Survival, morphology and adhesion properties of cerebellar interneurones cultured in chemically defined and serum-supplemented medium

Sexually Diergic Trophic Effects of Estradiol Exposure on Developing Bovine Cerebellar Granule Cells

In the mammalian brain, the differentiation of neural cells and the developmental organization of the underlying circuitry are influenced by steroid hormones. The estrogen 17-β estradiol (E2) is one of the most potent regulators of neural growth during prenatal life, synthetized locally from steroid precursors including prenatal testicular testosterone. Estradiol promotes brain differentiation counting sexually dimorphic neural circuits by binding to the estrogen receptors, ER-α and ER-β. The cerebellum has been described as a site of estrogen action and a potentially sexually dimorphic area. The goal of this study was to analyze the capacity of E2 to affect the growth of male and female fetal bovine cerebellar granule. We performed primary cultures of fetal cerebellar granules, and verified the mRNA expression of the ER-α and ER-β in both sexes. Moreover, the distribution of ERs in the male and female cerebellar granules of the second fetal stage was characterized by immunohistochemistry. We measured morphological parameters in presence (or absence) of estradiol administration, focusing on the variations of the dendritic branching pattern of granule neurons. By using the nonparametric combination and permutation testing approach, we proposed a sophisticated multivariate statistical analysis to demonstrate that E2 induces multifarious and dimorphic changes in the granule cells. E2 exerts trophic effects in both female and male granules and this effect is stronger in female. Male granules treated with E2 became similar to female control granule. Bos taurus species has a long gestation and a large brain that offers an interesting alternative in comparative neuroscience.

Neurobasal media facilitates increased specificity of siRNA-mediated knockdown in primary cerebellar cultures

BACKGROUND

Efficient and specific knockdown of proteins in post-mitotic cells such as differentiated neurons can be difficult to achieve. Further, special care must be taken to maintain the health of neurons in vitro. We wanted to achieve knockdown in primary cerebellar granule neurons, which can be effectively grown in Neurobasal™ media.

NEW METHOD

We tested the efficiency of siRNA from the Accell range from Dharmacon™ when delivered in Neurobasal™ media in contrast to the recommended Accell Delivery media provided by the manufacturer.

RESULTS

We observed a more specific knockdown of target in Neurobasal™ media, than in Accell Delivery media when using cerebellar granule neurons. Transfection efficiency and cell viability was comparable between the two media.

COMPARISON WITH EXISTING METHODS

Delivery of siRNA in Neurobasal™ media facilitates increased specificity of the knockdown compared to delivery in Accell Delivery media. The off-target effect observed in Accell Delivery media was not a secondary biological response to downregulation of target, but rather a mixture of specific and non-specific off-target effects.

CONCLUSIONS

Specific knockdown of target can be achieved in primary cerebellar granule cells using Accell siRNAs in Neurobasal™ media. This method ensures specific knockdown in post-mitotic neurons without the need for biosafety level 2 laboratories, additional reagents, or instruments needed by other transfection.

Microglia in Glia-Neuron Co-cultures Exhibit Robust Phagocytic Activity Without Concomitant Inflammation or Cytotoxicity

A simple method to co-culture granule neurons and glia from a single brain region is described, and microglia activation profiles are assessed in response to naturally occurring neuronal apoptosis, excitotoxin-induced neuronal death, and lipopolysaccharide (LPS) addition. Using neonatal rat cerebellar cortex as a tissue source, glial proliferation is regulated by omission or addition of the mitotic inhibitor cytosine arabinoside (AraC). After 7-8 days in vitro, microglia in AraC(-) cultures are abundant and activated based on their amoeboid morphology, expressions of ED1 and Iba1, and ability to phagocytose polystyrene beads and the majority of neurons undergoing spontaneous apoptosis. Microglia and phagocytic activities are sparse in AraC(+) cultures. Following exposure to excitotoxic kainate concentrations, microglia in AraC(-) cultures phagocytose most dead neurons within 24 h without exacerbating neuronal loss or mounting a strong or sustained inflammatory response. LPS addition induces a robust inflammatory response, based on microglial expressions of TNF-α, COX-2 and iNOS proteins, and mRNAs, whereas these markers are essentially undetectable in control cultures. Thus, the functional effector state of microglia is primed for phagocytosis but not inflammation or cytotoxicity even after kainate exposure that triggers death in the majority of neurons. This model should prove useful in studying the progressive activation states of microglia and factors that promote their conversion to inflammatory and cytotoxic phenotypes.

Ammonia-induced oxidative damage in neurons is prevented by resveratrol and lipoic acid with participation of heme oxygenase 1

Ammonia is a metabolite that, at high concentrations, is implicated in neurological disorders, such as hepatic encephalopathy (HE), which is associated with acute or chronic liver failure. Astrocytes are considered the primary target of ammonia toxicity in the central nervous system (CNS) because glutamine synthetase (GS), responsible for ammonia metabolism in CNS, is an astrocytic enzyme. Thus, neuronal dysfunction has been associated as secondary to astrocytic impairment. However, we demonstrated that ammonia can induce direct effects on neuronal cells. The cell viability was decreased by ammonia in SH-SY5Y cells and cerebellar granule neurons. In addition, ammonia induced increased reactive oxygen species (ROS) production and decreased GSH intracellular content, the main antioxidant in CNS. As ammonia neurotoxicity is strongly associated with oxidative stress, we also investigated the potential neuroprotective roles of the antioxidants, resveratrol (RSV) and lipoic acid (LA), against ammonia toxicity in cerebellar granule neurons. RSV and LA were able to prevent the oxidative damage induced by ammonia, maintaining the levels of ROS production and GSH close to basal values. Both antioxidants also decreased ROS production and increased GSH content under basal conditions (in the absence of ammonia). Moreover, we showed that heme oxygenase 1 (HO1), a protein associated with protection against stress conditions, is involved in the beneficial effects of RSV and LA in cerebellar granule neurons. Thus, this study reinforces the neuroprotective effects of RSV and LA. Although more studies in vivo are required, RSV and LA could represent interesting therapeutic strategies for the management of HE.

A systems biology approach to investigating apoptotic stimuli as effectors of cell metabolism: practical application of top-down control analysis to attached neurons

Reduced glycolytic and mitochondrial respiration rates are common features of apoptosis that may reflect key events contributing to cell death. However, it is unclear to what extent the rate changes can be explained by direct alterations in the kinetics of the participating reactions, as changes in the concentrations of intermediates also affect reaction rates. Direct kinetic changes can be identified, ranked, and compared to the indirect effects mediated by the intermediates using top-down control analysis. Flux changes that are explained primarily by direct effects are likely to be prime targets of the pathways that signal death, and thus important contributors to apoptosis. Control analysis concepts relevant to identifying such effects are reviewed. Metabolic flux measurements are essential for this approach, but can be technically difficult, particularly when using adherent cells such as neurons. A simple method is described that renders such measurements feasible.

Synthesis and Structure-Activity Relationship Studies of 3-Substituted Indolin-2-ones as Effective Neuroprotective Agents

Neurodegenerative diseases are a major health problem particularly among the elderly. Drugs to prevent or slow down the death of neurons are urgently needed but are currently unavailable. We previously reported that the c-Raf inhibitor, GW5074 {5-iodo-3-[(3′,5′-dibromo-4′-hydroxyphenyl) methylene]-2-indolinone}, is protective in tissue culture and in vivo paradigms of neurodegeneration. However, at doses slightly higher than those at which it is protective, GW5074 displays toxicity when tested in neuronal cultures. We report herein the synthesis, biological evaluation, and structure-activity relationship (SAR) studies of novel 3-substituted indolin-2-one compounds that are highly neuroprotective but lack the toxicity of GW5074. Of the 45 analogs tested in this study, compounds 7, 37, 39, and 45 were found to be the most potent neuroprotective and thus represent promising lead compounds for preclinical development for the treatment of neurodegenerative disorders.

Assessment of cell viability in primary neuronal cultures

This unit contains five protocols for assaying cell viability in vitro using primary neuronal cultures, including a novel method for use with transfected neurons. Three of the assays are based on the principle that cell death cascades alter membrane permeability. The lactate dehydrogenase (LDH) release assay measures the amount of the cytoplasmic enzyme released into the bathing medium, while the trypan blue and propidium iodide assays measure the ability of cells to exclude dye from their cytoplasm. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay measures the mitochondrial activity of viable cells by quantifying the conversion of the tetrazolium salt to its formazan product. Finally, the fifth assay details the measurement of luciferase expression as an indication of neuronal viability within a relatively small population of transfected neurons.

Developmental neurotoxicity testing in vitro: models for assessing chemical effects on neurite outgrowth

In vitro models may be useful for the rapid toxicological screening of large numbers of chemicals for their potential to produce toxicity. Such screening could facilitate prioritization of resources needed for in vivo toxicity testing towards those chemicals most likely to result in adverse health effects. Cell cultures derived from nervous system tissue have proven to be powerful tools for elucidating cellular and molecular mechanisms of nervous system development and function, and have been used to understand the mechanism of action of neurotoxic chemicals. Recently, it has been suggested that in vitro models could be used to screen for chemical effects on critical cellular events of neurodevelopment, including differentiation and neurite growth. This review examines the use of neuronal cell cultures as an in vitro model of neurite outgrowth. Examples of the cell culture systems that are commonly used to examine the effects of chemicals on neurite outgrowth are provided, along with a description of the methods used to quantify this neurodevelopmental process in vitro. Issues relating to the relevance of the methods and models currently used to assess neurite outgrowth are discussed in the context of hazard identification and chemical screening. To demonstrate the utility of in vitro models of neurite outgrowth for the evaluation of large numbers of chemicals, efforts should be made to: (1) develop a set of reference chemicals that can be used as positive and negative controls for comparing neurite outgrowth between model systems, (2) focus on cell cultures of human origin, with emphasis on the emerging area of neural progenitor cells, and (3) use high-throughput methods to quantify endpoints of neurite outgrowth.

Effect of the L- or D-aspartate on ecto-5'nucleotidase activity and on cellular viability in cultured neurons: participation of the adenosine A(2A) receptors

Glutamate increases the extracellular adenosine levels, an important endogenous neuromodulator. The neurotoxicity induced by glutamate increases the ecto-5'-nucleotidase activity in neurons, which produces adenosine from AMP. L- and D-aspartate (Asp) mimic most of the actions of glutamate in the N-methyl-D-aspartate (NMDA) receptors. In the present study, both amino acids stimulated the ecto-5'-nucleotidase activity in cerebellar granule cells. MK-801 and AP-5 prevented the L- and D-Asp-evoked activation of ecto-5'-nucleotidase. Both NMDA receptor antagonists prevented completely the damage induced by L-Asp, but partially the D-Asp-induced damage. The antagonist of adenosine A(2A) receptors (ZM 241385) prevented totally the L- Asp-induced cellular death, but partially the neurotoxicity induced by D-Asp and the antagonist of adenosine A(1) receptors (CPT) had no effect. The results indicated a different involvement of NMDA receptors on the L- or D-Asp-evoked activation of ecto-5'-nucleotidase and on cellular damage. The adenosine formed from ecto-5'-nucleotidase stimulation preferentially acted on adenosine A(2A) receptor which is probably co-operating with the neurotoxicity induced by amino acids.

Efflux of sphingolipids metabolically labeled with [1-3H]sphingosine, L-[3-3H]serine and [9,10-3H]palmitic acid from normal cells in culture

The membrane complex lipids of human fibroblasts and differentiated rat cerebellar granule cells in culture were metabolically radiolabeled with [1-(3)H]sphingosine, L-[3-(3)H]serine and [9,10-(3)H]palmitic acid. A relevant efflux of radioactive sphingolipids and phosphatidylcholine was observed from cells to the culture medium in the presence of fetal calf serum. This event was independent of the concentration and structure of the metabolic precursor administered to cells, and it was linearly time-dependent. The radioactive lipid patterns present in the medium were different from those present in the cells. Radioactive sphingomyelin and ganglioside GM3 containing short acyl chains were the main species present in the medium from human fibroblasts, while sphingomyelin and GD3 ganglioside in that from neuronal cells. In the absence of proteins in the culture medium, the efflux of complex lipids was much lower than in the presence of serum, and the patterns of released molecules were again different from those of cells.

Neurotoxicity of the pentabrominated diphenyl ether mixture, DE-71, and hexabromocyclododecane (HBCD) in rat cerebellar granule cells in vitro

Polybrominated diphenyl ethers (PBDE) and hexabromocyclododecane (HBCD) are compounds used as additive flame retardants in plastics, electronic equipment, and textiles. The aim of the present study was to investigate the in vitro effects of the pentabrominated diphenyl ether mixture, DE-71, and HBCD on cerebellar granule cells (CGC). Both DE-71 and HBCD induced death of CGC in low micromolar concentrations. The NMDA receptor antagonist MK801 (3 microM), and the antioxidant alpha-tocopherol (50 microM) significantly reduced the cell death. Incubation of the compounds together with the rat liver post-mitochondrial (S9) fraction reduced cell death by 58 and 64% for DE-71 and HBCD, respectively. No ROS formation and no elevation in intracellular calcium were observed. We further demonstrated apoptotic morphology (Hoechst straining) after exposure to low levels of the two brominated flame retardants and signs of DNA laddering were found after DE-71 exposure. However, other hallmarks of apoptosis, like caspase activity, were absent indicating an atypical form of apoptosis induced by DE-71. After intraperitoneal injection of the two compounds both DE-71 and HBCD were found in significant amounts in brain (559 +/- 194 and 49 +/- 13 microg/kg, respectively) and liver (4,010 +/- 2,437 and 1,248 +/- 505 microg/kg, respectively) 72 h after injection. Our results indicate that the lower brominated PBDEs have a higher potency of bioaccumulation than HBCD, and that both compounds have a neurotoxic potential in vitro.

In vitro toxicological evaluation of a dinuclear platinum(II) complex with acetate ligands

In the present study the toxicological potential of a tumor-inhibiting dinuclear platinum(II) complex (bis(acetato)diammine-bis-micro-acetato diplatinum(II) dihydrate (BAP)) was evaluated, utilizing in vitro models of nephrotoxicity, myelosuppression and neurotoxicity. Regarding the discrepancies between the hallmark toxicity of the clinically utilized platinum drugs, we used three distinct referent compounds as follows cisplatin for the assessment of in vitro nephrotoxicity, carboplatin in case of cultured bone marrow cells and oxaliplatin for the determination of the in vitro neurotoxicty, respectively. The results obtained indicate that the investigated dinuclear complex is endowed by a lower potential to induce detrimental effects upon these typically susceptible platinum toxicity cellular populations as compared to the corresponding referent drugs. These findings, together with the previously encountered profound cytotoxic efficiency of this dinuclear platinum(II) complex against human tumor cell lines, recall for a further detailed evaluation of BAP as potential antineoplastic agent.

Effects of glutamate transporter and receptor ligands on neuronal glutamate uptake

The excitatory amino acids (EAAs) transporters regulate the balance between physiological and pathological signaling over stimulation of the glutamatergic system pathway. The effect of transportable substrates and glutamate (Glu) receptor agonists on Glu uptake in neuronal cells was assessed at different conditions. Cells pre-incubated with Glu, L- or D-aspartate (Asp) and washed presented an inhibition on [(3)H]-Glu uptake and this effect was not mimicked by Glu receptors agonists. The effects of L- and D-Asp were not altered by the presence of N-methyl-d-aspartate (NMDA) receptor antagonists. Thus, the reduction on Glu uptake induced by EAAs is probably linked to the transporter activity. In contrast, the presence of NMDA or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) during the pre-incubation and the [(3)H]-Glu uptake assay period increased Glu uptake, whilst kainic acid (KA) had no effect. The NMDA effect was not altered by its antagonists (+/-)-2-amino-5-phosphonopentanoic acid (AP-5) or dizocilpine (MK-801). The SR-ACPD effect was due to the activation of metabotropic Glu receptor, since it was abolished by its antagonist, L(+/-)-2-amino-3-phosphonopropionic acid (L-AP3). Thus, the current studies suggest that the neuronal EAAs transporter is regulated in different manner by transportable substrates and Glu receptor agonists. The possible involvement of this modulation after certain neurotoxicity insults is discussed.

Adenosine receptors co-operate with NMDA preconditioning to protect cerebellar granule cells against glutamate neurotoxicity

N-Methyl-D-aspartate (NMDA) preconditioning is evoked by subtoxic concentrations of NMDA (50 microM), which has been shown previously to lead to transient resistance to subsequent lethal dose of glutamate or NMDA in cultured neurons. The purpose of this study was to investigate the participation of adenosine A1 and A2A receptors on NMDA preconditioning against glutamate-induced cellular damage in cerebellar granule cells. NMDA preconditioning prevented the stimulatory effect induced by glutamate on AMP hydrolysis, but not on ADP hydrolysis. The neuroprotection evoked by NMDA preconditioning against glutamate-induced cellular damage was prevented by the presence of adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT, 100 nM), but not by the adenosine A2A receptors antagonist, (4-(2[7-amino-2-(2-furyl {1,2,4}-triazolo{2,3-a{1,3,5}triazian-5-yl-aminoethyl)phenol (ZM 241385, 50 nM). Interestingly, a long-term treatment with CPT or ZM 241385 alone protected cells against glutamate-induced neurotoxicity. Moreover, the functionality of adenosine A1 receptor was not affected by NMDA preconditioning, but this treatment promoted adenosine A2A receptor desensitization, measured by cAMP accumulation. Taken together, the results described herein suggest that the neuroprotection evoked by NMDA preconditioning against cellular damage elicited by glutamate occurs through mechanisms involving adenosine A2A receptors desensitization co-operating with adenosine A1 receptors activation in cerebellar granule cells.

Microglia in Culture: What Genes Do They Express?

The cell culture model utilized in this study represents one of the most widely used paradigms of microglia in vitro. After 14 days, microglia harvested from the neonatal rat brain are considered 'mature'. However, it is clear that this represents a somewhat arbitrary definition. In this paper, we provide a transcriptome definition of such microglial cells. More than 7,000 known genes and 1,000 expressed sequence tag clusters were analysed. 'Microglia genes' were defined as sequences consistently expressed in all microglia samples tested. Accordingly, 388 genes were identified as microglia genes. Another 1,440 sequences were detected in a subset of the cultures. Genes consistently expressed by microglia included genes known to be involved in the cellular immune response, brain tissue surveillance, microglial migration as well as proliferation. The expression profile reported here provides a baseline against which changes of microglia in vitro can be examined. Importantly, expression profiling of normal microglia will help to provide the presently purely operational definition of 'microglial activation' with a molecular biological correlate. Furthermore, the data reported here add to our understanding of microglia biology and allow projections as to what functions microglia may exert in vivo, as well as in vitro.

Microglia release activators of neuronal proliferation mediated by activation of mitogen-activated protein kinase, phosphatidylinositol-3-kinase/Akt and delta-Notch signalling cascades

Microglia, the resident macrophage of the brain, can release substances that aid neuronal development, differentiation and survival. We have investigated the effects of non-activated microglia on the survival of cultured rat cerebellar granule neurones. Microglial-conditioned medium, collected from primary rat microglial cultures, was used to treat 7-day-in-vitro neurones, and neuronal viability and proliferation was assessed following a further 1 or 7 days in culture. Microglial-conditioned medium enhanced neuronal survival by up to 50% compared with untreated neurones and this effect was completely abated by pretreatment of the microglia with l-leucine methyl ester. The expression of the proliferation marker Ki-67 increased in neuronal cultures treated with microglial-conditioned medium suggesting enhanced proliferation of precursor neurones. Microglial-induced neuronal proliferation could be attenuated by specific inhibition of mitogen-activated protein kinase or phosphatidylinositol-3-kinase/Akt signalling pathways, and by selective fractionation and immunodepletion of the microglial-conditioned medium. Activation of the Notch pathway was enhanced as antibody against the Notch ligand, delta-1, prevented the microglial-induced neuronal proliferation. These results show that microglia release stable neurotrophic factors that can promote neuronal precursor cell proliferation.

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.

Dynamics of membrane lipid domains in neuronal cells differentiated in culture

Treatment with methyl-beta-cyclodextrin (MCD) induced a time- and dose-dependent efflux of cholesterol, sphingolipids, and phosphatidylcholine (PC) from cerebellar neurons differentiated in culture. With a "mild" treatment, the loss of cell lipids induced a deep reorganization of the remaining membrane lipids. In fact, the amount of PC associated with a Triton X-100-insoluble membrane fraction (highly enriched in sphingolipids and cholesterol in nontreated cells) was lowered by the treatment. This suggested a reduction of the lipid domain area. However, the cholesterol and sphingolipid enrichment of this fraction remained substantially unchanged, suggesting the existence of dynamic processes aimed at preserving the segregation of cholesterol and sphingolipids in membrane domains. Under these conditions, the lipid membrane domains retained the ability to sort signaling proteins, such as Lyn and c-Src, but cells displayed deep alterations in their membrane permeability. However, normal membrane permeability was restored by loading cells with cholesterol. When MCD treatment was more stringent, a large loss of cell lipids occurred, and the lipid domains were much less enriched in cholesterol and lost the ability to sort specific proteins. The loss of the integrity and properties of lipid domains was accompanied by severe changes in the membrane permeability, distress, and eventually cell death.

Evaluation of the therapeutic usefulness of botulinum neurotoxin B, C1, E, and F compared with the long lasting type A. Basis for distinct durations of inhibition of exocytosis in central neurons

Seven types (A-G) of botulinum neurotoxin (BoNT) target peripheral cholinergic neurons where they selectively proteolyze SNAP-25 (BoNT/A, BoNT/C1, and BoNT/E), syntaxin1 (BoNT/C1), and synaptobrevin (BoNT/B, BoNT/D, BoNT/F, and BoNT/G), SNARE proteins responsible for transmitter release, to cause neuromuscular paralysis but of different durations. BoNT/A paralysis lasts longest (4-6 months) in humans, hence its widespread clinical use for the treatment of dystonias. Molecular mechanisms underlying these distinct inhibitory patterns were deciphered in rat cerebellar neurons by quantifying the half-life of the effect of each toxin, the speed of replenishment of their substrates, and the degradation of the cleaved products, experiments not readily feasible at motor nerve endings. Correlation of target cleavage with blockade of transmitter release yielded half-lives of inhibition for BoNT/A, BoNT/C1, BoNT/B, BoNT/F, and BoNT/E (31, 25, approximately 10, approximately 2, and approximately 0.8 days, respectively), equivalent to the neuromuscular paralysis times found in mice, with recovery of release coinciding with reappearance of the intact SNAREs. A limiting factor for the short neuroparalytic durations of BoNT/F and BoNT/E is the replenishment of synaptobrevin or SNAP-25, whereas pulse labeling revealed that extended inhibition by BoNT/A, BoNT/B, or BoNT/C1 results from longevity of each protease. These novel findings could aid development of new toxin therapies for patients resistant to BoNT/A and effective treatments for human botulism.

Pituitary adenylate cyclase-activating polypeptide and sonic hedgehog interact to control cerebellar granule precursor cell proliferation

Although positive and negative signals control neurogenesis in the embryo, factors regulating postnatal proliferation are less well characterized. In the vertebrate cerebellum, Sonic Hedgehog (Shh) is an efficacious mitogen for cerebellar granule neuron precursors (GNPs), and mutations activating the Shh pathway are linked to medulloblastoma, a tumor derived from GNPs. Although the mitogenic effects of Shh can be blocked by increasing cAMP or protein kinase A activity, the physiological factors antagonizing this stimulation are undefined. In the embryo, pituitary adenylate cyclase-activating polypeptide (PACAP) receptor 1 (PAC1) signaling regulates neural precursor proliferation. We now show that in the developing cerebellum, PAC1 mRNA colocalizes with gene transcripts for Shh receptor Patched 1 and target gene Gli1 in the external germinal layer. We consequently investigated the interactions of PACAP and Shh in proliferation of purified GNPs in culture. Shh exhibited mitogenic activity in both rat and mouse cultures, stimulating DNA synthesis approximately 10-fold after 48 hr of exposure. PACAP markedly inhibited Shh-induced thymidine incorporation by 50 and 85% in rat and mouse GNPs, respectively, but did not significantly affect the stimulation induced by other mitogens. This selective effect was reproduced by the specific PAC1 agonist maxadilan, as well as by the adenylate cyclase activator forskolin, suggesting that PAC1 provides a potent inhibitory signal for Shh-induced proliferation in developing cerebellum. In contrast, in the absence of Shh, PACAP and maxadilan modestly stimulated DNA synthesis, an effect reproduced by activating protein kinase C. These observations suggest that G-protein-coupled receptors, such as PAC1, serve as sensors of environmental cues, coordinating diverse neurogenetic signals.

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.

Neuronal expression of synaptotagmin-related gene 1 is regulated by thyroid hormone during cerebellar development

Thyroid hormone (TH) is essential for proper brain development, acting through nuclear receptors that modulate the expression of specific genes in response to hormone binding. In a screen for genes regulated by TH in the rat cerebellum, we recently identified a novel gene, synaptotagmin-related gene 1 (Srg1). The Srg1 protein is structurally similar to synaptotagmins, a family of proteins involved in regulating neurotransmission. To elucidate a potential role of Srg1 in brain development, we have investigated the developmental and TH-regulated expression of Srg1 in the neonatal rat brain. We show that expression of both Srg1 RNA and protein is detected only in the brain and specifically in neurons. Srg1 mRNA and protein levels increase postnatally, nearing adult levels after the third postnatal week. Neonatal TH deficiency results in a significant reduction and delay in expression of both Srg1 RNA and protein. Using immunohistochemistry, we were able to detect Srg1 protein in numerous brain regions. In the cerebellum, Srg1 protein is localized to the molecular layer, indicating that it is highly expressed in granule cell axons. To further examine Srg1 expression in cerebellar granule cells (CGCs), we used an in vitro cell culture model. In primary cultures of CGCs, Srg1 expression is significantly reduced in the absence of TH. Srg1 mRNA is rapidly upregulated in cultured CGCs, suggesting a direct response to TH. Neuronal and TH-regulated expression of Srg1, together with its localization to neurites, implicates Srg1 as an important component of the program of gene expression induced by TH in the developing brain.

Changes in the lipid turnover, composition, and organization, as sphingolipid-enriched membrane domains, in rat cerebellar granule cells developing in vitro

In the present paper, we report on the properties of sphingolipid-enriched domains of rat cerebellar granule cells in culture at different stages of neuronal development. The major lipid components of these domains were glycerophospholipids and cholesterol. Glycerophospholipids were 45-75% and cholesterol 15-45% of total lipids of the domains. This corresponded to 5-17% of total cell glycerophospholipids and 15-45% of total cell cholesterol. Phosphatidylcholine, mainly dipalmitoylphosphatidylcholine, was 66-85% of all the glycerophospholipids associated with these domains. Consequently, the palmitoyl residue was significantly enriched in the domains. The surface occupied by these structures increased during development. 40-70% of cell sphingolipids segregated in sphingolipid-enriched membrane domains, with the maximum ganglioside density in fully differentiated neurons. A high content of ceramide was found in the domains of aging neurons. Then, the sphingolipid/glycerophospholipid molar ratio was more than doubled during the initial stage of development, whereas the cholesterol/glycerophospholipid molar ratio gradually decreased during in vitro differentiation. Phosphorylated phosphoinositides, which were scant in the domains of undifferentiated cells, dramatically increased during differentiation and aging in culture. Proteins were minor components of the domains (0.1-2.8% of all domain components). Phosphotyrosine-containing proteins were selectively recovered in the sphingolipid-enriched domain. Among these, Src family protein-tyrosine kinases, known to participate to the process of neuronal differentiation, were associated with the sphingolipid-enriched domains in a way specific for the type of kinase and for the developmental stage of the cell. Proteins belonging to other signaling pathways, such as phosphoinositide 3-kinase and its downstream target, Akt, were not associated with the domains.

Expression and regulation of the LIM-class homeobox gene rlim-1 in neuronal progenitors of the rat cerebellum

To investigate LIM gene function in the rat cerebellar system, we analyzed expression and regulation of the rat homologue of frog Xlim-1 (rlim-1) in vivo and in cultured cells. In developing cerebellum, peak levels of rlim-1 mRNA at postnatal day 8 (p8) are coincident with the peak period of granule cell proliferation. Analysis of rlim-1 protein with a specific antibody showed that expression was also maximal at p8. In situ hybridization showed that at p8 rlim-1 mRNA was expressed in Purkinje and granule cells. Both the proliferative and the premigratory granule cells in the external germinal zone displayed high levels of rlim-1 mRNA expression. Immunocytochemical staining demonstrated that at p8 rlim-1 protein was also present in proliferative and premigratory granule cells. In adult cerebellum (p30), rlim-1 mRNA and protein expression in granule cells was strongly attenuated. The down-regulation of rlim-1 mRNA occurred in granule cells just after the time of final division, coinciding with the onset of their migration. rlim-1 protein was detected in migratory granule neurons. The developmental decrease in rlim-1 mRNA and protein found in vivo was reproduced in pure cerebellar granule cell cultures. In these cultures, granule neurons were postmitotic 1 day after plating but still displayed high levels of rlim-1 protein expression up to 3 days in vitro. Our findings indicate that 1) rlim-1 is likely to act in concert with other genes to specify granule cell fate, 2) rlim-1 expression in granule neurons is regulated autonomously, and 3) rlim-1 protein may also play an important role in granule neuron differentiation and survival. Published 2001 Wiley-Liss, Inc.

The chemokine growth-related gene product beta protects rat cerebellar granule cells from apoptotic cell death through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors

Cultured cerebellar granule neurons are widely used as a cellular model to study mechanisms of neuronal cell death because they undergo programmed cell death when switched from a culture medium containing 25 mM to one containing 5 mM K(+). We have found that the growth-related gene product beta (GRObeta) partially prevents the K(+)-depletion-induced cell death, and that the neuroprotective action of GRObeta on granule cells is mediated through the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type of ionotropic glutamate receptors. GRObeta-induced survival was suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione, which is a specific antagonist of AMPA/kainate receptors; it was not affected by the inhibitor of N-methyl-D-aspartate receptors, 2-amino-5-phosphonopentanoic acid, and was comparable to the survival of granule cells induced by AMPA (10 microM) treatment. Moreover, GRObeta-induced neuroprotection was abolished when granule cells were treated with antisense oligonucleotides specific for the AMPA receptor subunits, which significantly reduced receptor expression, as verified by Western blot analysis with subunit-specific antibodies and by granule cell electrophysiological sensitivity to AMPA. Our data demonstrate that GRObeta is neurotrophic for cerebellar granule cells, and that this activity depends on AMPA receptors.

Prosaposin prevents programmed cell death of rat cerebellar granule neurons in culture

Prosaposin, the precursor of sphingolipid activator proteins (saposin A-D), has been reported to be a neurotrophic factor in vitro and in vivo. Prosaposin mRNA is transiently expressed at a high level in developing cerebellum during the period of granule cell proliferation and maturation, suggesting its significance during development of cerebellum. Here we investigated the neuroprotective effect of prosaposin on cerebellar granule neurons, exposing primary cerebellar granule cells to low K+ which induced programmed cell death. Prosaposin rescued mature cerebellar granule neurons in a bimodal manner. A similar neuroprotective effect was obtained using TX14(A), a 14-mer neurotrophic peptide derivative of prosaposin. An additive neuroprotective effect was observed between BDNF and TX14(A), but not between IGF-1 and TX14(A). Prosaposin rescued 60% of nifedipine sensitive cerebellar granule neurons as well as IGF-1, while BDNF did not. Furthermore, the neuroprotective action of prosaposin was inhibited by LY294002, a specific inhibitor of PI 3-kinase. These findings indicated that prosaposin had a trophic effect upon newborn cerebellar granule cells and that the neuroprotective action was similar to that of IGF-1 rather than BDNF. Prosaposin may play a role in cerebellar development during programmed cell death of cerebellar neurons.

Extracellular potassium concentration regulates proliferation of immature cerebellar granule cells

The present study examines the effect of depolarizing potassium concentrations on the proliferation of immature rat cerebellar neurons. Cells inoculated in serum free medium and 5 mM KCl (5 K) showed a high degree of 3H-thymidine incorporation that decreased 24-48 h after plating as differentiation began. During the first 24 h after inoculation, cells grown in high potassium (25 K), showed a 34 +/- 3% increase (mean +/- S.E.M., n = 12) in 3H-thymidine incorporation as compared with the values observed in 5 K. After 24 h in vitro, cells grown in 25 K showed 23 +/- 3% (mean +/- S.E.M., n = 3) less DNA synthesis than those inoculated in 5 K. The increase in DNA synthesis due to 25 K was blocked by MgCl2 and nifedipine, but not by omega-conotoxin GVIA, suggesting that it is mediated by a Ca2+ influx via voltage-gated calcium channels (VGCC) of the L-subtype. High potassium-induced cell proliferation was blocked by the mitogen-activated protein kinase kinase (MEK1) inhibitor (PD98059, 75 microM). The number of neurons counted after 48 h in vitro in 25 K was 35-100% above of the number obtained with 5 K and this increase also was blocked by MgCl2 and nifedipine. These data support the hypothesis that depolarizing activity during neurogenesis plays a role in the modulation of cerebellar granule cells proliferation.

Bax deletion further orders the cell death pathway in cerebellar granule cells and suggests a caspase-independent pathway to cell death

Dissociated cerebellar granule cells maintained in medium containing 25 mM potassium undergo an apoptotic death when switched to medium with 5 mM potassium. Granule cells from mice in which Bax, a proapoptotic Bcl-2 family member, had been deleted, did not undergo apoptosis in 5 mM potassium, yet did undergo an excitotoxic cell death in response to stimulation with 30 or 100 microM NMDA. Within 2 h after switching to 5 mM K+, both wild-type and Bax-deficient granule cells decreased glucose uptake to <20% of control. Protein synthesis also decreased rapidly in both wild-type and Bax-deficient granule cells to 50% of control within 12 h after switching to 5 mM potassium. Both wild-type and Bax -/- neurons increased mRNA levels of c-jun, and caspase 3 (CPP32) and increased phosphorylation of the transactivation domain of c-Jun after K+ deprivation. Wild-type granule cells in 5 mM K+ increased cleavage of DEVD-aminomethylcoumarin (DEVD-AMC), a fluorogenic substrate for caspases 2, 3, and 7; in contrast, Bax-deficient granule cells did not cleave DEVD-AMC. These results place BAX downstream of metabolic changes, changes in mRNA levels, and increased phosphorylation of c-Jun, yet upstream of the activation of caspases and indicate that BAX is required for apoptotic, but not excitotoxic, cell death. In wild-type cells, Boc-Asp-FMK and ZVAD-FMK, general inhibitors of caspases, blocked cleavage of DEVD-AMC and blocked the increase in TdT-mediated dUTP nick end labeling (TUNEL) positivity. However, these inhibitors had only a marginal effect on preventing cell death, suggesting a caspase-independent death pathway downstream of BAX in cerebellar granule cells.

Media composition modulates excitatory amino acid-induced death of rat cerebellar granule cells

This study examined the effects of maintaining cells in different media and the role of serum in glutamate and NMDA-induced neurotoxicity in rat cerebellar granule cells. Glutamate stimulated a concentration-dependent cell death with similar potency in cerebellar granule cells grown in BME and Neurobasal media without serum. However, the maximal cell death to glutamate and N-methyl-D-aspartate (NMDA) varied in the different media compositions. In the presence of serum, glutamate and NMDA-induced excitotoxicity was abolished, suggesting a factor(s) in serum which influences glutamate-receptor mediated death. The protective effect of serum could be overcome by chronic stimulation with high doses of glutamate. The glutamate-stimulated increase in intracellular calcium load was attenuated in the presence of serum, resulting from an elevated basal calcium level, suggesting an association between raised basal calcium and neuroprotection.

Cell-specific expression of beta-amyloid precursor protein isoform mRNAs and proteins in neurons and astrocytes

The abnormal accumulation of beta-amyloid (A beta) in senile plaques appears to be a central pathological process in Alzheimer's disease. A beta is formed by proteolysis of beta-amyloid precursor protein (APP) with several isoforms generated by alternative splicing of exons 7, 8 and 15. A semi-quantitative reverse transcription (RT)-polymerase chain reaction (PCR) analysis showed that APP695 mRNA lacking exon 7 and 8 was most abundant in primary cultures of rat neurons, while APP770 and APP751 representing, respectively, the full length and exon 8 lacking isoforms predominated in cultured astroglial cells. Antisera AP-2 and AP-4 were produced by immunizing rabbits with keyhole limpet haemocyanin coupled with synthetic peptides representing KPI region APP301-316 and A beta region APP670-686 of APP770, respectively. These polyclonal antisera were purified against the corresponding peptide using affinity chromatography. Western blot analysis of homogenates of relatively enriched neuronal and astroglial cultures showed that these antibodies discretely stained bands of proteins in a cell-specific manner. Dot-blot analysis using AP-2, AP-4 and 22C11 antibodies indicated that, in comparison with neurons, cultured astrocytes contained 3-fold greater KPI-containing APP isoform proteins. The amount of total APP proteins, which include both KPI-containing and KPI-lacking APP isoforms, was approximately 90% higher in astrocytes than in neurons. Consistent with these in vitro findings in cultured astrocytes, in fimbria-fornix lesioned rat hippocampus, labelling with AP-2 antibody, which specifically reacts with KPI-containing APP proteins, was mainly observed in glial fibrillary acidic protein-positive reactive astrocytes in vivo. The results showed that APP isoforms are expressed in a cell type-specific manner in the brain and, since deposition of A beta is closely associated with the expression of KPI-containing APP isoforms, provide further evidence for the involvement of astrocytes in plaque biogenesis.

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.

Inhibition of phosphatidylinositol 3-kinase activity blocks depolarization- and insulin-like growth factor I-mediated survival of cerebellar granule cells

Depolarizing concentrations of potassium promote the survival of many neuronal cell types including cerebellar granule cells. To begin to understand the intracellular mediators of neuronal survival, we have tested whether the survival-promoting effect of potassium depolarization on cerebellar granule cells is dependent on either mitogen-activated protein (MAP) kinase or phosphatidylinositol 3-kinase (PI-3-K) activity. In 7-day cerebellar granule cell cultures, potassium depolarization activated both MAP kinase and PI-3-K. Preventing the activation of MAP kinase with the MEK1 inhibitor PD98059 did not affect potassium saving. In contrast, the survival-promoting effect of 25 mM potassium was negated by the addition of 30 microM LY 294002 or 1 microM wortmannin, two distinct inhibitors of PI-3-K. The cell death induced by PI-3-K inhibition was indistinguishable from the cell death caused by potassium deprivation; LY 294002-induced death included nuclear condensation, was blocked by cycloheximide, and had the same time course as potassium deprivation-induced cell death. Cerebellar granule cells can also be maintained in serum-free medium containing either 100 ng/ml insulin-like growth factor I (IGF-I) or 800 microM cAMP. PI-3-K inhibition completely blocked the survival-promoting activity of IGF-I, but had no effect on cAMP-mediated survival. These data indicate that the survival-promoting effects of depolarization and IGF-I, but not cAMP, require PI-3-K activity.

Metabolic and genetic analyses of apoptosis in potassium/serum-deprived rat cerebellar granule cells

Cerebellar granule cells maintained in medium containing serum and 25 mM potassium undergo an apoptotic death within 96 hr when switched to serum-free medium with 5 mM potassium. Because large numbers of apparently homogeneous neurons can be obtained, this represents a potentially useful model of neuronal programmed cell death (PCD). Analysis of the time course and extent of death after removal of either serum or K+ alone demonstrated that a fast-dying (T(1/2) = 4 hr) population (20%) responded to serum deprivation, whereas a slow-dying (T(1/2) = 25 hr) population (80%) died in response to K+ deprivation. Taking advantage of the complete death after removing both K+ and serum, changes in metabolic events and mRNA levels were analyzed in this model. Glucose uptake, protein synthesis, and RNA synthesis fell to <35% of control by 9 hr after potassium/serum deprivation, a time when 85% of the cells were still viable. The pattern of the fall in these metabolic parameters was similar to that reported for trophic factor-deprived sympathetic neurons. Most mRNAs decreased markedly after K+/serum deprivation. Levels of c-jun mRNA increased fivefold in potassium/serum-deprived granule cells; c-jun is required for cell death of sympathetic neurons. mRNA levels of cyclin D1, c-myb, collagenase, and transin remained relatively constant in potassium/serum-deprived granule cells. These data demonstrate the existence of two populations of granule cells with respect to cell death and define common metabolic and genetic events involved in neuronal PCD.

Brain-derived neurotrophic factor suppresses programmed death of cerebellar granule cells through a posttranslational mechanism

Cerebellar granule cells isolated from 7-d-old rats have been shown to die in vitro unless they are continuously exposed to elevated K+ (25 mM). Here we have characterized this neuronal death, and examined whether its major features are shared with those of sympathetic neurons following nerve growth factor (NGF) deprivation. Granule cells underwent active cell death accompanied by morphological features of apoptosis. Brain-derived neurotrophic factor (BDNF), but not NGF, was capable of preventing this neuronal death by acting posttranslationally. Moreover, semiquantitative RT-PCR, Northern blot, and immunoblot analyses showed that trkB, the signal-transducing receptor for BDNF, was upregulated during neuronal death of granule cells in vitro. These results extend recent findings for the role of BDNF in granule cell development, and suggest that BDNF plays a pivotal role on the regulation of the neuronal death/survival of granule cells.

TGF-beta-induced apoptosis of cerebellar granule neurons is prevented by depolarization

The regulation of programmed cell death in the developing nervous system involves target-derived survival factors, afferent synaptic activity, and hormone- and cytokine-dependent signaling. Cultured immature cerebellar granule neurons die by apoptosis within several days in vitro unless maintained in depolarizing (high) concentrations of potassium (25 mM K+). Here we report that transforming growth factors (TGF)-beta1, -beta2, and -beta3 accelerate apoptosis of these neurons when maintained in physiological (low) K+ medium (5mM K+) as assessed by measures of viability, quantitative DNA fragmentation, and nuclear morphology. TGF-beta-induced apoptosis of these neurons is not blocked by CNTF and LIF, cytokines that enhance neuronal survival when applied alone, or by IGF-I, which prevents apoptosis upon potassium withdrawal. In contrast, neurons that differentiate in high K+ medium for several days in vitro acquire resistance to TGF-beta-mediated cell death. Granule neurons maintained in either low or high K+ medium produce latent, but not bioactive, TGF-beta1 and -beta2. Because neutralizing TGF-beta antibodies fail to augment survival of low K+ neurons, the cerebellar neurons are apparently unable to activate latent TGF-beta. Thus, apoptosis of low K+ neurons is not attributable to endogenous production of TGF-beta. Taken together, our data suggest that TGF-beta may limit the expansion of postmitotic neuronal precursor populations by promoting their apoptosis but may support survival of those neurons that have maturated, differentiated, and established supportive synaptic connectivity.

Parathyroid hormone-related peptide is produced by cultured cerebellar granule cells in response to L-type voltage-sensitive Ca2+ channel flux via a Ca2+/calmodulin-dependent kinase pathway

Parathyroid hormone (PTH)-related peptide (PTHrP) is expressed in the adult mammalian brain, but its function is unknown. Here we show that PTHrP and the PTH/PTHrP receptor are products of cerebellar granule cells in primary culture. Granule cells maintained under depolarizing conditions (25 mM K+) make and release PTHrP. Further, PTHrP-(1-36) stimulates cAMP accumulation in granule neurons in a dose-dependent manner with half-maximal activation at approximately 16 nM. Granule cell PTHrP mRNA is activity-dependent, and the pathway of regulation depends absolutely on the flux of Ca2+ ions through the L-type voltage-sensitive Ca2+ channel and the Ca2+/calmodulin kinase cascade. PTHrP is therefore a neuropeptide whose regulation depends upon L-type voltage-sensitive Ca2+ channel activity, and the gene is expressed under conditions that promote granule cell survival.

Carbon monoxide: an endogenous modulator of the nitric oxide-cyclic GMP signaling system

Carbon monoxide (CO) is an activator of soluble guanylyl cyclase and is implicated as a neuronal messenger. CO production, nitric oxide synthase (NOS) activity, and guanosine 3',5'-monophosphate (cGMP) levels were quantitated in cerebellar granule cell cultures. Metabolic labeling experiments enabled the direct measurement of neuronal CO production in vitro. CO production is significant, and peaked during early stages of culture. NOS activity and cGMP levels synchronously increased as cells matured. Whereas inhibition of NOS depleted cGMP in mature cultures, inhibitors of CO production potentiated the nitric oxide (NO)-mediated cGMP increase. Exogenous CO at similar concentrations to endogenous levels blocked the NO-mediated cGMP increase. These results directly demonstrate that endogenous neuronal CO production is high and indicate that while NO is the major regulator of cGMP in these neurons, CO may modulate the NO-cGMP signaling system.

Neurite outgrowth of murine cerebellar granule cells can be enhanced by aniracetam with or without alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)

To assess the neurotrophic effects of a nootropic drug, aniracetam, we studied neurite extension of mouse cerebellar granule cells in culture with low or with high K+ under different combinations of drugs and then immunohistochemically stained the cells with an antibody against L1, a neural cell adhesion molecule on cerebellar granule cells. Quantitative analyses using parameters of the total neurite length, maximal neurite length and number of branches disclosed that aniracetam, in the presence of high K+ and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), significantly enhanced neurite extension of cultured granule neurons. Aniracetam alone also stimulated neurite extension of cerebellar granule cells at a longer period of culture with low K+ showing a bell-shaped dose response curve with maximal effects at 10 microM. Aniracetam may influence remodeling of the neural network after injury.

Maturation of cerebellar granule cells is delayed in cultures derived from ethanol-treated chick embryos: survival and proliferation studies

Previous studies from this laboratory have shown that ethanol administration to chick embryos during embryonic days 1-3, a critical period of neuroembryogenesis, differentially affects primordial CNS structures. In this study, chick embryos were treated in ovo with ethanol (10 mg/50 microliter/day) at E1 to E3. At 14 days of embryonic age cerebellar (E14CE) granule cell cultures were prepared from both control and ethanol-treated embryos. Growth patterns were evaluated morphologically and the neuronal nature of these cultures was evaluated immunocytochemically. E14CE granule cell cultures exhibited neurofilament immunoreactivity demonstrating the neuronal-nature of these cultures. In addition E14CE granule cultures contained numerous glutamatergic neurons as assessed by positive glutamate immunoreactivity and also some GABAergic neurons as assessed by positive GABA immunoreactivity. Cultures derived from both control and ethanol-treated embryos were labeled with 3H-thymidine and assessed for effects on survival and proliferation in culture. Cultures derived from ethanol-treated embryos showed a higher rate of proliferation and survival during the first 3 days in culture as compared to those derived from controls. However, after 3 days in culture, survival was lower in the cultures from ethanol-treated embryos as compared to those derived from controls. We interpret these findings to mean that (a) ethanol arrested cerebellar granule cell development at an immature state; (b) immature neurons have a higher survival capacity than differentiated neurons; and (c) ethanol accelerates normal neuronal cell death as previously reported.

Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells

The functions of the D4 receptor, a newly cloned D2-like receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-like receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine. These results indicate the presence in the neonatal cerebellum of a functional D4 receptor inhibiting an L-type calcium current, an action involving a Gi/Go protein but independent from adenylate cyclase inhibition.

Monitoring intracellular nitric oxide formation by dichlorofluorescin in neuronal cells

A method for rapid fluorometric assay of intracellular nitric oxide (NO) formation was developed for use in cultured neuronal cells. In a cell-free system 2,7-dichlorofluorescin (DCF), a non-fluorescent species, is oxidized by NO to dichlorofluorescein, a fluorescent compound. Addition of NO to a solution containing DCF increased the fluorescent signal within 10 s and continued to increase slowly over a 10-min period. The intensity of the fluorescence was dependent upon the concentration of NO. In DCF-loaded PC12 cells, addition of NO markedly increased fluorescence (limit of detection = 16 microM NO) and pretreatment with reduced hemoglobin (Hb) inhibited the NO-mediated increase of fluorescence in both the cell-free system and PC12 cells. In PC12 cells loaded with DCF, the NO generator sodium nitroprusside (SNP) produced a rapid increase of fluorescence. To rule out the possibility that reactive oxygen species (ROS) mediated the increased of fluorescence, superoxide dismutase (SOD) and catalase were added to the cuvette. The enzymes did not alter the fluorescence generated after addition of NO to PC12 cells. This assay was used to determine the ability of glutamate to stimulate NO production in cerebellar granule cells. When 10 microM glutamate was added to DCF-loaded cerebellar granule cells, a rapid increase in fluorescence was noted. The fluorescence was blocked approximately 50% after addition of either Hb or SOD, or by pretreatment with NG-nitro-L-arginine methyl ester (300 microM), a nitric oxide synthase (NOS) inhibitor. It was concluded that glutamate stimulated intracellular generation of both NO and ROS, and at least 50% of the oxidation of DCF was attributed to intracellular generation of NO. These results demonstrate that oxidation of DCF by NO can be used to measure intracellular generation of NO and by adding either Hb or SOD to the cell system, the extent of oxidation of DCF attributed to NO and ROS can be determined.

Opposing effects of thapsigargin on the survival of developing cerebellar granule neurons in culture

Elevated levels of potassium (K+) promote maturation and survival of cerebellar granule neurons in culture. When switched from a culture medium containing high K+ (25 mM) to one with low K+ (5 mM) mature granule neurons undergo death by apoptosis. The mechanism by which high K+ promotes neuronal survival (and conversely inhibits apoptosis) is unclear. Several pieces of evidence indicate that an increase in intracellular calcium (Ca2+) resulting from depolarization mediated-influx of extracellular Ca2+ is necessary. We examined the effect of thapsigargin on granule neuron cultures. Thapsigargin is an inhibitor of the endoplasmic reticular Ca2+ ATPase causing a depletion of Ca2+ from internal stores. This treatment would therefore be expected to raise intracellular cytosolic Ca2+ without membrane depolarization. We find that treatment of mature neurons with thapsigargin at doses > or = 5 nM inhibits death resulting from the lowering of extracellular K+. The survival effect of thapsigargin was not affected by inhibitors of extracellular Ca2+ influx including nifedipine, verapamil, methoxyverapamil, Mg2+, and Ni2+, nor was it inhibited by the NMDA receptor antagonist, MK801. We have further examined whether thapsigargin could substitute for elevated K+ during the maturation of granule cells. Unexpectedly, treatment of younger (immature) neuronal cultures with the same dose of thapsigargin (5 nM) induced cell death. DNA fragmentation analysis suggested that death was due to apoptosis and not toxicity. As observed with the survival effect on mature neurons, the lethal effect of thapsigargin on immature granule cells was not prevented by inhibitors of Ca2+ influx.

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)

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.