Effect of potassium depolarization on phosphate-activated glutaminase activity in primary cultures of cerebellar granule neurons and astroglial cells during development

Fraping: A computational tool for detecting slight differences in fluorescence recovery after photobleaching (FRAP) data for actin polymerization analysis

Fluorescence recovery after photobleaching (FRAP) is a laser method of light microscopy to evaluate the rapid movement of fluorescent molecules. To have a more reliable approach to analyze data from FRAP, we designed Fraping, a free access R library to data analysis obtained from FRAP. Unlike other programs, Fraping has a new form of analyzing curves of FRAP using statistical analysis based on the average curve difference. To evaluate our library, we analyzed the differences of actin polymerization in real time between dendrites and secondary neurites of cultured neuron transfected with LifeAct to track F-actin changes of neurites. We found that Fraping provided greater sensitivity than the conventional model using mobile fraction analysis. Likewise, this approach allowed us to normalize the fluorescence to the size area of interest and adjust data curves choosing the best parametric model. In addition, this library was supplemented with data simulation to have a more significant enrichment for the analysis behavior. We concluded that Fraping is a method that reduces bias when analyzing two data groups as compared with the conventional methods. This method also allows the users to choose a more suitable analysis approach according to their requirements. RESEARCH HIGHLIGHTS: Fraping is a new programming tool to analyze FRAP data to normalize fluorescence recovery curves. The conventional method uses one-point analysis, and the new one compares all the points to define the similarity of the fluorescence recovery.

Txnip expression promotes JNK-mediated neuronal death in response to reactive oxygen species

TXNIP is a protein sensitive to oxidant conditions whose expression is related to the progression of death in cancer, diabetes, ischemia, and neurodegenerative diseases, among others. Because of this, many studies propose TXNIP as a therapeutic target in several diseases. Exposure of cerebellar granule neurons to staurosporine or low potassium leads to apoptotic death. Both conditions generate an early production of reactive oxygen species (ROS) that induces the activation of the ASK1 pathway and the apoptotic machinery. In these models, it has been shown an increase in TXNIP protein mediated by ROS. Here, we evaluated the molecular mechanisms involved in the regulation of the Txnip expression during neuronal death, as well as the role of the protein in the progression of cell death induced by these two apoptotic conditions. In cultured cerebellar granule neurons, we observed that low potassium and staurosporine induced an early increase in ROS that correlated with an increase in Txnip mRNA. When we evaluated the promoter of the gene, we found that the JASPAR-reported FOXO1/3 transcription factor motifs are close to the transcription start site (TSS). We then verified through the Chromatin immunoprecipitation technique (ChIP) that FOXO3 interacts with the Txnip promoter after 1 h of low potassium treatment. We also detected FOXO3 nuclear translocation by low potassium and staurosporine treatments. Finally, by using shRNA in the neuroblastoma MSN cell line, we found that Txnip downregulation decreased neuronal death induced by staurosporine stimulus. Together, these results suggest that ROS promotes the expression of Txnip through the activation of the FOXO3 transcription factor mediated by Akt inhibition. We also demonstrated that TXNIP is necessary for neuronal death progression.

Differential ROS-Mediated Phosphorylation of Drp1 in Mitochondrial Fragmentation Induced by Distinct Cell Death Conditions in Cerebellar Granule Neurons

Reactive oxygen species (ROS) production has been associated with neuronal death. ROS are also involved in mitochondrial fission, which is mediated by Dynamin-related protein 1 (Drp1). The regulation of mitochondrial fragmentation mediated by Drp1 and its relationship to mitochondrial ROS (mtROS) in neuronal death have not been completely clarified. The aim of this study is to evaluate the role of mtROS in cell death and their involvement in the activation of Drp1 and mitochondrial fission in a model of cell death of cultured cerebellar granule neurons (CGN). Neuronal death of CGN induced by potassium deprivation (K5) and staurosporine (ST) triggers mitochondrial ROS production and mitochondrial fragmentation. K5 condition evoked an increase of Drp1 phosphorylation at Ser616, but ST treatment led to a decrease of Drp1 phosphorylation. Moreover, the death of CGN induced by both K5 and ST was markedly reduced in the presence of MitoTEMPO; however, mitochondrial morphology was not recovered. Here, we show that the mitochondria are the initial source of ROS involved in the neuronal death of CGN and that mitochondrial fragmentation is a common event in cell death; however, this process is not mediated by Drp1 phosphorylation at Ser616.

Oleamide Induces Cell Death in Glioblastoma RG2 Cells by a Cannabinoid Receptor-Independent Mechanism

The endocannabinoid system has been associated with antiproliferative effects in several types of tumors through cannabinoid receptor-mediated cell death mechanisms. Oleamide (ODA) is a CB1/CB2 agonist associated with cell growth and migration by adhesion and/or ionic signals associated with Gap junctions. Antiproliferative mechanisms related to ODA remain unknown. In this work, we evaluated the effects of ODA on cell viability and morphological changes in a rat RG2 glioblastoma cell line and compared these effects with primary astrocyte cultures from 8-day postnatal rats. RG2 and primary astrocyte cultures were treated with ODA at increasing concentrations (25, 50, 100, and 200 μM) for different periods of time (12, 24, and 48 h). Changes in RG2 cell viability and morphology induced by ODA were assessed by viability/mitochondrial activity test and phase contrast microscopy, respectively. The ratios of necrotic and apoptotic cell death, and cell cycle alterations, were evaluated by flow cytometry. The roles of CB1 and CB2 receptors on ODA-induced changes were explored with specific receptor antagonists. ODA (100 μM) induced somatic damage, detachment of somatic bodies, cytoplasmic polarization, and somatic shrinkage in RG2 cells at 24 and 48 h. In contrast, primary astrocytes treated at the same ODA concentrations exhibited cell aggregation but not cell damage. ODA (100 μM) increased apoptotic cell death and cell arrest in the G1 phase at 24 h in the RG2 line. The effects induced by ODA on cell viability of RG2 cells were independent of CB1 and CB2 receptors or changes in intracellular calcium transient. Results of this novel study suggest that ODA exerts specific antiproliferative effects on RG2 glioblastoma cells through unconventional apoptotic mechanisms not involving canonical signals.

Protein-conformational diseases in childhood: Naturally-occurring hIAPP amyloid-oligomers and early β-cell damage in obesity and diabetes

BACKGROUND AND AIMS

This is the first time that obesity and diabetes mellitus (DM) as protein conformational diseases (PCD) are reported in children and they are typically diagnosed too late, when β-cell damage is evident. Here we wanted to investigate the level of naturally-ocurring or real (not synthetic) oligomeric aggregates of the human islet amyloid polypeptide (hIAPP) that we called RIAO in sera of pediatric patients with obesity and diabetes. We aimed to reduce the gap between basic biomedical research, clinical practice-health decision making and to explore whether RIAO work as a potential biomarker of early β-cell damage.

MATERIALS AND METHODS

We performed a multicentric collaborative, cross-sectional, analytical, ambispective and blinded study; the RIAO from pretreated samples (PTS) of sera of 146 pediatric patients with obesity or DM and 16 healthy children, were isolated, measured by sound indirect ELISA with novel anti-hIAPP cytotoxic oligomers polyclonal antibody (MEX1). We carried out morphological and functional studied and cluster-clinical data driven analysis.

RESULTS

We demonstrated by western blot, Transmission Electron Microscopy and cell viability experiments that RIAO circulate in the blood and can be measured by ELISA; are elevated in serum of childhood obesity and diabetes; are neurotoxics and works as biomarkers of early β-cell failure. We explored the range of evidence-based medicine clusters that included the RIAO level, which allowed us to classify and stratify the obesity patients with high cardiometabolic risk.

CONCLUSIONS

RIAO level increases as the number of complications rises; RIAOs > 3.35 μg/ml is a predictor of changes in the current indicators of β-cell damage. We proposed a novel physio-pathological pathway and shows that PCD affect not only elderly patients but also children. Here we reduced the gap between basic biomedical research, clinical practice and health decision making.

Caspase-3 Activation Correlates With the Initial Mitochondrial Membrane Depolarization in Neonatal Cerebellar Granule Neurons

In this study we evaluated the effect of the reduction in the endoplasmic reticulum calcium concentration ([Ca²⁺]_ER), changes in the cytoplasmic calcium concentration ([Ca²⁺]_i), alteration of the mitochondrial membrane potential, and the ER stress in the activation of caspase-3 in neonatal cerebellar granule cells (CGN). The cells were loaded with Fura-2 to detect changes in the [Ca²⁺]_i and with Mag-fluo-4 to measure variations in the [Ca²⁺]_ER or with TMRE to follow modifications in the mitochondrial membrane potential in response to five different inducers of CGN cell death. These inducers were staurosporine, thapsigargin, tunicamycin, nifedipine and plasma membrane repolarization by switching culture medium from 25 mM KCl (K25) to 5 mM KCl (K5). Additionally, different markers of ER stress were determined and all these parameters were correlated with the activation of caspase-3. The different inducers of cell death in CGN resulted in three different levels of activation of caspase-3. The highest caspase-3 activity occurred in response to K5. At the same time, staurosporine, nifedipine, and tunicamycin elicited an intermediate activation of caspase-3. Importantly, thapsigargin did not activate caspase-3 at any time. Both K5 and nifedipine rapidly decreased the [Ca²⁺]_i, but only K5 immediately reduced the [Ca²⁺]_ER and the mitochondrial membrane potential. Staurosporine and tunicamycin increased the [Ca²⁺]_i and they decreased both the [Ca²⁺]_ER and mitochondrial membrane potential, but at a much lower rate than K5. Thapsigargin strongly increased the [Ca²⁺]_i, but it took 10 min to observe any decrease in the mitochondrial membrane potential. Three cell death inducers -K5, staurosporine, and thapsigargin- elicited ER stress, but they took 30 min to have any effect. Thapsigargin, as expected, displayed the highest efficacy activating PERK. Moreover, a specific PERK inhibitor did not have any impact on cell death triggered by these cell death inducers. Our data suggest that voltage-gated Ca²⁺ channels, that are not dihydropyridine-sensitive, load the ER with Ca²⁺ and this Ca²⁺ flux plays a critical role in keeping the mitochondrial membrane potential polarized. A rapid decrease in the [Ca²⁺]_ER resulted in rapid mitochondrial membrane depolarization and strong activation of caspase-3 without the intervention of the ER stress in CGN.

Diabetes Drug Discovery: hIAPP1-37 Polymorphic Amyloid Structures as Novel Therapeutic Targets

Human islet amyloid peptide (hIAPP_1–37) aggregation is an early step in Diabetes Mellitus. We aimed to evaluate a family of pharmaco-chaperones to act as modulators that provide dynamic interventions and the multi-target capacity (native state, cytotoxic oligomers, protofilaments and fibrils of hIAPP_1–37) required to meet the treatment challenges of diabetes. We used a cross-functional approach that combines in silico and in vitro biochemical and biophysical methods to study the hIAPP_1–37 aggregation-oligomerization process as to reveal novel potential anti-diabetic drugs. The family of pharmaco-chaperones are modulators of the oligomerization and fibre formation of hIAPP_1–37. When they interact with the amino acid in the amyloid-like steric zipper zone, they inhibit and/or delay the aggregation-oligomerization pathway by binding and stabilizing several amyloid structures of hIAPP_1–37. Moreover, they can protect cerebellar granule cells (CGC) from the cytotoxicity produced by the hIAPP_1–37 oligomers. The modulation of proteostasis by the family of pharmaco-chaperones A–F is a promising potential approach to limit the onset and progression of diabetes and its comorbidities.

Reactive Oxygen Species Evoked by Potassium Deprivation and Staurosporine Inactivate Akt and Induce the Expression of TXNIP in Cerebellar Granule Neurons

The reactive oxygen species (ROS) play a critical role in neuronal apoptosis; however, the mechanisms are not well understood. It has been shown that thioredoxin-interacting protein (TXNIP) overexpression renders cells more susceptible to oxidative stress and promotes apoptosis and that the activation of PI3K/Akt pathway leads to a downregulation of TXNIP. Here, we evaluated the role of ROS in the regulation of Akt activity and the subsequent regulation of the TXNIP expression in a model of apoptotic death of cerebellar granule neurons (CGN). We observed that two apoptotic conditions that generate ROS at short times led to an increase in the expression of TXNIP in a time-dependent manner; antioxidants significantly reduced this expression. Also, H₂O₂ caused an increase in TXNIP expression. Moreover, apoptotic conditions induced inactivation of Akt in a time-dependent manner similar to TXNIP expression and H₂O₂ treatment led to Akt inactivation. Besides, the pharmacological inhibition of Akt increases TXNIP expression and induces CGN cell death. Together, these results suggest that ROS promote neuronal apoptosis through the Akt-TXNIP signaling pathway, supporting the idea that the PI3K/Akt pathway regulates the TXNIP expression. This study highlights the potential importance of this mechanism in neuronal death.

Peroxynitrite is Involved in the Apoptotic Death of Cultured Cerebellar Granule Neurons Induced by Staurosporine, but not by Potassium Deprivation

Nitric oxide (NO) regulates numerous physiological process and is the main source of reactive nitrogen species (RNS). NO promotes cell survival, but it also induces apoptotic death having been involved in the pathogenesis of several neurodegenerative diseases. NO and superoxide anion react to form peroxynitrite, which accounts for most of the deleterious effects of NO. The mechanisms by which these molecules regulate the apoptotic process are not well understood. In this study, we evaluated the role of NO and peroxynitrite in the apoptotic death of cultured cerebellar granule neurons (CGN), which are known to experience apoptosis by staurosporine (St) or potassium deprivation (K5). We found that CGN treated with the peroxynitrite catalyst, FeTTPs were completely rescued from St-induced death, but not from K5-induced death. On the other hand, the inhibition of the inducible nitric oxide synthase partially protected cell viability in CGN treated with K5, but not with St, while the inhibitor L-NAME further reduced the cell viability in St, but it did not affect K5. Finally, an inhibitor of the soluble guanylate cyclase (sGC) diminished the cell viability in K5, but not in St. Altogether, these results shows that NO promotes cell survival in K5 through sGC-cGMP and promotes cell death by other mechanisms, while in St NO promotes cell survival independently of cGMP and peroxynitrite results critical for St-induced death. Our results suggest that RNS are differentially handled by CGN during cell death depending on the death-inducing conditions.

ROS produced by NOX2 control in vitro development of cerebellar granule neurons development

Reactive oxygen species (ROS) act as signaling molecules that regulate nervous system physiology. ROS have been related to neural differentiation, neuritogenesis, and programmed cell death. Nevertheless, little is known about the mechanisms involved in the regulation of ROS during neuronal development. In this study, we evaluated the mechanisms by which ROS are regulated during neuronal development and the implications of these molecules in this process. Primary cultures of cerebellar granule neurons (CGN) were used to address these issues. Our results show that during the first 3 days of CGN development in vitro (days in vitro; DIV), the levels of ROS increased, reaching a peak at 2 and 3 DIV under depolarizing (25 mM KCl) and nondepolarizing (5 mM KCl) conditions. Subsequently, under depolarizing conditions, the ROS levels markedly decreased, but in nondepolarizing conditions, the ROS levels increased gradually. This correlated with the extent of CGN maturation. Also, antioxidants and NADPH-oxidases (NOX) inhibitors reduced the expression of Tau and MAP2. On the other hand, the levels of glutathione markedly increased at 1 DIV. We inferred that the ROS increase at this time is critical for cell survival because glutathione depletion leads to axonal degeneration and CGN death only at 2 DIV. During the first 3 DIV, NOX2 was upregulated and expressed in filopodia and growth cones, which correlated with the hydrogen peroxide (H2O2) distribution in the cell. Finally, NOX2 KO CGN showed shorter neurites than wild-type CGN. Taken together, these results suggest that the regulation of ROS is critical during the early stages of CGN development.

Endogenous XIAP, but not other members of the inhibitory apoptosis protein family modulates cerebellar granule neurons survival

Programmed cell death plays a critical role during cerebellar development. In particular, it has been shown in vivo and in vitro that developing cerebellar granule neurons (CGN) die apoptotically. Apoptosis involves a series of morphological changes and the activation of caspases. Inhibitor of apoptosis proteins (IAPs) is implicated in negative regulation of caspase activation and apoptotic cell death. Although apoptotic death of CGN has been extensively studied, there is no information about the role of IAPs in the developing cerebellum. Here, we studied the participation of some members of IAPs in the survival of the developing rat CGN in culture and under physiological conditions. Under these conditions, we found a differential expression pattern of cIAP-1, cIAP-2, XIAP and survivin during cerebellar development in an age-dependent manner, highlighting the significant increase of XIAP levels. We also detected an interaction between XIAP and caspase 3 at postnatal day (P) 12 and 16. On the other hand, we found a significant decrease of XIAP levels in cultured CGN maintained in chronic potassium deprivation, an apoptotic condition, suggesting a possible relationship between XIAP levels and neuronal viability. Under these conditions, we also detected the interaction of XIAP with active caspase-3. The down-regulation of XIAP in CGN cultured under survival conditions (chronic potassium depolarization) induced a reduction of cell viability and an increment of apoptotic cells. These findings support the idea that IAPs could be involved in the survival of CGN and that XIAP might be critical for neuronal survival in cerebellar development and during chronic depolarization in cultured CGN through a mechanism involving caspase inhibition.

CYP2E1 induction leads to oxidative stress and cytotoxicity in glutathione-depleted cerebellar granule neurons

Increasing evidence suggests that brain cytochrome P450 (CYP) can contribute to the in situ metabolism of xenobiotics. In the liver, some xenobiotics can be metabolized by CYPs into more reactive products that can damage hepatocytes and induce cell death. In addition, normal CYP activity may produce reactive oxygen species (ROS) that contribute to cell damage through oxidative mechanisms. CYP2E1 is a CYP isoform that can generate ROS leading to cytotoxicity in multiple tissue types. The aim of this study was to determine whether CYP2E1 induction may lead to significant brain cell impairment. Immunological analysis revealed that exposure of primary cerebellar granule neuronal cultures to the CYP inducer isoniazid, increased CYP2E1 expression. In the presence of buthionine sulfoximine, an agent that reduces glutathione levels, isoniazid treatment also resulted in reactive oxygen species (ROS) production, DNA oxidation and cell death. These effects were attenuated by simultaneous exposure to diallyl sulfide, a CYP2E1 inhibitor, or to a mimetic of superoxide dismutase/catalase, (Euka). These results suggest that in cases of reduced antioxidant levels, the induction of brain CYP2E1 could represent a risk of in situ neuronal damage.

Novel TASK channels inhibitors derived from dihydropyrrolo[2,1-a]isoquinoline

TASK channels belong to the family of K(+) channels with 4 transmembrane segments and 2 pore domains (4TM/2P) per subunit. These channels have been related to apoptosis in cerebellar granule neurons (CGN), as well as cancer in other tissues. TASK current is regulated by hormones, neurotransmitters, anesthetics and divalent cations, which are not selective. Recently, there has been found some organic compounds that inhibit TASK current selectively. In order to find other modulators, we report here a group of five dihydropyrrolo[2,1-a]isoquinolines (DPIs), four of them with putative anticancer activity, that were evaluated on TASK-1 and TASK-3 channels. The compounds 1, 2 and 3 showed IC50 < 320 μM on TASK-1 and TASK-3, intermediate activity on TASK-1/TASK-3 heterodimer, moderate effect over hslo and TREK-1 (500 μM), and practically not inhibition on Shaker-IR, herg and IRK2.1 potassium channels, when they were expressed heterologously in Xenopus laevis oocytes. In rat CGN, 500 μM of these three compounds induced a decrement by >39% of the TASK-carried leak current. Finally, only compound 1 showed significant protection (∼36%) against apoptotic death of CGN induced by K(+) deprivation. These results suggest that DPI compounds could be potential candidates for designing new selective inhibitors of TASK channels.

Novel multi-functional nitrones for treatment of ischemic stroke

Ischemic stroke resulting from obstruction of blood vessels is an enormous public health problem with urgent need for effective therapy. The co-administration of thrombolytic/antiplatelet agent and neuroprotective agent improves therapeutic efficacy and agent possessing both thrombolytic/antiplatelet and antiradical activities provides a promising strategy for the treatment of ischemic stroke. We have previously reported a novel compound, namely TBN, possessing both antiplatelet and antiradical activities, showed significant neuroprotective effect in a rat stroke model. We herein report synthesis of a series of new pyrazine derivatives, and evaluation of their biological activities. Their mechanisms of action were also investigated. Among these new derivatives, compound 21, armed with two nitrone moieties, showed the greatest neuroprotective effects in vitro and in vivo. Compound 21 significantly inhibited ADP-induced platelet aggregation. In a cell free antiradical assay, compound 21 was the most effective agent in scavenging the three most damaging radicals, namely (·)OH, O(2)(·-) and ONOO(-).

Reactive oxygen species are related to ionic fluxes and volume decrease in apoptotic cerebellar granule neurons: role of NOX enzymes

Reactive oxygen species (ROS) are produced early during apoptosis of cerebellar granule neurons induced by low potassium (K5) and staurosporine (Sts). In addition, K5 and Sts activate NADPH oxidases (NOX). Recently, we described that K5 and Sts induce apoptotic volume decrease (AVD) at a time when ROS generation and NOX activity occur. In the present study, we evaluated the relationship between ROS generation and ionic fluxes during AVD. Here, we showed that K5- and Sts-induced AVD was inhibited by antioxidants and that direct ROS production induced AVD. Moreover, NOX inhibitors eliminated AVD induced by both K5 and Sts. Sts, but not K5, failed to induce AVD in cerebellar granule neurons from NOX2 knockout mice. These findings suggest that K5- and Sts-induced AVD is largely mediated by ROS produced by NOX. On the other hand, we also found that the blockage of ionic fluxes involved in AVD inhibited both ROS generation and NOX activity. These findings suggest that ROS generation and NOX activity are involved in ionic fluxes activation, which in turn could maintain ROS generation by activating NOX, leading to a self-amplifying cycle.

Role for apoptosis-inducing factor in the physiological death of cerebellar neurons

Apoptosis-inducing factor (AIF) is implicated in caspase-independent apoptotic-like death. AIF released from mitochondria translocates to the nucleus, where it mediates some apoptotic events such as chromatin condensation and DNA degradation. Here, the role of AIF in the neuronal death was studied under physiological conditions. When we analyzed the cellular localization of AIF during cerebellar development, we found a significant increase in the number of neurons with nuclear AIF localization in an age-dependent manner. On the other hand, cerebellar granule neurons (CGN) chronically cultured in low concentration of potassium (5 mM; K5) die with apoptotic-like characteristics after five days. In the present study we found that K5 induces a caspase-dependent apoptotic-like death of CGN as well as a late nuclear translocation of AIF. When CGN death induced by K5 was carried out in the presence of a general inhibitor of caspases, there was a slight decrement of cell death, but neurons eventually died by showing apoptotic-like features such as phosphatidylserine translocation and nuclear condensation. Besides, there was a significant increment of nuclear AIF translocation. These findings support the idea that AIF could be involved in apoptotic-like death of CGN and that it could be an alternative mechanism of neuronal death during cerebellar development.

NOX2 mediates apoptotic death induced by staurosporine but not by potassium deprivation in cerebellar granule neurons

Neuronal apoptotic death involves the participation of reactive oxygen species (ROS), but their sources have not been completely elucidated. Previous studies have demonstrated that the ROS-producing enzyme NADPH oxidase is present in neuronal cells and that this enzyme could participate in the apoptotic neuronal death. Cerebellar granule neurons (CGN) undergo apoptosis when cells are transferred from a medium with 25 mM KCl (K25) to a 5 mM KCl (K5) medium or when they are treated with staurosporine (ST). Under these conditions, apoptotic death of CGN is dependent on ROS production. In this study, we evaluated the role of NOX2, an NADPH oxidase homolog, in the apoptotic death of CGN induced by two different conditions. In CGN from NOX2-deficient (ko) mice, a significantly lower rate of apoptotic death occurs compared with wild-type (wt) CGN. Also, caspase-3 activation, NADPH oxidase activity, and superoxide anion production induced by ST were markedly lower in ko neurons than in wt CGN. In contrast to the case with ST, when CGN were treated with K5, no differences were observed between ko and wt cells in any of the parameters measured. However, all NADPH oxidase inhibitors tested noticeably reduced cell death and apoptotic parameters induced by K5 in both wt and ko CGN. These results suggest that NOX2 could be necessary for apoptotic death induced by ST, but not by K5, which could require other member of the NOX family in the apoptotic process.

Tetanus toxin H(C) fragment reduces neuronal MPP+ toxicity

The non-toxic carboxi-terminal domain of the heavy chain of tetanus toxin (H(C)) elicits neuroprotection of cerebellar granule neurons against apoptotic death induced by potassium deprivation. In this study we sought to determine whether H(C) also prevents the apoptosis induced by the mitochondrial poison 1-methyl-4-phenylpyridinium (MPP+), which induces a form of Parkinsonism. Pre-treatment of cultures with H(C) prevented MPP+-induced cell death, as well as impaired the MPP+-triggered apoptotic cascade. Cytochrome c release from the mitochondria, procaspase-3 activation and chromatin condensation, were significantly reduced in H(C)-pre-treated neurons. Moreover, H(C) induced Ser(112) and Ser(136) BAD phosphorylation, which correlated with the detachment of BAD from Bcl-X(L) and its association to 14-3-3. In turn, Bcl-X(L) remained bound to Bax, impairing its translocation to mitochondria of stressed neurons. The use of Wortmannin or PD98059 demonstrated the involvement of the PI3K/Akt as well as the ERK1/2 transduction pathways in the H(C) fragment effect. Interestingly, the H(C) fragment also induces an increase in the DNA binding activity of NF-kappaB, a well-established transcription factor involved in the prevention of neuronal death. Taken together, our results strongly suggest that the recombinant H(C) fragment of tetanus toxin can act as a neuroprotector in a model of MPP(+)-triggered apoptosis.

Role of brain-derived neurotrophic factor in the protective action of N-methyl-D-aspartate in the apoptotic death of cerebellar granule neurons induced by low potassium

Several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), and neurotransmitters, such as glutamate, may influence neuronal apoptotic death. Rat cerebellar granule neurons (CGN) cultured in low potassium (5 or 10 mM KCl) for more than 5 days in vitro (DIV) die apoptotically. These cells survive in the presence of high potassium (25 mM KCl, K25) or N-methyl-D-aspartate (NMDA), an agonist of glutamatergic receptors. CGN transferred from high to low potassium die apoptotically. Here, we characterized the effect of BDNF and NMDA on the apoptotic death induced by low potassium in CGN. Cell death of CGN by culturing in low potassium for 6 DIV was inhibited by BDNF and NMDA. When CGN were cultured in K25 and transferred to a low-potassium medium, 65% of neurons died after 48 hr. Under these conditions, BDNF, NMDA, or BDNF + NMDA increased CGN survival. Both BDNF and NMDA decreased caspase-9 activity and mRNA caspase-3 levels and activity induced by low potassium. CGN survival induced by BDNF is mediated by TrkB activation, whereas that induced by NMDA is mediated by NMDA receptor and TrkB activation. NMDA, but not BDNF, raised [Ca(2+)](i), which was reduced by low-potassium treatment. These results suggest that NMDA receptor stimulation induces CGN survival through the influx of extracellular Ca(2+) that may evoke the release of BDNF and the activation of TrkB. Complementary mechanisms induced by depolarization and changes in Ca(2+) levels would also contribute to the neuroprotection exerted by NMDA and potassium.

Effect of NMDA antagonists on the death of cerebellar granule neurons at different ages

Cerebellar granule neurons (CGN) are the most abundant neuronal type in the cerebellum. During development, these cells migrate from the external to the internal granule layer (IGL), where they receive excitatory glutamatergic and cholinergic contacts from mossy fibers. During this period of development a large proportion of CGN are eliminated via apoptosis. In vitro studies have demonstrated that when CGN are obtained from rats at postnatal day 8 (P8), the sustained activation of N-methyl-D-aspartate (NMDA) receptor at 2-4 days in vitro rescues neurons from cell death. The NMDA action on cultured CGN could mimic the in vivo actions of the transient activation of the glutamate receptors by the transmitter released by mossy fibers by P12. However, some results suggest that glutamate stimulation could be relevant for CGN at earlier stages of development. In this study we evaluated the effect of NMDA receptor stimulation or blockade on the cell death of both in vivo and cultured CGN obtained from P2 to P8 rats. Our results showed that the blockade of NMDA receptors with the antagonists D,L-2-amino-5-phosphonovaleric acid or dizocilpine (MK-801) reduces cell survival to 20-40%, whereas NMDA treatment increases neuronal survival by approximately 50-60%. In vivo, the treatment with MK-801 reduced the number of apoptotic CGN in the molecular layer (ML) from P5 to P8. These results suggest that NMDA receptor stimulation plays a critical role in the regulation of CGN death during the first week of rat cerebellar development.

Myosin Va is proteolysed in rat cerebellar granule neurons after excitotoxic injury

Cerebellar granule neurons when exposed to glutamate die through an excitotoxic mechanism induced by overactivation of glutamate receptors. This kind of cell death is mediated by an overload of intracellular calcium involving calpain activation, a Ca2+ -dependent intracellular cysteine protease, among other intracellular responses. On the other hand, class V myosins are proteins that move cargo along actin filaments and one of its members, myosin Va, is involved in vesicles transport. Here we studied the effect of excitotoxicity on myosin Va in cultured cerebellar granule neurons. Western blot analysis of control cultures shows a band corresponding to myosin Va as well as an 80 kDa band corresponding to its proteolytic product by calpain. When cells are exposed to glutamate (500 microM), kainate (100 microM) or NMDA (150 microM) during 3-24 h, the proteolytic processing of myosin Va is markedly increased. This proteolysis is inhibited by leupeptin (100 microM) and calpain inhibitor I (50 microM). These inhibitors also significantly improve the morphological appearance of the neurons possibly through the preservation of the cytoskeleton integrity. Our results suggest that myosin Va is a target for calpain I during an excitotoxic injury and could lead to a new area of research to address the participation of molecular motors in neurotoxicity.

Role of ionic fluxes in the apoptotic cell death of cultured cerebellar granule neurons

Cultured cerebellar granule neurons (CGC) increase survival in a medium containing 25 mM KCl (K25), and they die apoptotically when cultures are treated with staurosporine (St) or are transferred to a 5-mM KCl containing medium (K5). Apoptotic CGC show nuclear condensation and caspase-3 activation. Cell death induced by these conditions was partially prevented when cultures were maintained under alkaline conditions, which also induced a marked reduction of the caspase-3 activation. The acidification of the medium further increased cell death induced by both stimuli. Cultures transferred to K5 suffered an immediate intracellular alkalinization that remained constant during the time K5 was present. In contrast, St did not modify cytosolic pH at any of the evaluated times. On the other hand, DIDS, furosemide, and bumetanide prevented CGC death induced by K5 and St. Other drugs such as amiloride, EIPA, tamoxifen, NEM, or NPPB did not modify cell death induced by these conditions. Both DIDS and bumetanide markedly inhibited the processing and activation of caspase-3, and DIDS prevented the nuclear condensation induced by K5 and St. These findings suggest that pH is a condition that could contribute to the modulation of cell death induced by some stimuli and that other ions, such as potassium, could have a role in the initial phase of apoptotic death of CGC.

Reactive oxygen species induce different cell death mechanisms in cultured neurons

Apoptosis is characterized by chromatin condensation, phosphatidylserine translocation, and caspase activation. Neuronal apoptotic death involves the participation of reactive oxygen species (ROS), which have also been implicated in necrotic cell death. In this study we evaluated the role of different ROS in neuronal death. Superoxide anion was produced by incubating cells with xanthine and xanthine oxidase plus catalase, singlet oxygen was generated with rose Bengal and luminic stimuli, and hydrogen peroxide was induced with the glucose and glucose oxidase. Cultured cerebellar granule neurons died with the characteristics of apoptotic death in the presence of superoxide anion or singlet oxygen. These two conditions induced caspase activation, nuclear condensation, phosphatidylserine translocation, and a decrease in intracellular calcium levels. On the other hand, hydrogen peroxide led to a necrosis-like cell death that did not induce caspase activation, phosphatidylserine translocation, or changes in calcium levels. Cell death produced by both singlet oxygen and superoxide anion, but not hydrogen peroxide, was partially reduced by an increase in intracellular calcium levels. These results suggest that formation of specific ROS can lead to different molecular cell death mechanisms (necrosis and apoptosis) and that ROS formed under different conditions could act as initiators or executioners on neuronal death.

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.

Mechanisms of cell death by deprivation of depolarizing conditions during cerebellar granule neurons maturation

Cerebellar granule cells (CGC) cultured under 5mM KCl (K5) undergo apoptosis after 5 days in vitro (DIV). CGC death is reduced by chronic treatment with 25 mM KCl (K25) or NMDA. Also, when CGC cultured for 6-8 DIV in K25 are transferred to a K5 medium, cells die apoptotically. Moreover, Bcl-2 and Bcl-xL protect neurons from apoptosis, while Bax and Bcl-xS may act as proapototic proteins. It is suggested that these members of the Bcl-2 family may be involved in the cytochrome-c (cyt-c) release to the cytosol. Cytochrome-c is able to form a complex with other proteins to activate a cascade of proteases. In this work, we found that Bcl-2 levels in K5 cells did not show any change during 2-7 days in vitro (DIV); but cells grown with NMDA and K25 displayed an increase (55% approximately) of Bcl-2 from 4 DIV, as compared to control. Under these conditions, Bax levels showed a tendency to decrease with age under control cells and NMDA/K25 induced a reduction of approximately 10% in Bax levels from 4 DIV. On the other hand, in cells maintained in K25 during 7 DIV and then switched to a K5 medium, the levels of Bax showed a consistent decrease (30% after 8h). Under these conditions, the Bcl-2 levels did not show any significant change after 24h. Cytochrome-c levels were unaffected under K5, NMDA and K25 and only a marginal increase of cytochrome-c in the cytosol was detected at 6h after switching. We also found that caspase-9 was only activated under K25-deprivation meanwhile caspase-3 was involved in both protocols. These results suggest that the Bcl-2 family members, caspases activation and cytochrome-c release are involved in CGC death induced by K5 and their participation in this process could be different depending on neuronal maturation in culture.

Caspase activation pathways induced by staurosporine and low potassium: role of caspase-2

Apoptotic death is a physiological process with regulatory mechanisms that are under the control of different molecules such as caspases. These are classified as initiators, such as caspases-8 and -9, and effectors, such as caspases-3 and -7. The participation of caspase-2 in the effector phase of apoptosis has been commonly observed in many cell types; however, it is able to act as an initiator caspase, depending on the apoptotic stimulus. Cerebellar granule cells (CGCs) undergo apoptosis when they are transferred from high potassium (K25) to low potassium (K5); this process seems to be mediated by caspase-3 activation. Staurosporine (STS), a full strength inhibitor of kinase proteins, also induces apoptosis in these cells. To characterize the caspase cascade induced by two stimuli in the same cell type we studied the activation of different caspases in CGCs treated with STS or K5. We found that both K5 and STS induce the activation of caspase-3. This result was confirmed by the proteolytic cleavage of poly (ADP-ribose) polymerase (PARP), an endogenous caspase-3 substrate. Caspase-2 was activated preferentially by STS, which showed a temporal course suggesting that this caspase was induced before caspase-3. The initiator caspase-9 was also activated by both K5 and STS, as well as cytochrome-c release. The results obtained in this study suggest that STS and K5 induced different activation caspase pathways for apoptotic cell death of CGCs.

Role of oxidative stress in the apoptotic cell death of cultured cerebellar granule neurons

When cultured cerebellar granule neurons (CGN) are transferred from 25 mM KCl (K25) to 5 mM KCl (K5) caspase-3 and caspase-8, but not caspase-1 or caspase-9,activities are induced and cells die apoptotically. CGN death was triggered by a [Ca(2+)](i) modification when [Ca(2+)](i) was reduced from 300 nM to 50 nM in a K5 medium. The [Ca(2+)](i) changes were followed by an increase in ROS levels. The generation of both cytosolic and mitochondrial reactive oxygen species (ROS) occurred at three different times, 10 min, 30 min and 3--4 hr but only those ROS produced after 3--4 hr are involved in the process of cell death. When CGN cultured in a K5 medium are treated with different antioxidants like scavengers of ROS (mannitol, DMSO) or antioxidant enzymes (superoxide dismutase and catalase) phosphatidylserine translocation, caspase activity, chromatin condensation and cell death is markedly diminished. The protective effect of antioxidants is not mediated through a modification in [Ca(2+)](i). Caspase activation, PS translocation and chromatin condensation were downstream of ROS production. In contrast to H(2)O(2), ROS produced by a xanthine/xanthine oxidase system in CGN cultured in K25 were able to directly induce caspase-3 activation and death that resulted sensitive to z-VAD, a caspase inhibitor. These findings indicate that a reduction in [Ca(2+)](i) triggers CGN death by inducing a generation of ROS after 3--4 hr, which could play a critical role in the initial phases of the apoptotic process including PS translocation, chromatin condensation and the activation of initiator and executor caspases.

Role of heat shock proteins in the effect of NMDA and KCl on cerebellar granule cells survival

Cerebellar granule cells (CGC) die apoptotically after five days in culture (DIV) at physiological concentrations of potassium (5 mM; K5). When CGC are depolarized (K25) or treated with NMDA (150 microM) cell survival is increased. CGC changed from K25 to K5 die after 24-48 h. It is known that heat shock protein (HSP) may protect from cell death. Here, we found that cells in K5 showed an increase in HSP-70 levels after 3 DIV. Similarly, in cells changed from K25 to K5, HSP-70 levels were increased after 6 h. Neither NMDA nor K25 treatment affected HSP-70 levels from 2-7 DIV. Ethanol or thermal stress induced HSP-70, but cell survival was not affected in K5 medium. These results suggest that HSP, particularly HSP-70, are not involved in the mechanisms by which NMDA and KCl promote cell survival.

Caspase-3 expression by cerebellar granule neurons is regulated by calcium and cyclic AMP

Caspase-3 enzyme activity is induced, and cell death follows, when cerebellar granule neurons (CGNs) from 8-day-old rats are transferred from an extracellular concentration of 25 mM K+ (25 mM [K+]e) to 5 mM [K+]e. Death of these neurons is diminished by an inhibitor of caspase-3 but not by an inhibitor of caspase-1. Actinomycin D and cycloheximide inhibit induction of caspase-3 and prevent death. Experiments in which CGN intracellular Ca2+ concentration ([Ca2+]i) was manipulated by either changing [K+]e or adding a voltage-gated Ca2+ channel antagonist or a Ca2+ ionophore to the medium showed that caspase-3 mRNA rises 2.5-fold when [Ca2+]i is diminished from 300 to 150 nM, with a corresponding rise in peak caspase enzyme activity. Whereas the caspase-3 mRNA level does not rise further with a still greater diminution in [Ca2+]i, peak caspase enzyme activity continues to increase, reaching sevenfold induction when [Ca2+]i is reduced to 55 nM. In CGNs in which [Ca2+]i is set at 55 nM by incubation in 5 mM [K+]e, treatment with forskolin or dibutyryl 3',5'-cyclic adenosine-5'-monophosphate delays caspase-3 induction and diminishes death but does not alter [Ca2+]i. We conclude that, in immature CGNs, both caspase-3 transcription and the subsequent processing of caspase-3 are induced by a fall in [Ca2+]i. Elevating cyclic AMP content delays caspase-3 induction by a mechanism that does not require an increase in [Ca2+]i.

Effect of NMDA antagonists on the activity of glutaminase and aspartate aminotransferase in the developing rat cerebellum

Chronic treatment of rats from postnatal day 6 to 25 with drugs that interact with the N-methyl-D-aspartate (NMDA) receptor induced a differential effect on the activity of some enzymes involved in neurotransmitter synthesis. Two of these drugs ((5R,10S)-(+)-5-methyl-10,11 -dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801) and 3-(2-carboxypiperazin-4-yl)propyl-1phosphonic acid (CPP)) caused a marked reduction (20-40%) of glutaminase and aspartate aminotransferase activity in the cerebellum. These changes were observed only at a very precise time of development (i.e. 10 to 19 postnatal day). The competitive antagonist, amino phosphonovaleric acid (APV), did not affect any of the enzymes studied at all tested ages. When animals were treated with NMDA only a slight, but significant, increase in the activity of glutaminase was observed at 9-11 postnatal day only. Any of the agonists or antagonists tested significantly affected the activity of lactate dehydrogenase as compared to control animals. Histologic observations of cerebella treated with the indicated drugs showed that only MK-801, and CPP to a lesser extent, induced a small reduction in the width of the internal granule layer. The body weight of animals treated with MK-801 was clearly reduced, but only in more mature rats (> 16 postnatal day), when animals did not show any alteration in the enzymes tested. These results support the suggestion that presynaptic influences, particularly from glutamatergic neurons, are critical to promote cerebellar granule neurons differentiation during critical periods of the cerebellar development.

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.

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

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

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.

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.

Effect of potassium and N-methyl-D-aspartate on the aspartate aminotransferase activity in cultured cerebellar granule cells

The effect of potassium depolarization and N-methyl-D-aspartate (NMDA) on the activity of aspartate aminotransferase (AAT; EC 2.6.1.1), an enzyme suggested to be involved in neurotransmitter glutamate synthesis, was studied in cultured cerebellar granule neurons. Both KCl and NMDA increased AAT activity in a dose-dependent manner. When cells were treated 48-72 hr with 40 mM KCl or 150 microM NMDA the AAT was enhanced about 65-75%. The EC50 for NMDA and KCl were 25 microM and 17 mM, respectively. The effect of NMDA and KCl was specific for AAT without affecting the activity of other enzymes like lactate dehydrogenase or protein content and it was observed only in granule cells but not in astrocytes or cortical neurons. The effect of KCl was not mediated by an activation of excitatory amino acid receptors and was Ca(++)-dependent. The effect of NMDA was completely blocked by Mg++ and NMDA antagonists. The increase of AAT induced by AAT and KCl was blocked by cycloheximide and actinomycin D, suggesting an involvement of de novo synthesis of proteins and RNA. Kainic acid and quinolinic acid were also effective in increasing the AAT activity. The action of kainate was less effective than that of NMDA and it was observed only at relatively low concentrations (10 microM). Quinolinic acid raised the activity of AAT about 45% at a concentration of 500 microM. Other non-NMDA agonists did not modify the AAT activity. From these findings we can conclude that NMDA and KCl exert a trophic action on cerebellar granular neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of neurotransmitter enzyme by quisqualate subtype glutamate receptors in cultured cerebellar and hippocampal neurons

The possible involvement of ionotropic and metabotropic quisqualate (QA) receptors in neuronal plasticity was studied in cultured glutamatergic cerebellar or hippocampal cells in terms of the specific activity of phosphate-activated glutaminase, an enzyme important in the synthesis of the putative neurotransmitter pool of glutamate. When cerebellar or hippocampal neurons were treated with QA, it elevated the specific activity of glutaminase in a dose-dependent manner. The half-maximal effect was obtained at about 0.1 microM, the maximum increase was at about 1 microM, but levels higher than 10 microM QA produced progressive reduction in glutaminase activity. In contrast, QA had little effects on the activities of lactate dehydrogenase and aspartate aminotransferase and the amount of protein, indicating that the increase in glutaminase was relatively specific. The QA-mediated increase in glutaminase was mimicked by the ionotropic QA receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; EC50, about 0.5 microM), but not by the metabotropic QA receptor agonist trans-(+-)-1-amino-cyclopentyl-1,3,dicarboxylate (t-ACPD; up to 0.5 mM). The specific ionotropic QA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) inhibited QA- and AMPA-mediated increases in glutaminase activity in a dose-dependent manner, whereas other glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate, gamma-D-glutamyl aminomethyl sulphonic acid and gamma-D-glutamyl diethyl ester were ineffective. The elevation of neurotransmitter enzyme was Ca(2+)-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term expression of the c-fos protein during the in vitro differentiation of cerebellar granule cells induced by potassium or NMDA

Levels of the c-fos protein were assayed in mouse cerebellar granule cells during their in vitro development under different culture conditions. When grown in media favoring both their survival and differentiation, i.e. in the presence of 30 mM K+ or 12.5 mM K+ plus 100 microM N-methyl-D-aspartate (NMDA), the c-fos protein becomes detectable in the nucleus of granule cells on and after 6 days and persists to high levels until the culture begins to decline. The protein c-fos appears therefore after the critical period described for the survival effect of K+ depolarization or NMDA receptor stimulation which corresponds to days 2-5 after plating. The c-fos protein remains however scarcely detectable or undetectable throughout the life-span of cells cultured under conditions providing poor survival and differentiation, i.e. in the presence of low K+ (5 or 12.5 mM) alone or when the effect of NMDA is blocked by the NMDA receptor antagonist MK-801. Interestingly, in cortical and striatal neurons, the survival and differentiation of which being not affected by depolarizing media, no c-fos protein is detected whatever the culture conditions tested at least during the first 18 days in vitro. This suggests that long-term expression of the c-fos gene might be related to some aspect of the late in vitro differentiation process of cerebellar granule cells.

Motoneuron survival in vitro: effects of pyruvate, alpha-ketoglutarate, gangliosides and potassium

When grown in a serum and muscle extract containing medium, about 20% of chick motoneurons survived for one week. The presence of pyruvate during the first hour of culture doubles the number of motoneurons that survive. The subsequent addition of 5 mM alpha-ketoglutarate, 10-20 mM potassium, or 100 microM GM1 ganglioside results in a further increase in motoneurons surviving. These effects of alpha-ketoglutarate, potassium and GM1 ganglioside are not additive. Using such compounds in the culture medium, one can have a three-fold increase in the survival of motoneurons.

Differences in transmitter release, morphology, and ischemia-induced cell injury between cerebellar granule cell cultures developing in the presence and in the absence of a depolarizing potassium concentration

Release of glutamate and aspartate was measured in mouse cerebellar granule cells in primary cultures grown for 4-16 days in serum-containing tissue culture medium with either a partially depolarizing (25 mM) or a physiological concentration of potassium (5.4 mM). The cells migrated to form aggregates connected by a network of processes during the first week in culture and both groups of cultures survived for at least 2 weeks. In cultures grown in the presence of 25 mM potassium for at least 8 days there was a large (approximately 10 nmol/min/mg protein), calcium-dependent glutamate release and a smaller aspartate release during superfusion with 50 mM potassium. This response was not present in cultures grown in the physiological medium. Nevertheless, exposure to an elevated potassium concentration caused a normal, or even enhanced calcium entry into the cells. Phase contrast microscopy showed a similar appearance of the cellular aggregates under each of the two conditions. Electron microscopy revealed that the aggregates consisted of a centrally located neuropil and peripherally located granule cell bodies. The morphology of the cell bodies and the neuropil in the cells grown at the high potassium concentration closely resembled that of cerebellar granule cells in vivo. In the cells grown at the low potassium concentration, cell bodies, axons and synaptic vesicles looked normal, but the remainder of the neuropil, especially dendrites, showed massive degeneration. Immunochemical measurements demonstrated similar amounts of synaptophysin under each of the two culturing conditions, thus confirming our impression that there were similar numbers of synaptic vesicles and hence presynaptic elements in the two types of cultures. Fluorescence microscopy, using fluorescein diacetate to stain living cells and propidium iodide to stain dead cells, indicated a much greater resistance to ischemic cell injury in the cells cultured at the low potassium concentration. Possible reasons for this difference are discussed.

Quinolinic acid promotes the biochemical differentiation of cerebellar granule neurons

The possible involvement of quinolinic acid in the biochemical differentiation of cultured glutamatergic cerebellar granule neurons was studied in terms of the activity of phosphate-activated glutaminase (GLNase) and aspartate aminotransferase (ASP-AT). Treatment with quinolinate elevated the specific activity of GLNase and amount of protein per culture dish in a dose-dependent manner. The half maximal effect was obtained at about 0.5 mM quinolinate, whereas the maximum concentration, which produced about a 2.3-fold increase in GLNase activity, was about 2 mM. Quinolinate, like N-methyl-D-aspartate (NMDA), had no significant effects on the activities of ASP-AT and lactate dehydrogenase enzymes. The increases in the activity of GLNase and amount of protein were completely blocked by the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid. The result would indicate that, (a) contrary to an earlier proposal, ASP-AT does not appear to be a good marker for studying dynamic responses of glutamatergic neurons, and (b) the trophic effect of quinolinic acid on the development of cerebellar granule neurons is mediated by selective activation of NMDA subtype excitatory amino acid receptors.

Histamine increases ornithine decarboxylase activity in primary cultures of cerebellar granule cells

The effect of histamine on the activity of ornithine decarboxylase (ODC) of cerebellar granule neurons was studied using primary cultures grown both in serum-containing medium and in chemically defined medium. In comparison with granule neurons grown in chemically defined medium, the activity of ODC was about twice as great in the neurons grown in serum-containing medium. Treatment of cultured cerebellar neurons with histamine caused a dose-dependent increase in ODC activity. The maximum elevation was observed at 500 nM of histamine, when the increase in ODC activity was about 50% and 120% over controls in granule cells grown in serum-containing medium and in chemically defined medium, respectively. Histamine had no significant effect on the activity of lactate dehydrogenase in these cultures. The present findings provided direct evidence for the involvement of histamine in the regulation of ODC-related non-mitotic growth of granule neurons in the cerebellum.

Death of subcortical cholinergic neurons in certain neurodegenerative disorders may not be due to an overstimulation of N-methyl-D-aspartate receptors

Stimulation of N-methyl-D-aspartate (NMDA) receptors increases the activity of enzymes involved in the synthesis of nerve cell specific neurotransmitters. In the present study this phenomenon has been used to identify the neurons in the septal-diagonal band region having NMDA receptors. Exposure of cultures relatively enriched in subcortical cholinergic neurons to a depolarizing concentration of K+ (40 mM) significantly enhanced the expression of choline acetyltransferase (ChAT) activity. In contrast, when these septal cells were treated with as much as 100 microM NMDA no significant increase was observed in the activity of choline acetyltransferase, although there was a marked enhancement in glutamate decarboxylase activity. The results would indicate that subcortical cholinergic neurons do not possess excitatory amino acid receptors of the NMDA subtype, and that therefore neurotoxicity mediated through NMDA receptors may not be involved in the death of cholinergic neurons in degenerative disorders of the brain, such as Down's syndrome and Alzheimer's disease, which entail major losses of these neurons.

The effects of potassium-induced depolarization, glutamate receptor antagonists and N-methyl-D-aspartate on neuronal survival in cultured neocortex explants

The effects of elevating the potassium concentration of the growth medium of neocortical explants was studied. Under control conditions, 10 mM potassium resulted in ca 20% decrease in the number of surviving neurons. The same potassium concentration, however, was clearly neurotrophic in tetrodotoxin-grown cultures: tetrodotoxin-induced neuronal death was significantly reduced. Both effects could be mimicked by the addition of 10 microM N-methyl-D-aspartate (NMDA); lower concentrations were without effect; higher concentrations were neurotoxic under both control and tetrodotoxin conditions. The neurotoxic, as well as the neurotrophic effects of 10 mM potassium appear to be mediated through depolarization-induced glutamate release since they could be influenced by the application of glutamate receptor antagonists. The addition of the NMDA receptor antagonist D-2-amino-7-phosphonoheptanoate (APH) blocked the trophic effect of 10 mM potassium in tetrodotoxin-grown cultures, resulting in low survival. On the other hand, the addition of the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) resulted in neuronal survival similar to control cultures, indicating that it blocked the toxic effects of glutamate, leaving the trophic effects on the NMDA receptor untouched. Under control (non-TTX) conditions, neither DNQX nor APH showed significant effects on 10 mM potassium-induced cell death, indicating that stimulation of the non-NMDA, as well as the NMDA receptors is neurotoxic. This differential effect of NMDA receptor stimulation on neuronal survival is discussed with respect to the maturational and/or functional state of the neurons in the culture.