Effect of depolarization on the maturation of cerebellar granule cells in culture

BNN27, a 17-Spiroepoxy Steroid Derivative, Interacts With and Activates p75 Neurotrophin Receptor, Rescuing Cerebellar Granule Neurons from Apoptosis

Neurotrophin receptors mediate a plethora of signals affecting neuronal survival. The p75 pan-neurotrophin receptor controls neuronal cell fate after its selective activation by immature and mature isoforms of all neurotrophins. It also exerts pleiotropic effects interacting with a variety of ligands in different neuronal or non-neuronal cells. In the present study, we explored the biophysical and functional interactions of a blood-brain-barrier (BBB) permeable, C17-spiroepoxy steroid derivative, BNN27, with p75^NTR receptor. BNN27 was recently shown to bind to NGF high-affinity receptor, TrkA. We now tested the p75^NTR-mediated effects of BNN27 in mouse Cerebellar Granule Neurons (CGNs), expressing p75^NTR, but not TrkA receptors. Our findings show that BNN27 physically interacts with p75^NTR receptors in specific amino-residues of its extracellular domain, inducing the recruitment of p75^NTR receptor to its effector protein RIP2 and the simultaneous release of RhoGDI in primary neuronal cells. Activation of the p75^NTR receptor by BNN27 reverses serum deprivation-induced apoptosis of CGNs resulting in the decrease of the phosphorylation of pro-apoptotic JNK kinase and of the cleavage of Caspase-3, effects completely abolished in CGNs, isolated from p75^NTR null mice. In conclusion, BNN27 represents a lead molecule for the development of novel p75^NTR ligands, controlling specific p75^NTR-mediated signaling of neuronal cell fate, with potential applications in therapeutics of neurodegenerative diseases and brain trauma.

Redox signal regulation via nNOS phosphorylation at Ser847 in PC12 cells and rat cerebellar granule neurons

Phosphorylation is considered a main mechanism modulating nNOS (neuronal nitric oxide synthase) function to reduce NO production. In the present study, the effects of nNOS phosphorylation on redox signalling, including that of NO, ROS (reactive oxygen species), and 8-nitro-cGMP (8-nitroguanosine 3',5'-cyclic monophosphate), a downstream messenger of redox signalling, were investigated. In vitro experiments revealed that a phosphorylation-mimic mutant of nNOS (Ser847 replaced with aspartic acid, 847D) increased uncoupling to produce a superoxide. In addition, nicotine, which triggers an influx of Ca2+, induced more ROS and 8-nitro-cGMP production in 847D-expressing PC12 cells than WT (wild-type)-expressing cells. Additionally, nicotine-induced phosphorylation of nNOS at Ser847 and increased ROS and 8-nitro-cGMP production in rat CGNs (cerebellar granule neurons). In CGNs, the NOS (nitric oxide synthase) inhibitor L-NAME (NG-nitro-L-arginine methyl ester) and superoxide dismutase completely inhibited ROS and 8-nitro-cGMP production, whereas the CaMK (Ca2+/calmodulin-dependent protein kinase) inhibitor KN93 mildly reduced this effect. Nicotine induced HO-1 (haem oxygenase 1) expression in CGNs and showed cytoprotective effects against apoptosis. Moreover, 8-nitro-cGMP treatment showed identical effects that were attenuated by KN93 pre-treatment. The present paper provides the first substantial corroboration for the biological effects of nNOS phosphorylation at Ser847 on redox signalling, including ROS and intracellular 8-nitro-cGMP generation in neurons, which possibly play roles in neuroprotection.

Ethanol neurotoxicity in the developing cerebellum: underlying mechanisms and implications

Ethanol is the main constituent of alcoholic beverages that exerts toxicity to neuronal development. Ethanol affects synaptogenesis and prevents proper brain development. In humans, synaptogenesis takes place during the third trimester of pregnancy, and in rodents this period corresponds to the initial few weeks of postnatal development. In this period neuronal maturation and differentiation begin and neuronal cells start migrating to their ultimate destinations. Although the neuronal development of all areas of the brain is affected, the cerebellum and cerebellar neurons are more susceptible to the damaging effects of ethanol. Ethanol’s harmful effects include neuronal cell death, impaired differentiation, reduction of neuronal numbers, and weakening of neuronal plasticity. Neuronal development requires many hormones and growth factors such as retinoic acid, nerve growth factors, and cytokines. These factors regulate development and differentiation of neurons by acting through various receptors and their signaling pathways. Ethanol exposure during development impairs neuronal signaling mechanisms mediated by the N-methyl-d-aspartate (NMDA) receptors, the retinoic acid receptors, and by growth factors such as brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-I), and basic fibroblast growth factor (bFGF). In combination, these ethanol effects disrupt cellular homeostasis, reduce the survival and migration of neurons, and lead to various developmental defects in the brain. Here we review the signaling mechanisms that are required for proper neuronal development, and how these processes are impaired by ethanol resulting in harmful consequences to brain development.

Mechanisms of ethanol-induced death of cerebellar granule cells

Maternal ethanol exposure during pregnancy may cause fetal alcohol spectrum disorders (FASD). FASD is the leading cause of mental retardation. The most deleterious effect of fetal alcohol exposure is inducing neuroapoptosis in the developing brain. Ethanol-induced loss of neurons in the central nervous system underlies many of the behavioral deficits observed in FASD. The cerebellum is one of the brain areas that are most susceptible to ethanol during development. Ethanol exposure causes a loss of both cerebellar Purkinje cells and granule cells. This review focuses on the toxic effect of ethanol on cerebellar granule cells (CGC) and the underlying mechanisms. Both in vitro and in vivo studies indicate that ethanol induces apoptotic death of CGC. The vulnerability of CGC to ethanol-induced death diminishes over time as neurons mature. Several mechanisms for ethanol-induced apoptosis of CGC have been suggested. These include inhibition of N-methyl-D-aspartate receptors, interference with signaling by neurotrophic factors, induction of oxidative stress, modulation of retinoid acid signaling, disturbance of potassium channel currents, thiamine deficiency, and disruption of translational regulation. Cultures of CGC provide an excellent system to investigate cellular/molecular mechanisms of ethanol-induced neurodegeneration and to evaluate interventional strategies. This review will also discuss the approaches leading to neuroprotection against ethanol-induced neuroapoptosis.

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.

Apoptosis and autophagy in rat cerebellar granule neuron death: Role of reactive oxygen species

Programmed cell death (PCD) has been defined as an active, controlled process in which cells participate in their own demise. Apoptosis, or type I PCD, has been widely characterized, both morphologically and biochemically. More recently, autophagy, the self-digesting mechanism involved in the removal of cytoplasmic long-lived proteins, has been involved in cell death, and type II PCD is defined as cell death occurring with autophagic features. Neurons can undergo more than one type of PCD as a backup mechanism when the traditional death pathway is inhibited or in response to a particular death-inducing stimulus. Reactive oxygen species (ROS) have been shown to be important signaling molecules in the execution of apoptosis and, more recently, in the autophagic pathway. In this work, we characterize apoptotic and autophagic cell death in rat cerebellar granule neuron (CGN) culture, a widespread model for the study of neuronal death. Potassium deprivation (K5) and staurosporine (STS) were used for death induction. We found apoptotic and autophagic features under both conditions. Caspase inhibition as well as autophagy inhibition by 3-methyl adenine decreased cell death. Moreover, CGN can undergo the alternative type of cell death when the other one is inhibited. An antioxidant or NADPH oxidase inhibitors delayed apoptosis and had no effect in autophagic features. Thus, we found that autophagy plays a role in cell death of CGN and that, when cells are treated with K5 or STS, both autophagy and ROS seem to promote apoptosis by independent mechanisms.

Paradoxical effect of ethanol on potassium channel currents and cell survival in cerebellar granule neurons

Transient exposure to ethanol (EtOH) results in a massive neurodegeneration in the developing brain leading to behavioral and cognitive deficits observed in fetal alcohol syndrome. There is now compelling evidence that K+ channels play an important role in the control of programmed cell death. The aim of the present work was to investigate the involvement of K+ channels in the EtOH-induced cerebellar granule cell death and/or survival. At low and high concentrations, EtOH evoked membrane depolarization and hyperpolarization, respectively. Bath perfusion of EtOH (10 mM) depressed the I(A) (transient K+ current) potassium current whereas EtOH (400 mM) provoked a marked potentiation of the specific I(K) (delayed rectifier K+ current) current. Pipette dialysis with GTPgammaS or GDPbetaS did not modify the effects of EtOH (400 mM) on both membrane potential and I(K) current. In contrast, the reversible depolarization and slowly recovering inhibition of I(A) induced by EtOH (10 mM) became irreversible in the presence of GTPgammaS. EtOH (400 mM) induced prodeath responses whereas EtOH (10 mM) and K+ channel blockers promoted cell survival. Altogether, these results indicate that in cerebellar granule cells, EtOH mediates a dual effect on K+ currents partly involved in the control of granule cell death.

A rapid screening method for population-specific neuronal motogens, substrates and associated signaling pathways

We developed and characterized an assay that allows for rapid examination of migration of specific neuronal populations within a mixed population using the Boyden chamber principle. Migration of cerebellar interneurons and granule cells was examined using mice expressing enhanced green fluorescent protein (eGFP) under the glutamate decarboxylase (GAD(65)) and growth-associated protein-43 (GAP43) promoters, respectively. Brain-derived neurotrophic factor (BDNF) was used as the prototypic motogen for both populations. Fluorescent light-blocking inserts (FluoroBlok) with different pore sizes and densities were compared in a two-compartment assay. Immunodetection of polarity markers and nuclear staining indicated that dendrites and somata are preferentially extended through the pores in response to BDNF. Inserts coated with extracellular matrix (ECM) proteins were used to examine interactions between BDNF and the ECM during migration. ECM proteins alone stimulated migration when the lower side of the insert was coated, however coating of both sides of the insert slowed migration when compared to poly-D-lysine. Addition of a PI 3-kinase inhibitor to the lower compartment blocked BDNF-stimulated migration of both populations while a Src inhibitor reduced laminin-stimulated migration of interneurons, but not granule cells. We also examined use of neurons cultured from GAD(65)-eGFP mice as a reporter system for promoter activity. GAD(65)-eGFP mice may also be useful as a model for promoter regulation and the potential confounding effects of eGFP induction by the stimuli are also addressed. This assay allows for rapid analysis of motogens, substrates and signaling pathways that regulate migration of selected neuronal populations.

Membrane potential-regulated transcription of the resting K+ conductance TASK-3 via the calcineurin pathway

The 2P domain K(+) channel TASK-3 is highly expressed in cerebellar granule neurons where it has been proposed to underlie the K(+) leak conductance, IKso. In a previous work we showed that expression of TASK-3 increases in cerebellar granule neurons as they mature in culture. Here we show that within the cerebellum, levels of TASK-3 mRNA increase as granule neurons migrate to their adult positions and receive excitatory mossy fiber input. To understand the mechanism of this increase in TASK-3 expression we used an in vitro model culturing the neurons in either depolarizing conditions mimicking neuronal activity (25K, 25 mm KCl) or in conditions which approach deafferentation (5K, 5 mm KCl). An important increase in TASK-3 mRNA is uniquely observed in 25K and is specific since other background K(+) channel levels remain unchanged or decrease. The rise in TASK-3 mRNA leads to an increase in TASK-3 protein and the IKso conductance resulting in hyperpolarization. Blocking L-type calcium channels or their downstream effector calcineurin, abrogates TASK-3 expression and IKso, leading to hyperexcitability. This is the first study demonstrating that depolarization-induced Ca(2+) entry can directly regulate cellular excitability by dynamically regulating the transcription of a resting K(+) conductance. The appearance of this conductance may play an important role in the transition of depolarized immature neurons to their mature hyperpolarized state during neuronal development.

AMPA/kainate receptor-mediated up-regulation of GABAA receptor δ subunit mRNA expression in cultured rat cerebellar granule cells is dependent on NMDA receptor activation

We have studied the effects of AMPA/kainate receptor agonists on GABA(A) receptor subunit mRNA expression in vitro in cultured rat cerebellar granule cells (CGCs). Kainate (KA) (100 microM) and high K(+) (25 mM) dramatically up-regulated delta subunit mRNA expression to 500-700% of that in control cells grown in low K(+) (5 mM). KA or high K(+) had no effect on the expression of the other major GABA(A) receptor subunits alpha1, alpha6, beta2, beta3 or gamma2. Up-regulation of delta mRNA was also detected with the AMPA receptor-selective agonist CPW-399 and to a lesser extent with the KA receptor-selective agonist ATPA. AMPA/kainate receptor-selective antagonist DNQX completely inhibited KA-, CPW-399- and ATPA-induced delta mRNA up-regulation indicating that the effects were mediated via AMPA and KA receptor activation. NMDA receptor-selective antagonist MK-801 inhibited 76% of the KA- and 57% of the CPW-399-induced delta up-regulation suggesting that KA and CPW-399 treatments may induce glutamate release resulting in NMDA receptor activation, and subsequently to delta mRNA up-regulation. In CGCs, delta subunit is a component of extrasynaptic alpha6betadelta receptors that mediate tonic inhibition. Up-regulation of delta during prolonged glutamate receptor activation or cell membrane depolarization may be a mechanism to increase tonic inhibition to counteract excessive excitation.

Robust axonal sprouting and synaptogenesis in organotypic slice cultures of rat cerebellum exposed to increased potassium chloride

Organotypic slices of the rat cerebellum, cultured in physiological levels [K+]o (5 mM) for 14 days, loose the majority of granule cells in the anterior lobe resulting in few axons and atypical Purkinje cell dendrites with vacant spines. When the culture medium was switched from 5 mM to 20, 30 or 40 mM [K+]o during the last 7 days of cultures, slices developed axons with numerous vesicle-filled boutons that made synaptic contact with Purkinje cell spines. Most boutons had one or two spine profile contacts, while some were unusually large. Enlarged boutons abutted Purkinje cell somata or their dendrites, causing intervening spines to invaginate terminals to form rosette synaptic complexes. Calbindin immuno-labeling excluded Purkinje cell axonal collaterals as the source of rosette boutons and suggested a granule cell origin. Quantification of vacant spines as compared to those on boutons revealed a threshold for potassium, between 10 and 20 mM, where the number of synaptic spines increased and vacant spines decreased drastically. These findings suggest that elevated [K+]o triggers an activity-dependent plasticity in rat cerebellar slice cultures by promoting axonal sprouting with formation of vesicle-filled boutons and synaptogenesis on open receptor sites of Purkinje cell spines.

pH is an intracellular effector controlling differentiation of oligodendrocyte precursors in culture via activation of the ERK1/2 pathway

We reported previously that onset of oligodendrocyte precursor cell (OPC) differentiation is accompanied by an increase in intracellular pH (pH(i)). We show that OPC differentiation is dependent primarily on a permissive pH(i) value. The highest differentiation levels were observed for pH(i) values around 7.15 and inhibition of differentiation was observed at slightly more acidic or alkaline values. Clamping the pH(i) of OPCs at 7.15 caused a transient activation of ERK1/2 that was not observed at more acidic or alkaline values. Furthermore, inhibition of ERK activation with the UO126 compound totally prevented OPC differentiation in response to pH(i) shift. These results indicate that pH(i), acting through the ERK1/2 pathway, is a key determinant for oligodendrocyte differentiation. We also show that this pH(i) pathway is involved in the process of retinoic acid-induced OPC differentiation.

N-methyl-D-aspartate blocks activation of JNK and mitochondrial apoptotic pathway induced by potassium deprivation in cerebellar granule cells

During the postnatal development of cerebellum, lack of excitatory innervation from the mossy fibers results in cerebellar granule cell (CGC) apoptosis during the migration of the cells toward the internal granule cell layer. Accordingly, CGCs die by apoptosis when cultured in physiological KCl concentrations (5 mm; K5), and they survive in the presence of depolarizing conditions such as high KCl concentration (25 mm; K25) or N-methyl-D-aspartate (NMDA). We have recently shown that NMDA is able to exert a long lasting neuroprotective effect when added to immature (2 days in vitro) CGC cultures by inhibition of caspase-3 activity. Here we show that NMDA- and K25-mediated neuroprotection is associated with an increase in the levels of Bcl-2, an inhibition of K5-mediated increase in Bax, and the inhibition of the release of apoptogenic factors from mitochondria such as Smac/DIABLO and cytochrome c. Moreover, we have shown that similar effects are observed when c-Jun N-terminal kinases (JNKs) are inhibited and that treatment of CGC cultures with NMDA blocks K5-mediated JNK activation. These results allow us to postulate that the inhibition of JNK-mediated release of apoptogenic factors from mitochondria is involved in the NMDA protection from K5-mediated apoptosis of CGCs.

Interleukin-6 produces neuronal loss in developing cerebellar granule neuron cultures

CNS levels of the cytokine interleukin-6 (IL-6) are elevated during CNS injury and disease, but it is unclear if IL-6 contributes to the pathologic process. Our studies show that in a well-characterized CNS developmental model system, primary cultures of rodent cerebellar granule neurons, chronic exposure to IL-6 during neuronal development can result in cell damage and death in a subpopulation of developing granule neurons. Chronic exposure to IL-6 also increased the susceptibility of the granule neurons to a toxic insult produced by excessive activation of NMDA receptors. These results are consistent with a role for IL-6 in the neuropathology observed in the developing CNS during injury and disease.

Activation of Fas receptor is required for the increased formation of the disialoganglioside GD3 in cultured cerebellar granule cells committed to apoptotic death

Apoptosis was induced in cultured cerebellar granule cells by lowering extracellular K+ concentrations (usually from 25 to 10 mM). The apoptotic phenotype was preceded by an early and transient increase in the intracellular levels of the disialoganglioside, GD3, which behaves as a putative pro-apoptotic factor. We examined whether activation of Fas receptor mediates the increase in GD3 formation in granule cells committed to die. Degenerating granule cells showed increased expression of both Fas receptor and its ligand (Fas-L), at times that coincided with the increase in GD3 levels and the induction of GD3 synthase mRNA. Addition of neutralizing anti-Fas-L antibodies reduced the extent of 'low-K+'-induced apoptosis and abolished the increase in GD3 levels and GD3 synthase mRNA. Similar reductions were observed in cultures prepared from gld or lpr mice, which harbor loss-of-function mutations of Fas-L and Fas receptor, respectively. In addition, exogenous application of soluble Fas-L further enhanced both the increase in GD3 formation and cell death in cultured granule cells switched from 25 into 10 mM K+. We conclude that activation of Fas receptor is entirely responsible for the increase in GD3 levels and contributes to the development of apoptosis by trophic deprivation in cultured cerebellar granule cells.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

Regulation of glutamate-synthesizing enzymes by NMDA and potassium in cerebellar granule cells

The presence of 25 mm potassium (KCl) or N-methyl-d-aspartate (NMDA) in cultured cerebellar granule neurons (CGN) induces a trophic effect, including a specific regulation of the enzymes involved in the glutamate neurotransmitter synthesis. In this study we explored the effect of these conditions on the cytosolic and mitochondrial isoenzymes of aspartate aminotransferase (AAT), and phosphate-activated glutaminase (PAG) in CGN. We found that NMDA and KCl increased the AAT total activity by 40% and 70%, respectively. This effect was mediated by an augmentation in the protein levels (68% by NMDA, 58% by KCl). NMDA raised the Vmax and KCl raised both the maximol velocity (Vmax) and Michaelis constant (Km) of AAT. NMDA increased cytosolic AAT activity by 30% and mitochondrial activity by 70%; KCl increased cytosolic and mitochondrial AAT activity by 60% and 100%, respectively. This activation was also related to an increase in the protein levels. The effect of both conditions on the activity and protein levels were more pronounced in mitochondrial than cytosolic AAT and the increment elicited by KCl was higher in both isoforms than that produced by NMDA. The PAG and AAT mRNA levels were also regulated by incubation with NMDA and KCl similarly to the observed changes in the protein levels. These results suggest that NMDA receptor stimulation during CGN development differentially regulates the two AAT isoenzymes involved in the maturation of CGN and that the regulation of both AAT and PAG occurs also at the mRNA expression level, suggesting the involvement of a mechanism of gene expression regulation.

Inhibition of protein kinase C promotes neuronal survival in low potassium through an Akt-dependent pathway

Cerebellar granule cell neurons undergo apoptotic cell death when subjected to serum-free conditions at physiological concentrations of potassium (5 mM). Protein kinase C (PKC) is known to play a role in preventing neuronal apoptosis under trophic factor deprivation, but its role in protecting cerebellar neurons from cell death under conditions of low potassium is unknown. This study sought to determine the involvement of PKC in neuronal survival and to determine if PKC regulated the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway in low physiologic concentrations of potassium. Incubation with a pan-PKC inhibitor, Ro-31-8220 (2 microm), or a specific PKCAlpha inhibitor, Gö6976, protected cerebellar granule cell neurons from low potassium-mediated cell death. In contrast, phorbol ester (TPA, 100 nm), a PKC activator, increased cell death. Incubation with, Ro-31-8220 rescued neurons from cell death induced by the PI 3-K inhibitor, LY294002, suggesting that Ro-31-8220 may affect Akt phosphorylation. Western blot analysis showed that serum-free, low potassium conditions decreased Akt phosphorylation, which was exacerbated by treatment with LY294002. In contrast, PKC inhibitors, Gö6976 or Ro-31-8220, increased Akt phosphorylation approximately two and four-fold, respectively in low potassium conditions. Because Akt activation appears to be critical in promoting neuronal survival under these culture conditions, increased Akt phosphorylation brought about by inhibiting PKC promotes neuronal survival.

Changes of KCl sensitivity of proliferating neural progenitors during in vitro neurogenesis

The effects of KCl-treatment on the survival and proliferation of NE-4C self-renewing neural progenitor cells were investigated during early phases of in vitro induced neurogenesis. NE-4C cells, derived from the anterior brain vesicles of embryonic mouse (E9), divided continuously under non-inducing conditions, but acquired neuronal features within 6 days, if induced by all-trans retinoic acid (RA). During the first 2 days of induction, the cells went on proliferating and did not show signs of morphological differentiation. In this stage, the resting membrane potential of RA-induced cells adopted more negative values in comparison to non-induced ones. Despite the increased membrane polarity and K+ conductance, addition of 20-50 mM KCl failed to elicit inward Na+ currents and did not induce an increase in the intracellular Ca+ level. Long-term treatment with 25 mM KCl, on the other hand, resulted in a selective loss of cells committed to neuronal fate by both decreasing the rate of cell proliferation and increasing the rate of cell death. The data indicate that the viability and proliferation of neural progenitors are influenced by extracellular K+-level in a differentiation stage-dependent manner.

High extracellular potassium protects against the toxicity of cytosine arabinoside but is not required for the survival of cerebellar granule cells in vitro

Depolarization of cerebellar granule cells with elevated potassium has been described as essential to maintain their survival in culture. There are several reports that this is only specific for rat cerebellar granule cells and not those of mouse. We reinvestigated this issue and found that although high potassium enhanced the survival of cerebellar granule cells from both rat and mouse it was not essential for the survival of those cultures. Further analysis of the culture system indicated that high potassium offered protection against the toxicity of glutamate and cytosine arabinose (Ara C), a standard antimitotic additive to cultures of granule cells. Ara C was found to be toxic to cerebellar cells after potassium withdrawal at concentrations standardly used in culturing these cells (10 microM). High potassium was found to diminish the expression of p53. Ara C toxicity is known to utilize the p53-dependent signaling pathway to initiate apoptosis. Another depolarizing agent, veratridine, offers no protection against Ara C but we provide evidence that the protective effect of high potassium against Ara C is mediated through calcium balance within the cells. We suggest that there is no requirement for high potassium in terms of cerebellar granule cell survival. The previously proposed role for high potassium in the survival cerebellar granule cells is rather a protective effect against toxic substances in serum such as glutamate or against agents such as Ara C.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Roles of thapsigargin-sensitive Ca2+ stores in the survival of developing cultured neurons

The roles of the intracellular calcium pool involved in regulating the Ca2+ profile and the neuronal survival rate during development were studied by using thapsigargin (TG), a specific inhibitor of endoplasmic reticulum (ER) Ca2+-ATPase in cultured cerebellar granule neurons. Measuring the neuronal [Ca2+]i directly in the culture medium, we found a bell-shaped curve for [Ca2+]i versus cultured days in cerebellar granule neurons maintained in medium containing serum and 25 mM K+. The progressive increase in [Ca2+]i of the immature granule neurons (1-4 days in vitro) was abolished by TG, which resulted in massive neuronal apoptosis. When the [K+] was lowered from 25 to 5 mM, neither the progressively increasing [Ca2+]i nor the survival of immature granule neurons was significantly changed over 24-h incubation. Similarly, TG caused a dramatic decrease in the [Ca2+]i and survival rate of these immature neurons when switched to 5 mM K+ medium. Following maturation, the granule neurons became less sensitive to TG for both [Ca2+]i and neuronal survival. However, TG can protect mature granule neurons from the detrimental effect of switching to a 5 mM K+ serum-free medium by decreasing [Ca2+]i to an even lower level than in the respective TG-free group. Based on these findings, we propose that during the immature stage, TG-sensitive ER Ca2+-ATPase plays a pivotal role in the progressive increase of [Ca2+]i, which is essential for the growth and maturation of cultured granule neurons.

Calcineurin controls inositol 1,4,5-trisphosphate type 1 receptor expression in neurons

In the central nervous system, release of Ca2+ from intracellular stores contributes to numerous functions, including neurotransmitter release and long-term potentiation and depression. We have investigated the developmental profile and the regulation of inositol 1,4,5-trisphosphate receptor (IP3R) and ryanodine receptor (RyR) in primary cultures of cerebellar granule cells. The expression of both receptor types increases during development. Whereas the expression of type 1 IP3R appears to be regulated by Ca2+ influx through L type channels or N-methyl-D-aspartate (NMDA) receptors, RyR levels increase independently of Ca2+. The main target of Ca2+-influx-regulating IP3R expression is the Ca2+ calmodulin-dependent protein phosphatase calcineurin, because pharmacological blockade of this protein abolishes IP3R expression. Although calcineurin has been shown to regulate the phosphorylation state of the IP3R, the effect described here is at the transcriptional level because IP3R mRNA changes in parallel with protein levels. Thus, calcineurin plays a dual role in IP3R-mediated Ca2+ signaling: it regulates IP3R function by dephosphorylation in the short-term time scale and IP3R expression over more extended periods.

Expression of the GABAA receptor delta subunit is selectively modulated by depolarization in cultured rat cerebellar granule neurons

The levels of several GABAA receptor subunit mRNAs increase as cerebellar granule neurons migrate to their adult positions and receive excitatory mossy fiber input. Despite the temporal similarity of these increases in transcript expression in vivo, studies in cultured granule neurons demonstrated that the subunit mRNAs are differentially regulated. To address the possibility that neuronal activity regulates transcript expression, GABAA receptor subunit mRNA levels were assessed in cultured granule neurons grown in chemically defined, serum-free medium containing either nondepolarizing (5 mM) or depolarizing (25 mM) KCl concentrations. Whereas the delta subunit mRNA was almost undetectable in cultures maintained in nondepolarizing medium, an eightfold increase occurred between days 2 and 4 in cultures grown in depolarizing medium. Furthermore, delta subunit transcript expression was reduced by 76 +/- 6% when neurons in depolarizing medium were switched into nondepolarizing medium. The importance of depolarization in the initiation and maintenance of subunit transcript expression in granule neurons was selective for the GABAA receptor delta subunit. These changes in transcript expression involved calcium entry through L-type calcium channels. Nifedipine treatment (1 microM) both reduced intracellular calcium and decreased delta subunit mRNA expression by 79 +/- 4%. Furthermore, inhibition of Ca2+/calmodulin-dependent protein kinases (CaM kinases) by KN-62 (1 microM) also reduced delta subunit transcript expression. These studies demonstrate that KCl-induced depolarization, a condition that mimics the effects of neuronal activity, selectively modulates GABAA receptor delta subunit mRNA expression through a pathway involving calcium entry and activation of a CaM kinase.

The expression of plasma membrane Ca2+ pump isoforms in cerebellar granule neurons is modulated by Ca2+

Plasma membrane Ca2+ ATPase (PMCA) pump isoforms 2, 3, and 1CII are expressed in large amounts in the cerebellum of adult rats but only minimally in neonatal cerebellum. These isoforms were almost undetectable in rat neonatal cerebellar granule cells 1-3 days after plating, but they became highly expressed after 7-9 days of culturing under membrane depolarizing conditions (25 mM KCl). The behavior of isoform 4 was different: it was clearly detectable in adult cerebellum but was down-regulated by the depolarizing conditions in cultured cells. 25 mM KCl-activated L-type Ca2+ channels, significantly increasing cytosolic Ca2+. Changes in the concentration of Ca2+ in the culturing medium affected the expression of the pumps. L-type Ca2+ channel blockers abolished both the up-regulation of the PMCA1CII, 2, and 3 isoforms and the down-regulation of PMCA4 isoform. When granule cells were cultured in high concentrations of N-methyl-D-aspartic acid, a condition that increased cytosolic Ca2+ through the activation of glutamate-operated Ca2+ channels, up-regulation of PMCA1CII, 2, and 3 and down-regulation of PMCA4 was also observed. The activity of the isoforms was estimated by measuring the phosphoenzyme intermediate of their reaction cycle: the up-regulated isoforms, the activity of which was barely detectable at plating time, accounted for a large portion of the total PMCA activity of the cells. No up-regulation of the sarcoplasmic/endoplasmic reticulum calcium pump was induced by the depolarizing conditions.

Neurotrophins protect cultured cerebellar granule neurons against the early phase of cell death by a two-component mechanism

Cerebellar granule neurons cultured with serum develop a mature neuronal phenotype, including stimulus-coupled release of glutamate, and depend on elevated potassium for survival. We find that cells cultured with serum undergo two phases of cell death. By 6 d in vitro, 30-50% of the cells present are dead; after this time the remaining cells die. Elevated potassium prevents only this later phase of death, whereas neurotrophins protect these cells against the early phase of death. Factors that bind p75(NTR) or TNF-R, members of the same receptor family, exhibit voltage-sensitive calcium channel-dependent protection, whereas ligands of expressed Trk receptors show additional calcium channel-independent protection. The cells express TrkB protein and show elevated c-Fos and c-Jun levels in response to BDNF. No TrkA is detected, although p75(NTR) protein is expressed and NGF induces depolarization-dependent elevation of c-Jun levels. In the presence of the protein kinase C inhibitor bisindolylmaleimide, BDNF-induced survival promotion is reduced partially, whereas NGF-induced death is unmasked. Basal survival mechanisms are insensitive to inhibition of PK-C or PI-3 kinase. We conclude that BDNF promotes survival in part via its TrkB receptor, whereas there is an additional pathway promoting survival and elevating c-Jun evoked by both NGF and BDNF via a non-Trk receptor.

Weaver mutant mouse cerebellar granule cells respond normally to chronic depolarization

We studied the effects of chronic K(+)-induced membrane depolarization and treatment with N-methyl-D-aspartate (NMDA) on cerebellar granule cells (CGCs) from weaver mutant mice and non-weaver litter-mates. The weaver mutation is a Gly-to-Ser substitution in a conserved region of the Girk2 G protein-coupled inward rectifying potassium channel [Patil N., Cox D. R., Bhat D., Faham M., Myers R. M. and Peterson A. S. (1995) Nature Genet. 11, 126-129] which induces early death of CGCs. The biochemical differentiation of CGCs was estimated as the rate of 2-deoxy-D-glucose accumulation and the expression of neural cell adhesion molecule (NCAM). High (25 mM) K+ ion concentration or treatment with NMDA greatly promoted the biochemical differentiation of both weaver mutant and non-weaver litter-mate mouse CGCs. In contrast to the marked effect on biochemical differentiation in both weaver and non-weaver mice CGSs, chronic high K+ treatment only had limited effect on survival. The survival of weaver mutant mouse CGCs in medium containing 5 mM K+ ions was very low, only 20% of the plated cells surviving at 7 days after plating, as opposed to the 50% for non-weaver CGCs. Chronic high K+ treatment improved the relative survival of weaver mutant mouse CGCs 1.6 2.2-fold and that of non-weaver CGCs 1.2-1.4-fold; the same number of CGCs (about 20% of the plated cells) were rescued by high K+ in both types of culture. The findings indicate that, in culture weaver mutant mouse, CGCs have a normal response to membrane depolarization and that the normal function of the Girk2 potassium channel is not critical for the survival of differentiated CGCs.

The plasma membrane calcium pump: recent developments and future perspectives

The Ca2+ pump of the plasma membrane (PMCA) is regulated by a number of agents. The most important is calmodulin (CaM), which binds to a domain located in the C-terminal portion of the pump, removing it from an autoinhibitory site next to the active site. The CaM-binding domain is preceded by an acidic sequence which contains a hidden signal for endoplasmic reticulum (ER) retention. Chimeras of the PMCA and endoplasmic reticulum (SERCA) pumps have revealed the presence of a strong signal for ER retention in the first 45 residues of the SERCA pump. Four gene products of the PMCA pump are known: two of them (1 and 4) are ubiquitously expressed, two (2 and 3) are specific for nerve cells and may be induced by their activation. Mutagenesis work has identified four residues in three of the transmembrane domains of the pump which may be components of the trans-protein Ca2+ path. The mutation of two of these residues alters the membrane targeting of the pump.

Nitric oxide enhances amino acid release from immature chick embryo retina

Nitric oxide (NO) was investigated for its ability to induce amino acid release from immature chick retina. The production of endogenous NO by activation of NO synthase after stimulation of N-methyl-D-aspartate (NMDA) subtype of glutamate receptor caused a significant increase in basal release of gamma-aminobutyric acid (GABA) and glutamine, whereas a more modest increase in the glutamate release was also observed. The exposure of chick retina from 9-day-old embryos to NO-generating compounds, S-nitroso-N-acetylpe-nicillamine (SNAP) and sodium nitroprusside (SNP) produced a dose dependent increase in GABA, glutamine, and glutamate release. This effect was reduced by about 80% by haemoglobin. These results indicate that NO has a stimulatory effect on amino acid release from chick embryo immature retina. However, this effect does not appear to involve a cGMP-related mechanism because 8-bromo-cGMP, a stable analogue of cGMP, failed to affect spontaneous amino acid release and because zaprinast did not enhance NMDA-stimulated release. In conclusion, our present observations may account for a role of NMDA-mediated events in the biochemical maturation under depolarizing conditions.

Effect of K+- and kainate-mediated depolarization on survival and functional maturation of GABAergic and glutamatergic neurons in cultures of dissociated mouse cerebellum

The effect of the depolarizing agents, an elevated potassium concentration (25 mM) or kainic acid (50 microM) on neuronal survival and differentiation was investigated in cultures of dissociated neurons from cerebella of 7-day-old mice. When maintained in the presence of an antimitotic agent such cultures consist primarily of glutamatergic and GABAergic neurons. Cell survival was monitored by measurement of DNA, and differentiation by determining uptake and depolarization coupled release of glutamate (D-aspartate as label) and GABA. The depolarizing agents were added separately or together either from the start of the culture period (7-8 days) or at day 5 in culture. The main findings are that K+ depolarization is important for differentiation of glutamatergic neurons but not for GABAergic neurons. This depolarizing signal is important during the early phase of development in culture. For glutamatergic neurons, kainate may replace K+ as a depolarizing signal whereas in case of the GABAergic neurons, kainate was toxic particularly during the late phase of development. It was further observed that the glutamatergic neurons when maintained in a medium with 5 mM K+ during the first 5 days in culture became sensitive to kainate toxicity when this amino acid was added at day 5. This was not the case when the medium contained 25 mM K+ from the start of the culture period.

Balanced interaction of growth factors and taurine regulate energy metabolism, neuronal survival, and function of cultured mouse cerebellar cells under depolarizing conditions

The development of neuronal cells in a given cellular environment requires mechanisms that dynamically regulate the balanced interactions of multiple factors which are known to control maintenance and plasticity in function of neurons throughout constantly changing extracellular conditions. Periodic release of excitatory amino acids from both developing glial and neuronal cells into the extracellular environment and their uptake has been shown to stimulate neuronal function in concert with growth factors that control the degree of depolarization and, therefore, neuronal function. This study attempts to characterize the critical concentrations of these factors either alone or together in relation to energy metabolism, cell survival and function. We demonstrate a close correlation between energy metabolism of neuronal cells, controlled by the combination of growth-factors (beta FGF, BDNF), and glutamate-taurine as well as K+ in depolarizing concentrations (10-25 mM), during the balancing act of neuronal survival or death, and neuronal function. These functions depend on medium conditions (energy sources, ion composition), the ratio of glial cells versus neurons and cell density. Granule cell migration as a measure of developmental neuronal function was analyzed in the presence of various combinations of growth factors and taurine under various depolarizing conditions (glutamate, K+). We found that K+ concentrations > 7 mM in BME and 10% horse serum blocked migration in less than 30 min. Taurine did not prevent this effect. However, in the presence of HEPES as well as in F12-medium with HEPES, taurine restored granule cell migration. On the other hand, glutamate-or NMDA-mediated depolarization stopped migrating granule cells while NMDA antagonists extended the period of migration. Taurine amplified the stop-signal in the presence of glutamate agonists but increased the number of migrating cells in the absence of glutamate. Thus, the mechanisms of glutamate receptor mediated excitotoxicity, possibly by reducing Ca2+ influx under depolarizing conditions, but amplifies the stop-signal, Ca2+ levels may not control granule cell migration.

Depolarization and hypoxia-induced cell damage in serum-free cultures of the rat cortex, and related extracellular glutamate changes

Experiments were carried out to clarify the influence of K+-induced chronic membrane depolarization on cytotoxicity and changes in extracellular glutamate, as induced by hypoxia in serum-free cortical cultures. Excitotoxic cell death was examined by measuring lactic dehydrogenase (LDH) activity released into the culture medium. In culture grown in the presence of 25 mM K+, morphological injury occurred during a 4 h exposure to hypoxia, together with a substantial efflux of LDH. In hypoxic cultures, extracellular glutamate concentrations were elevated and these responses were absent in cultures grown in physiological medium (K+ = 5.4 mM), even with 16 h of hypoxia. In cultures at 25 mM K+, the cytotoxicity induced by hypoxia was attenuated by NMDA receptor antagonists, in a concentration dependent manner. We also examined the effects of excitatory amino acids, agonists of the main glutamate receptor classes (glutamate, NMDA, kainate, and AMPA). In both 5.4 nM and 25 mM K+ cultures, a dose dependent release of LDH was induced by a long exposure to glutamate receptor agonists, although the release of LDH in the 5.4 mM K+ was less than that in the 25 mM K+ cultures. Despite of the expression of the glutamate receptor in the 5.4 mM K+ cultures, hypoxic neuronal damage did not occur. These results suggest that when cultures grown in a chronically depolarizing environment are exposed to hypoxia, they are damaged by an excitotoxic mechanism in which the main cause seems to be the glutamate released into the medium at high extracellular levels.

Agonist-induced down-regulation of NMDA receptors in cerebellar granule cells in culture

In contrast to the acute toxic effect of NMDA on mature cerebellar granule cells, chronic treatment with NMDA (140 microM from 1 to 9 days in vitro) did not compromise cell survival. Such treatment markedly suppressed NMDA receptor activity: at 8 days in vitro NMDA-induced 45Ca2+ influx was reduced by approximately 60% and acute exposure to NMDA (highest concentration tested, 1 mM) at 9 days in vitro did not cause detectable toxicity. The reduction in NMDA receptor activity was accompanied by a significant decrease (approximately 80% at 9 days in vitro) in the level of the NR1 and the NR2A NMDA receptor subunit protein, detected using the selective photoaffinity ligand [125I]CGP55802A. It seems, therefore, that the agonist-induced decrease in NMDA receptor activity is due to receptor down-regulation. In contrast to the marked influence of chronic NMDA exposure on the cellular content of the NMDA receptor subunit proteins, mRNA levels of the different subunits (NR1, NR2A, NR2B and NR2C) were not significantly affected. It seems, therefore, that agonist-induced down-regulation of the NMDA receptor involves critically mRNA translation and/or post-translational regulation.

GABA attenuates the neurotoxic effects of ethanol in neuron-enriched cultures from 8-day-old chick embryo cerebral hemispheres

Neuron-enriched cultures were prepared from 8-day-old chick embryo cerebral hemispheres and exposed to ethanol (50 mM), GABA (10(-5) M) and ethanol (50 mM) + GABA (10(-5) M) from day 4 to 8 in culture. At day 8, control, ethanol, GABA and ethanol + GABA-treated cultures were examined morphologically and biochemically. Choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were used as markers for cholinergic and GABAergic neuronal phenotypic expression, respectively. Control cultures showed more numerous and large neuronal aggregates as well as prominent neuritic bundles. Moreover, cultures treated with GABA depicted even more numerous neuronal aggregates with interconnecting neurites as compared to control. In contrast, ethanol-treated cultures exhibited smaller neuronal aggregates with less prominent neuritic bundles than control. However, cultures treated concomitantly with ethanol + GABA exhibited numerous and larger aggregates than cultures treated with ethanol alone. Neuritic bundles which were highly reduced in ethanol-treated cultures became prominent in the presence of GABA. As previously reported, ethanol alone enhanced ChAT and reduced GAD activities. GABA given alone enhanced the expression of both neuronal phenotypes. When GABA was given concomitantly with ethanol the decline in GAD and the rise in ChAT observed in ethanol-treated cultures was restored by GABA to almost control levels. Thus, ethanol-induced alterations in morphology and neuronal phenotypes were counteracted by the neurontrophic effect of GABA.

Characterization of nicotinic acetylcholine receptors expressed in primary cultures of cerebellar granule cells

Nicotinic acetylcholine receptors (nAChRs), like other calcium permeable channel receptors, may play a crucial role during neuronal development. We have characterized nAChRs in developing mouse cerebellar granule cells in primary culture. L-[3H]Nicotine, [3H]cytisine and [125I]alpha-bungarotoxin binding experiments revealed the presence of a single class of saturable and specific high affinity binding sites for each ligand. The expression of these nicotinic binding sites followed a developmental pattern reaching a maximum during the establishment of excitatory amino acid synaptic contacts. Immunolabeling with monoclonal antibodies to nAChR subunits revealed the presence of alpha 4 and beta 2 subunits in most neurons. Moreover, some neuronal cells displayed a somatic as well as a neuritic localization for the alpha 7 subunit as shown by [125I]alpha-bungarotoxin autoradiography. The reverse transcription-polymerase chain reaction (RT-PCR) detected the presence of mRNAs for alpha 3, alpha 4, alpha 5, alpha 7, beta 2 and beta 4 nAChR subunits. Non-neuronal cells did not express nAChRs, as shown by [3H]nicotine and [125I]alpha-bungarotoxin binding, immunocytochemistry and PCR. Maximum Ca2+ influx elicited by nicotine, and partly sensitive to alpha-bungarotoxin, was observed around 10-14 days after plating. This correlated with the time period at which the highest number of nicotine binding sites was detected. Sensitivity to several NMDA receptor antagonists as well as to removal of endogenous glutamate by pyruvate transaminase treatment revealed a glutamatergic component in the nicotine stimulated calcium influx. The time-dependent specific nAChR expression and the potential association between nAChRs and NMDA receptor activation suggest that nAChRs may regulate glutamatergic activity during synaptogenesis in cerebellar granule cells.

Role of GABAB receptors in intracellular Ca2+ homeostasis and possible interaction between GABAA and GABAB receptors in regulation of transmitter release in cerebellar granule neurons

The expression of GABAB receptors in cultured mouse cerebellar granule cells was investigated in binding experiments using [3H](S,R)-baclofen as well as in functional assessment of the ability of (R)-baclofen to interact with depolarization (15-40 mM KCl) coupled changes in intracellular Ca2+ homeostasis and neurotransmitter release. In the latter case a possible functional coupling between GABAA and GABAB receptors was investigated. The binding studies showed that the granule cells express specific binding sites for (R)-baclofen. The number of binding sites could be increased by exposure of the cells to the GABAA receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) during the culture period. Pretreatment of the neurons with pertussis toxin showed that the GABAB receptors are coupled to G-proteins. This coupling was, however, less pronounced when the cells had been cultured in the presence of THIP. When 45Ca2+ uptake was measured or the intracellular Ca2+ concentration ([Ca2+]i) determined using the fluorescent Ca2+ chelator Fluo-3 it could be demonstrated that culturing the neurons in THIP influences intracellular Ca2+ homeostasis. Moreover, this homeostasis was found to be functionally coupled to the GABAB receptors as (R)-baclofen inhibited depolarization-induced increases in 45Ca2+ uptake and [Ca2+]i. (R)-Baclofen also inhibited K(+)-induced transmitter release from the neurons as monitored by the use of [3H]D-aspartate which labels the neurotransmitter pool of glutamate. Using the selective GABAA receptor agonist isoguvacine it could be demonstrated that the GABAB receptors are functionally coupled to GABAA receptors in the neurons leading to a disinhibitory action of GABAB receptor agonists.

Chronic mild acidosis specifically reduces functional expression of N-methyl-D-aspartate receptors and increases long-term survival in primary cultures of cerebellar granule cells

Previous studies suggest that chronic depolarization by addition of 25 mM KCl or N-methyl-D-aspartate to primary cultures of cerebellar granule cells promotes expression of the N-methyl-D-aspartate subtype of glutamate receptor, as determined by electrophysiological responsiveness and susceptibility to excitotoxicity. Recent studies have demonstrated that acute mild acidosis reduces N-methyl-D-aspartate receptor channel activity by a non-competitive action of H+ on an extracellular site of the receptor channel complex. Since the level of N-methyl-D-aspartate receptor expression in granule cell cultures is activity-dependent, we examined whether chronic mildly acidotic culture conditions would selectively diminish the level of N-methyl-D-aspartate responsiveness in granule cells, in effect producing a functional level of expression more comparable to that observed in vivo. To test this, cerebellar granule cells from eight-day neonatal rats were grown in an HCO3-buffered medium containing elevated K+ (25 mM KCl) either under standard conditions (95% air/5% CO2, pH 7.4), or under chronic mildly acidotic conditions (90% air/10% CO2, estimated pH of 7.1). Glutamate receptor subtype expression was subsequently assessed using standard neurotoxicity assays, a quantitative immunoblotting assay for N-methyl-D-aspartate receptors and whole cell patch clamp recordings. Cells grown in the 10% CO2 environment exhibited a significant reduction in susceptibility to L-glutamate neurotoxicity (at least 10-fold), but not kainate-induced neurotoxicity, relative to cells grown in 5% CO2. In both culture conditions, L-glutamate- and kainate-induced toxicity were mediated by activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, respectively, as determined by the sensitivity of agonist-induced toxicity to specific receptor antagonists. Using polyclonal antibodies generated against a peptide sequence recognizing five of eight splice variants in the common "R1" subunit of N-methyl-D-aspartate receptors, a 31% reduction in the amount of immunoreactive protein was observed in membrane preparations from cells grown in 10% CO2, relative to the amount detected in cells grown in 5% CO2. Moreover, perfusion of cells with glutamate (50 microM) in a nominally Mg(2+)-free solution containing glycine (2 microM) elicited N-methyl-D-aspartate antagonist-sensitive inward currents in proportionately fewer cells cultured in 10% CO2, relative to cells cultured in 5% CO2. Long-term survival was also significantly enhanced in cells exposed chronically to mild acidotic culture conditions, relative to cells grown under standard pH conditions (22 days, 10% CO2 vs 16 days, 5% CO2).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of magnesium on NMDA mediated toxicity and increases in [Ca2+]i and cGMP in cultured neocortical neurons: evidence for distinct regulation of different responses

The effect of varying external concentrations of Mg2+ has been studied on NMDA induced toxicity, increases in the intracellular calcium concentration, [Ca2+]i, and cGMP production in cultured neocortical neurons. The neurotoxic potency of NMDA during a 5 h exposure period in 7-day-old cultures was examined in three different exposure media: phosphate buffered saline, PBS (137 mM NaCl, 2.7 mM KCl, 7.3 mM Na2HPO4, 1.5 mM KH2PO4, 0.9 mM CaCl2, 0.6 mM MgCl2; pH 7.4); PBS without addition of Mg2+; and Neuronal Dulbecco's Minimal Essential Medium (NDMEM = DMEM with a final concentration of KCl of 25.5 mM KCl; cf. Experimental Procedures). In the presence of Mg2+, no toxicity of NMDA was observed in PBS (ED50 > 1000 microM) whereas omission of Mg2+ in PBS resulted in an ED50 value for NMDA of 9 +/- 3 microM. Using NDMEM as the exposure medium, an intermediate neurotoxic potency of NMDA (ED50 = 40 +/- 5 microM) was observed. This intermediate value is probably due to partial attenuation of the Mg2+ block of the NMDA associated cation channel by the depolarizing conditions in NDMEM (with 25.5 mM KCl). Diminishing the external Mg2+ concentration also potentiated the increase in [Ca2+]i after stimulation with NMDA. This potentiation was maximally 3-fold (obtained at 300 microM NMDA), and the IC50 was 15 +/- 2 microM. Surprisingly, the NMDA induced production of cGMP was not sensitive to variations in the external Mg2+ concentration. These findings of distinct regulatory mechanisms of different NMDA receptor coupled responses may indicate the existence of several types of NMDA receptors.

Growth conditions differentially modulate the vulnerability of developing cerebellar granule cells to excitatory amino acids

The survival of immature nerve cells in a cerebellar culture, predominantly excitatory granule cells, is known to be promoted by chronic exposure to high K+ (> 20 mM) or glutamate (Glu) receptor agonists. These treatments are believed to mimic the in vivo effect of the incoming glutamatergic afferents, the mossy fibres. Here we report that with maturation the cells become vulnerable to excitatory amino acids (EAAs) and that the characteristics of EAA sensitivity are dependent on the environmental influences being either "trophic" (25 mM K+ or 140 microM NMDA, K25 or K10 + NMDA) or "non-trophic" (10 mM K+, K10). Toxicity was assayed routinely at 9 days in vitro (DIV) after 24 h exposure to EAAs. Under all the tested conditions, the effect of Glu was mediated exclusively through NMDA receptors. However, the efficacy and potency of Glu were high in K25- and K10 + NMDA-grown cells compared with K10-grown cells. Growth conditions had the same influence on NMDA as on Glu-induced toxicity, but with the following special features: (1) in comparison with K25 cells, the potency of NMDA was significantly lower in K10 + NMDA cells. The K10 + NMDA cultures behaved as if they were completely insensitive to the NMDA which is present in their growth medium. (2) The K10-grown cells were not vulnerable to NMDA, unless the cell membrane was depolarised by shifting the cells into K25 medium. The efficacy of NMDA became then similar to that in K25 cultures, although the potency was about 7-fold less. Thus NMDA receptors can be activated by the depolarisation of K10 cells, implying the operation of Mg2+ blockade of the channel at normal resting membrane potential. Although non-NMDA receptors did not seem to be involved in Glu toxicity, cells were vulnerable to kainate, which killed significantly more cells than Glu (about 80% vs 70%). This was partly due to the resistance of GABAergic interneurons present in the cultures to Glu- or NMDA-induced toxicity. In contrast to the effects of Glu or NMDA, KA vulnerability was lower in cells grown in K25 or K40 than K10 medium (rank order K10 > K25 > K40). Under our experimental conditions, cultured cells were resistant to AMPA, quisqualate and the selective metabotropic Glu receptor agonist 1S,3R-ACPD. Collectively, the observations indicated that EAA sensitivity of cultured cerebellar interneurons is significantly and differentially influenced by environmental factors, believed to mimic in vivo trophic influences on these cells.

The survival of cultured mouse cerebellar granule cells is not dependent on elevated potassium-ion concentration

The effects of K(+)-induced membrane depolarization were studied on the survival and biochemical parameters in mouse and rat cerebellar granule cells grown in micro-well cultures. Cell numbers were determined by estimating DNA content using the Hoechst 33258 fluorochrome binding assay. DNA from degenerated cells was removed by prior DNAase treatment. These DNA estimates of cell numbers were comparable with values obtained by direct counting of fluorescein diacetate-stained viable cells. In agreement with previous studies, the survival of rat granule cells was promoted by increasing the concentration of K+ in the medium from 5 to 25 mM throughout a 7-day culture period. In contrast, mouse granule cells survived in culture containing 'low' K+ (5 or 10 mM), as well as in the presence of 'high' K+ (25 mM). On the other hand, several biochemical parameters in mouse granule cells were markedly increased by cultivation in 'high' as compared with 'low' K(+)-containing media, demonstrated by increased fluorescein diacetate esterase activity, enhanced rate of NADPH-dependent tetrazolium reduction, augmented 2-deoxy-D-glucose accumulation and increased N-methyl-D-aspartate-evoked 45Ca2+ influx. It was concluded that although cultivation in 'high' K+ promotes biochemical differentiation in mouse cerebellar granule cells, these cells differ from their rat counterparts in that they do not develop a survival requirement for K(+)-induced membrane depolarization.

Alteration in oxidative metabolism of alanine in cerebellar granule cell cultures as a consequence of the development of the ability to utilize alanine as an amino group donor for synthesis of transmitter glutamate

Formation of 14CO2 from labeled alanine was measured in cultured cerebellar granule cells grown in the combined presence of alanine, alpha-ketoglutarate and glutamine or in the presence of glutamine alone. This was done in order to study whether the utilization of alpha-ketoglutarate plus alanine as precursors of transmitter glutamate, induced by culturing in the presence of these compounds, is reflected by an increase of CO2 production from alanine during stimulation with an elevated extracellular potassium concentration. Potassium stimulated CO2 production from alanine was present only in the cells grown in the combined presence of alanine, alpha-ketoglutarate and glutamine. This stimulation was abolished by glutamine, but not by ouabain, indicating that the depolarizing-induced stimulation of alanine metabolism is a consequence of increased release of transmitter glutamate formed from alanine, not a simple result of an increased metabolic rate.

Glutamate effects on calcium homeostasis in cerebellar granule cells in primary cultures grown under depolarizing and nondepolarizing conditions

Cerebellar granule cell cultures are normally grown under partly depolarizing conditions (in a medium with approximately 25 mM K+), but these cultures can also be grown at a normal potassium concentration (5.4 mM K+), although some of their characteristics are altered. In this study, intracellular free calcium concentration and 45Ca uptake were measured in cerebellar granule cell cultures grown at either 25 or 5.4 mM extracellular potassium in the presence of glutamate, and/or some of its subtype-specific agonists and antagonists. Granule cells in cultures grown at 25 mM K+ responded to glutamate, but not to quisqualate, with an increase in free cytosolic calcium concentration and in 45Ca uptake. This increase in free cytosolic calcium concentration was dependent on extracellular calcium and it was antagonized by AP5 and ketamine, NMDA receptor antagonists. In contrast, granule cells in cultures grown at 5.4 mM K+ responded to both glutamate and quisqualate, and these responses were independent of extracellular calcium and not sensitive to AP5 and ketamine. In agreement with this, 45Ca uptake was not affected by glutamate. Neither of the two culture types responded to kainate with an increase in calcium concentration or uptake. These observations indicate that calcium uptake in granule cells in cultures grown at 25 mM K+ reflect NMDA activation of calcium influx, whereas the cells in cultures grown at 5 mM K+ increase cytosolic calcium concentration on account of intracellular release of bound calcium, caused by activation of the metabotropic receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic glutamate receptors in cultured cerebellar granule cells: developmental profile

Excitatory amino acid (EAA)-induced polyphosphoinositide (PPI) hydrolysis was studied during the development in culture of cerebellar granule cells. The developmental pattern was similar using metabotropic glutamate (Glu) receptor (mGluR) agonists, including L-Glu, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid: The stimulation of [3H]inositol monophosphate ([3H]-InsP) formation was low at 2 days in vitro (DIV), but the response increased steeply, reaching a peak at 4 DIV, followed by a progressive decline. In contrast, carbamylcholine-induced PPI hydrolysis exhibited a plateau after a pronounced increase during the first week in vitro. At 6 DIV, but not at 4 DIV, when the activity peaked, PPI hydrolysis elicited by Glu was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, indicating that in cultured granule cells, NMDA receptors contribute to [3H]-InsP formation and that this component of the response develops relatively late. Accordingly, NMDA-induced [3H]-InsP formation, estimated under Mg(2+)-free conditions, increased markedly from very low values at 2 DIV to a plateau at 8-10 DIV. The developmental pattern of EAA-induced PPI hydrolysis was paralleled by changes in the level of an mRNA for a specific mGluR subtype (mGluR1 mRNA). RNA blot analysis performed with the pmGR1 cDNA probe revealed that the hybridization signal in RNA extracts from cultures at 1 DIV was very weak, but mGluR mRNA levels increased dramatically between 1 and 3 DIV, followed by a progressive decrease, so that by 15 DIV the mRNA levels were only approximately 10% of the values at 3 DIV.(ABSTRACT TRUNCATED AT 250 WORDS)

35 mM K(+)-stimulated 45Ca2+ uptake in cerebellar granule cell cultures mainly results from NMDA receptor activation

In primary cultures of cerebellar granule cells, the Ca2+ influx resulting from K+ depolarization (35 mM) was equal to one-third of that observed with 100 microM N-methyl-D-aspartate (NMDA) and was reduced in a major part (90%) by NMDA receptor antagonists. The rank order of potency of these competitive and non-competitive NMDA receptor antagonists was very close to their affinity for the NMDA and phencyclidine sites respectively. Granular cell depolarization with 35 mM K+ also induced a large increase in the extracellular glutamate concentration. Repeated washes of the culture wells, addition of glutamate pyruvate transaminase (+2 mM pyruvate), or pretreatment of the cells with tetanus toxin resulted in a parallel reduction of the extracellular glutamate concentration and 45Ca2+ uptake measured after a 35 mM K+ stimulation. Dihydropyridine (BAY K-8644) stimulated the release of glutamate in a nifedipine-sensitive manner in the presence of 15 mM K+. However, nifedipine (1 microM), which decreased by 60% the K(+)-induced 45Ca2+ uptake, did not reduce the 35 mM K(+)-evoked glutamate release. Taken together, these results demonstrated that in cerebellar granule cell cultures, 90% of the 35 mM K(+)-stimulated 45Ca2+ influx resulted from the release of glutamate and the consecutive activation of NMDA receptors. Activation of these glutamate receptors then allows Ca2+ influx to occur through L-type voltage-operated Ca2+ channels.

Thyrotropin releasing hormone (TRH) and its analog, RGH-2202, accelerate maturation of cerebellar neurons in vitro

We have studied the "trophic" action of thyrotropin releasing hormone (TRH) in cultured cerebellar granule cells, a pure and homogeneous population of glutamatergic neurons. As an index of neuronal maturation, we have measured the uptake of D-[3H]aspartate (a non-metabolizable analog of glutamate) at different days of maturation in vitro (DIV). In control cultures, D-[3H]Aspartate increased linearly during maturation reaching plateau values between 7 and 9 DIV; daily addition of TRH tartrate (TRH-t) or RGH-2202 (a TRH analog) accelerated in a concentration-dependent manner the maturation profile of D-[3H]aspartate uptake. This effect was more pronounced for RGH-2202: in cultures treated daily with RGH-2202, D-[3H]aspartate uptake was fully expressed after 3 DIV. Neither TRH-t nor RGH-2202 significantly increased D-[3H]aspartate uptake in mature cells, excluding a direct action on the glutamate transport system. Both compounds specifically potentiated the increase in [3H]inositol monophosphate formation (but not the stimulation of 45Ca2+ influx) induced by N-methyl-D-aspartate (NMDA) receptor agonists, without affecting the stimulation of inositol phospholipid hydrolysis by quisqualate or carbamylcholine. We suggest that, in cultured cerebellar granule cells, TRH and RGH-2202 enhance the trophic action of endogenous glutamate by amplifying some of the intracellular events that follow the influx of extracellular Ca2+ through NMDA-gated ion channels.