrocyte precursors derived from neural stem cells through a mechanism that requires the activation of CREB by protein kinase A and C. In the neurons derived from these cells, either AMPA or kainate receptor stimulation produces neuritic growth and larger cell bodies.

Inducible cAMP early repressor (ICER)-evoked delayed neuronal death in the organotypic hippocampal culture

Programmed cell death involving gene regulation and de novo protein synthesis is a major component of both normal development and a number of disease conditions. Hence, knowledge of its mechanisms, especially transcription factors, that regulate expression of the genes involved in neurodegenerative disorders is of great importance. cAMP-responsive element-binding protein (CREB) has repeatedly been implicated in the neuronal survival. In the present study we showed that inducible cAMP early repressor (ICER), an endogenous CREB antagonist, is expressed during both excitotoxic and spontaneous neuronal cell death in organotypic hippocampal slice cultures in vitro. Furthermore, overexpression of ICER via an adenoviral vector evoked neuronal cell loss in such cultures. The time course of ICER-dependent cell death was hippocampal subdivision specific, with dentate gyrus neurons dying mostly 3-7 days after the adenovector infection, followed by CA3, where neuronal death peaked after 7 days, and then CA1, where most neuronal death occurred after 7-14 days. These results underscore the usefulness of the organotypic cultures for studies of neurodegeneration and point to neuronal loss having a multifaceted nature in a complex cellular environment.

Seizures and Antiepileptic Drugs: Does Exposure Alter Normal Brain Development?

Seizures and antiepileptic drugs (AEDs) affect brain development and have long-term neurological consequences. The specific molecular and cellular changes, the precise timing of their influence during brain development, and the full extent of the long-term consequences of seizures and AEDs exposure have not been established. This review critically assesses both the basic and clinical science literature on the effects of seizures and AEDs on the developing brain and finds that evidence exists to support the hypothesis that both seizures and antiepileptic drugs influence a variety of biological process, at specific times during development, which alter long-term cognition and epilepsy susceptibility. More research, both clinical and experimental, is needed before changes in current clinical practice, based on the scientific data, can be recommended.

Severe alcohol-induced neuronal deficits in the hippocampus and neocortex of neonatal mice genetically deficient for neuronal nitric oxide synthase (nNOS)

Alcohol can severely damage the developing brain, and neuronal loss is a critical component of this injury. Thus, identification of molecular factors that ameliorate alcohol-induced neuronal loss is of great importance. Previous in vitro work has demonstrated that nitric oxide (NO) protects neurons against alcohol toxicity. We tested the hypothesis that neonatal mice carrying a null mutation for neuronal nitric oxide synthase (nNOS), the enzyme that synthesizes NO in neurons, have an increased vulnerability to alcohol-induced neuronal loss in the neocortex and hippocampus. Wildtype mice and nNOS-/- mice received ethanol (0.0, 2.2, 3.3, or 4.4 g/kg) daily over postnatal days (P) 4-9 and were sacrificed on P10. The number of hippocampal CA1 and CA3 pyramidal cells, dentate gyrus granule cells, and neocortical neurons were determined using stereological methods. Alcohol pharmacokinetics did not differ between wildtype and nNOS-/- strains. Alcohol induced dose-dependent reductions in all four neuronal populations, and the losses were substantially more severe in the nNOS-/- mice than in wildtype. Furthermore, the threshold dose of alcohol to induce cell death was lower in the nNOS-/- mice than in the wildtype mice for all neuronal populations. While nNOS deficiency worsened alcohol-induced neuronal losses, the magnitude of this exacerbation varied among brain regions and depended on alcohol dose. These results demonstrate that nNOS deficiency decreases the ability of developing neurons in vivo to survive the toxic effects of alcohol and strengthen the hypothesis that NO exerts a neuroprotective effect against alcohol toxicity in the developing brain.

Preconditioning doses of NMDA promote neuroprotection by enhancing neuronal excitability

Neuroprotection can be induced by low doses of NMDA, which activate both synaptic and extrasynaptic NMDA receptors. This is in apparent contradiction with our recent findings that extrasynaptic NMDA receptor signaling exerts a dominant inhibitory effect on prosurvival signaling from synaptic NMDA receptors. Here we report that exposure to low preconditioning doses of NMDA results in preferential activation of synaptic NMDA receptors because of a dramatic increase in action potential firing. Both acute and long-lasting phases of neuroprotection in the face of apoptotic or excitotoxic insults are dependent on this firing enhancement. Key mediators of synaptic NMDA receptor-dependent neuroprotection, phosphatidylinositol 3 kinase-Akt (PI3 kinase-Akt) signaling to Forkhead box subgroup O (FOXO) export and glycogen synthase kinase 3beta (GSK3beta) inhibition and cAMP response element-binding protein-dependent (CREB-dependent) activation of brain-derived neurotrophic factor (BDNF), can be induced only by low doses of NMDA via this action potential-dependent route. In contrast, NMDA doses on the other side of the toxicity threshold do not favor synaptic NMDA receptor activation because they strongly suppress firing rates below baseline. The classic bell-shaped curve depicting neuronal fate in response to NMDA dose can be viewed as the net effect of two antagonizing (synaptic vs extrasynaptic) curves: via increased firing the synaptic signaling dominates at low doses, whereas firing becomes suppressed and extrasynaptic signaling dominates as the toxicity threshold is crossed.

Intracellular calcium regulation by burst discharge determines bidirectional long-term synaptic plasticity at the cerebellum input stage

Variations in intracellular calcium concentration ([Ca2+]i) provide a critical signal for synaptic plasticity. In accordance with Hebb's postulate (Hebb, 1949), an increase in postsynaptic [Ca2+]i can induce bidirectional changes in synaptic strength depending on activation of specific biochemical pathways (Bienenstock et al., 1982; Lisman, 1989; Stanton and Sejnowski, 1989). Despite its strategic location for signal processing, spatiotemporal dynamics of [Ca2+]i changes and their relationship with synaptic plasticity at the cerebellar mossy fiber (mf)-granule cell (GrC) relay were unknown. In this paper, we report the plasticity/[Ca2+]i relationship for GrCs, which are typically activated by mf bursts (Chadderton et al., 2004). Mf bursts caused a remarkable [Ca2+]i increase in GrC dendritic terminals through the activation of NMDA receptors, metabotropic glutamate receptors (probably acting through IP3-sensitive stores), voltage-dependent calcium channels, and Ca2+-induced Ca2+ release. Although [Ca2+]i increased with the duration of mf bursts, long-term depression was found with a small [Ca2+]i increase (bursts <250 ms), and long-term potentiation (LTP) was found with a large [Ca2+]i increase (bursts >250 ms). LTP and [Ca2+]i saturated for bursts >500 ms and with theta-burst stimulation. Thus, bursting enabled a Ca2+-dependent bidirectional Bienenstock-Cooper-Munro-like learning mechanism providing the cellular basis for effective learning of burst patterns at the input stage of the cerebellum.

cAMP response element-binding protein regulates differentiation and survival of newborn neurons in the olfactory bulb

The transcription factor cAMP response element-binding protein (CREB) is involved in multiple aspects of neuronal development and plasticity. Here, we demonstrate that CREB regulates specific phases of adult neurogenesis in the subventricular zone/olfactory bulb (SVZ/OB) system. Combining immunohistochemistry with bromodeoxyuridine treatments, cell tracer injections, cell transplants, and quantitative analyses, we show that although CREB is expressed by the SVZ neuroblasts throughout the neurogenic process, its phosphorylation is transient and parallels neuronal differentiation, increasing during the late phase of tangential migration and decreasing after dendrite elongation and spine formation. In vitro, inhibition of CREB function impairs morphological differentiation of SVZ-derived neuroblasts. Transgenic mice lacking CREB, in a null CREM genetic background, show reduced survival of newborn neurons in the OB. This finding is further supported by peripheral afferent denervation experiments resulting in downregulation of CREB phosphorylation in neuroblasts, the survival of which appears heavily impaired. Together, these findings provide evidence that CREB regulates differentiation and survival of newborn neurons in the OB.

Nuclear Ca2+ and the cAMP response element-binding protein family mediate a late phase of activity-dependent neuroprotection

The mechanism by which physiological synaptic NMDA receptor activity promotes neuronal survival is not well understood. Here, we show that that an episode of synaptic activity can promote neuroprotection for a long time after that activity has ceased. This long-lasting or "late phase" of neuroprotection is dependent on nuclear calcium signaling and cAMP response element (CRE)-mediated gene expression. In contrast, neuroprotection evoked acutely by ongoing synaptic activity relies solely on the activation of the phosphatidylinositol 3-kinase/Akt pathway. This "acute phase" does not require nuclear calcium signaling and is independent of activation of the CRE-binding protein (CREB) family of transcription factors. Thus, activity-dependent neuroprotection comprises two mechanistically distinct phases that differ in their spatial requirements for calcium and in their reliance on the CREB family.

NMDA receptor activation modulates programmed cell death during early post-natal retinal development: a BDNF-dependent mechanism

Glutamate is a classical excitotoxin of the central nervous system (CNS), but extensive work demonstrates neuroprotective roles of this neurotransmitter in developing CNS. Mechanisms of glutamate-mediated neuroprotection are still under scrutiny. In this study, we investigated mediators of glutamate-induced neuroprotection, and tested whether this neurotransmitter controls programmed cell death in the developing retina. The protective effect of N-methyl-d-aspartate (NMDA) upon differentiating cells of retinal explants was completely blocked by a neutralizing antibody to brain-derived neurotrophic factor (BDNF), but not by an antibody to neurotrophin-4 (NT-4). Consistently, chronic activation of NMDA receptor increased the expression of BDNF and trkB mRNA, as well as BDNF protein content, but did not change the content of NT-4 mRNA in retinal tissue. Furthermore, we showed that in vivo inactivation of NMDA receptor by intraperitoneal injections of MK-801 increased natural cell death of specific cell populations of the post-natal retina. Our results show that chronic activation of NMDA receptors in vitro induces a BDNF-dependent neuroprotective state in differentiating retinal cells, and that NMDA receptor activation controls programmed cell death of developing retinal neurons in vivo.

Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

Long-term potentiation of synaptic transmission at the mossy fiber-granule cell relay of cerebellum

In the last decade, the physiology of cerebellar neurons and synapses has been extended to a considerable extent. We have found that the mossy fiber-granule cell relay can generate a complex form of long-term potentiation (mf-GrC LTP) following high-frequency mf discharge. Induction. Mf-GrC LTP depends on NMDA and mGlu receptor activation, intracellular Ca(2+) increase, PKC activation, and NO production. The preventative action of intracellular agents (BAPTA, PKC-inhibitors) and of membrane hyperpolarization, and the correlated increase in intracellular Ca(2+) observed using fluorescent dyes, indicate that induction occurs postsynaptically. Expression. Expression includes three components: (a) an increase of synaptic currents, (b) an increase of intrinsic excitability in GrC, and (c) an increase of intrinsic excitability in mf terminals. Based on quantal analysis, the EPSC increase is mostly explained by enhanced neurotransmitter release. NO is a candidate retrograde neurotransmitter which could determine both presynaptic current changes and LTP. NO cascade blockers inhibit both presynaptic current changes and LTP. The increase in intrinsic excitability involves a raise in apparent input resistance in the subthreshold region and a spike threshold reduction. Together with other forms of cerebellar plasticity, mf-GrC LTP opens new hypothesis on how the cerebellum processes incoming information.

Increased neurotransmitter release during long-term potentiation at mossy fibre-granule cell synapses in rat cerebellum

During long-term potentiation (LTP) at mossy fibre-granule cell synapses in rat cerebellum synaptic transmission and granule cell intrinsic excitability are enhanced. Although it is clear that changes in granule cell excitability are mediated postsynaptically, there is as yet no direct evidence for the site and mechanism of changes in transmission. To approach this problem, evoked postsynaptic currents (EPSCs) and miniature synaptic currents (mEPSCs) were recorded by patch-clamp in cerebellar slices obtained from P17-P23 rats. LTP was induced by theta-burst stimulation paired with depolarization. During LTP, the EPSCs showed a significant decrease in the coefficient of variation (CV; 28.9 +/- 5.2%, n= 8; P < 0.002), the number of failures (87.1 +/- 41.9%, n= 8; P < 0.04), and the paired-pulse ratio (PPR; 25.5 +/- 4.1% n= 5; P < 0.02). Similar changes were observed by increasing neurotransmitter release (extracellular solutions with high Ca(2+)/Mg(2+) ratio), whereas increases in CV, numbers of failures and PPR occurred when release was decreased (extracellular solutions with low Ca(2+)/Mg(2+) ratio; 10 microm Cl-adenosine). No changes followed modifications of postsynaptic holding potentials, while CV and failures were reduced when the number of active synapses was increased. LTP was prevented by use of solutions with high Ca(2+)/Mg(2+) ratio. Moreover, LTP and the associated CV decrease were observed in the spillover-mediated component of AMPA EPSCs and in NMDA EPSCs. During LTP, mEPSCs did not change in amplitude or variability but significantly increased in frequency (47.6 +/- 16%, n= 4; P < 0.03). By binomial analysis changes in EPSCs were shown to be due to increased release probability (from 0.6 +/- 0.08 to 0.73 +/- 0.06, n= 7; P < 0.02) with a constant number of three to four releasing sites. These observations provide evidence for increased neurotransmitter release during LTP at mossy fibre-granule cell synapses.

PACAP inhibits delayed rectifier potassium current via a cAMP/PKA transduction pathway: evidence for the involvement of I k in the anti-apoptotic action of PACAP

Abstract Activation of potassium (K(+)) currents plays a critical role in the control of programmed cell death. Because pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to inhibit the apoptotic cascade in the cerebellar cortex during development, we have investigated the effect of PACAP on K(+) currents in cultured cerebellar granule cells using the patch-clamp technique in the whole-cell configuration. Two types of outward K(+) currents, a transient K(+) current (I(A)) and a delayed rectifier K(+) current (I(K)) were characterized using two different voltage protocols and specific inhibitors of K(+) channels. Application of PACAP induced a reversible reduction of the I(K) amplitude, but did not affect I(A), while the PACAP-related peptide vasoactive intestinal polypeptide had no effect on either types of K(+) currents. Repeated applications of PACAP induced gradual attenuation of the electrophysiological response. In the presence of guanosine 5'-[gammathio]triphosphate (GTPgammaS), PACAP provoked a marked and irreversible I(K) depression, whereas cell dialysis with guanosine 5'-[betathio]diphosphate GDPbetaS totally abolished the effect of PACAP. Pre-treatment of the cells with pertussis toxin did not modify the effect of PACAP on I(K). In contrast, cholera toxin suppressed the PACAP-induced inhibition of I(K). Exposure of granule cells to dibutyryl cyclic adenosine monophosphate (dbcAMP) mimicked the inhibitory effect of PACAP on I(K). Addition of the specific protein kinase A inhibitor H89 in the patch pipette solution prevented the reduction of I(K) induced by both PACAP and dbcAMP. PACAP provoked a sustained increase of the resting membrane potential in cerebellar granule cells cultured either in high or low KCl-containing medium, and this long-term depolarizing effect of PACAP was mimicked by the I(K) specific blocker tetraethylammonium chloride (TEA). In addition, pre-incubation of granule cells with TEA suppressed the effect of PACAP on resting membrane potential. TEA mimicked the neuroprotective effect of PACAP against ethanol-induced apoptotic cell death, and the increase of caspase-3 activity observed after exposure of granule cells to ethanol was also significantly inhibited by TEA. Taken together, the present results demonstrate that, in rat cerebellar granule cells, PACAP reduces the delayed outward rectifier K(+) current by activating a type 1 PACAP (PAC1) receptor coupled to the adenylyl cyclase/protein kinase A pathway through a cholera toxin-sensitive Gs protein. Our data also show that PACAP and TEA induce long-term depolarization of the resting membrane potential, promote cell survival and inhibit caspase-3 activity, suggesting that PACAP-evoked inhibition of I(K) contributes to the anti-apoptotic effect of the peptide on cerebellar granule cells.

Evidence for an axonal localization of the type 2 corticotropin-releasing factor receptor during postnatal development of the mouse cerebellum

Previous studies have described the embryonic and postnatal development of CRF, as well as the type 1 CRF receptor in the mouse cerebellum. The present immunohistochemical study localizes the cellular distribution of the type 2 CRF receptor (CRF-R2) during postnatal development of the mouse cerebellum. Western blot analysis indicates that the antibody used in this analysis recognizes both a full-length and a truncated isoform of the type 2 receptor. We propose that each isoform has a unique cellular distribution. In the present study, the postnatal (P) development (P0-P14) and cellular localization of CRF-R2 in different cell types was analyzed using PAP and double-label fluorescent immunohistochemistry; cell-specific antibodies were used to identify cells expressing CRF-R2 at different stages of postnatal development. At P0, CRF-R2 immunoreactivity was localized within the somata of Purkinje cells and migrating GABAergic interneurons. CRF-R2 was first observed in the initial axonal segments of some Purkinje cells at P5, and was evident in many Purkinje cell axon hillocks at P8. Punctate immunoreactivity is present in the molecular layer by P5 and is interpreted to be immunolabeled parallel fibers. Between P8 and P14, CRF-R2 immunostaining is present in the initial axonal segments of Golgi cells, within the internal granule cell layer. Finally, CRF-R2 is present in both radial glia in the molecular layer as well as in astrocytes in the white matter and internal granule cell layer from P5 to P14. The present results suggest that CRF-R2, both the truncated and the full-length isoforms, are present in the developing cerebellum, each with a unique cellular distribution. The immunohistochemical evidence indicates that the truncated isoform of the type 2 CRF receptor is in the axons of several different types of cerebellar cortical neurons, and suggests that CRF could play a role in cerebellar development by modulating the release of transmitters from excitatory and/or inhibitory interneurons, which in turn could directly alter the maturation of cerebellar circuits. In contrast, the binding of a ligand to the full-length isoform of CRF-R2 or to CRF-R1, both in a postsynaptic location, may have a more direct effect on regulating the responsiveness of these cells to growth factors or neurotransmitters released from afferent axons by regulating permeability of ion channels or altering second messenger systems.

Identification of a novel cyclic AMP-response element (CRE-II) and the role of CREB-1 in the cAMP-induced expression of the survival motor neuron (SMN) gene

Spinal muscular atrophy, an autosomal recessive disorder, is caused by loss of the SMN1 (survival motor neuron) gene while retaining the SMN2 gene. SMN1 produces a majority of full-length SMN transcript, whereas SMN2 generates mostly an isoform lacking exon 7. Here, we demonstrate a novel cAMP-response element, CRE-II, in the SMN promoter that interacts with the cAMP-response element-binding (CREB) family of proteins. In vitro DNase I protection analysis and in vivo genomic footprinting of the SMN promoter using the brain and liver nuclei from SMN2 transgenic mice revealed footprinting at the CRE-II site. Site-directed mutation of the CRE-II element caused a marked reduction in the SMN promoter activity revealed by transient transfection assay. Activation of the cAMP pathway by dibutyryl cAMP (0.5 mm) alone or in combination with forskolin (20 microm) caused a 2-5-fold increase in the SMN promoter activity but had no effect on the CRE-II mutated promoter. Electrophoretic mobility shift assay and a UV-induced DNA-protein cross-linking experiment confirmed that CREB1 binds specifically to the CRE-II site. Transient overexpression of CREB1 protein resulted in a 4-fold increase of the SMN promoter activity. Intraperitoneal injection of epinephrine in mice expressing two copies of the human SMN2 gene resulted in a 2-fold increase in full-length SMN transcript in the liver. Combined treatment with dibutyryl cAMP and forskolin significantly increased the level of both the full-length and exon 7-deleted SMN (exonDelta7SMN) transcript in primary hepatocytes from mice expressing two copies of human SMN2 gene. Similar treatments of type I spinal muscular atrophy mouse and human fibroblasts as well as HeLa cells resulted in an augmented level of SMN transcript. These findings suggest that the CRE-II site in SMN promoter positively regulates the expression of the SMN gene, and treatment with cAMP-elevating agents increases expression of both the full-length and exonDelta7SMN transcript.

High K+ and IGF-1 protect cerebellar granule neurons via distinct signaling pathways

In culture, cerebellar granule neurons die of apoptosis in serum-free media containing a physiologic level of K(+) but survive in a depolarizing concentration of K(+) or when insulin-like growth factor 1 (IGF-1) is added. Both Akt/PKB activation and caspase-3 inhibition were implicated as the underlying neuroprotective mechanisms. The duration of high K(+), however, induced survival effects that outlasted its transient activation of Akt, and granule neurons derived from caspase-3 knockout mice died to the same extent as did those from wild-type mice, suggesting that additional mechanisms are involved. To delineate these survival mechanisms, we compared the activities of two major survival pathways after high K(+)-induced depolarization or IGF-1 stimulation. Although IGF-1 promoted neuronal survival by activating its tyrosine kinase receptor, high K(+) depolarization provided the same effect by increasing the Ca(2+) influx through the L Ca(2+) channel. Moreover, high K(+)-induced depolarization resulted in sustained activation of MAP kinase, whereas IGF-1 activated Akt in 4 hr. Inhibition of MEK (MAP kinase kinase) by either PD98059 or UO126 abolished the protective effect of high K(+)-induced depolarization, but not that of IGF-1, suggesting that activation of the MAP kinase pathway is necessary for high K(+) neuroprotective effects. We demonstrated also that high K(+)-induced depolarization, but not IGF-1, increased phosphorylation of cAMP-response element-binding protein (CREB) and protein synthesis, both of which can be blocked by UO126. Overall, our findings suggested that high K(+)-induced depolarization, unlike IGF-1, promoted neuronal survival via activating MAP kinase, possibly by increasing CREB-dependent transcriptional activation of specific proteins that promote neuronal survival.

Synaptic and extrasynaptic neurotransmitter receptors in glial precursors' quest for identity

It is widely established that neurotransmitter receptors are expressed in non-neuronal cells, and particularly in neural progenitor cells in the postnatal central nervous system. The functional role of these receptors during development is unclear, but it needs to be revisited now that cells previously considered restricted to glial lineages have been shown to generate neurons. The present review integrates recent advances, to shed new light on how neurotransmitter receptors may, alternatively, serve as excitable mediators of neuron-glia and neuron-neuroblast interactions.

Lead exposure alters cyclic-AMP response element binding protein phosphorylation and binding activity in the developing rat brain

We examined the effect of lead (Pb(2+)) exposure during development on cyclic-AMP response element binding protein (CREB) expression and phosphorylation in cortical and hippocampal nuclear extracts at postnatal (PN) days 7, 14, 21 and 50. We also examined the binding of CREB family proteins to the cyclic-AMP response element (CRE) using a novel filter-binding assay that provides a quantitative measure of binding kinetics. In the hippocampus and cerebral cortex of control rats, CREB and phospho-CREB (pCREB; serine-133) expression is highest at PN7 and decreases steadily until PN50. Developmental Pb(2+) exposure does not affect total CREB levels but decreases pCREB levels at PN14 and PN50 in the cortex and at PN50 in the hippocampus. Using the filter-binding assay, we measured a 30% decrease in B(max) and 38% decrease in the Kd of CREB family proteins for the CRE in PN50 hippocampal nuclear fractions prepared from Pb(2+)-exposed rats. A similar, but nonsignificant, trend is observed in the cortex of PN50 lead-exposed rats. In addition, a 70% increase in the B(max) was observed in the cortex of PN14 lead-exposed rats without a significant change in the Kd. These disruptions in pCREB expression and binding activity of CREB family members during the ontogeny of the rat brain begin to decipher intracellular mechanisms of Pb(2+) neurotoxicity.