N-methyl-D-aspartate stimulation of the survival of rat cerebellar granule cells in culture is not dependent upon increased c-fos expression and is not mimicked by protein kinase C activation

Relationship Between Intracellular Free Calcium Concentration and NMDA-induced Cerebellar Granule Cell Survival In Vitro

The survival of cerebellar granule cells in culture is stimulated by activation of the N-methyl-d-aspartate (NMDA) class of glutamate receptors. Activation of these receptors at the key period for cell survival in vitro (3 days; 3DIV) resulted in a sustained elevation of intracellular free calcium concentration [Ca2+]i over the same concentration range of NMDA that led to granule cell survival. Agents that release Ca2+ from intracellular stores led to only small, transient elevations of [Ca2+]i and were unable to stimulate granule cell survival. Addition of the Ca2+ ionophore ionomycin to granule cell cultures at 3DIV resulted in increased granule cell number at 7DIV. The ability of ionomycin to stimulate granule cell survival was related to the [Ca2+]i elicited, indicating that a rise in [Ca2+]i is sufficient to activate the processes leading to granule cell survival and that the extent of the elevation in [Ca2+]i is crucially important in determining granule cell fate.

2,2',3,3',4,4'-Hexahydroxy-1,1'-biphenyl-6,6'-dimethanol dimethyl ether (HBDDE)-induced neuronal apoptosis independent of classical protein kinase C alpha or gamma inhibition

Protein kinase C (PKC) isozymes constitute a family of at least 12 structurally related serine-threonine kinases that are differentially regulated and localized, and are presumed to mediate distinct intracellular functions. To explore their roles in intact cells, investigators are developing cell-permeable, isoform-selective inhibitors. 2,2',3,3',4,4'-Hexahydroxy-1, 1'-biphenyl-6,6'-dimethanol dimethyl ether (HBDDE) is reported to be a selective inhibitor of PKC alpha and gamma with IC(50) values of 43 and 50 microM, respectively, using an in vitro assay. However, data examining the potency and selectivity of HBDDE in intact cells are lacking. Employing rodent cerebellar granule neurons as a model system, we investigated the effects of HBDDE using cell survival as a functional end-point. HBDDE induced an apoptotic form of cell death that was dependent upon protein synthesis and included activation of a terminal executioner of apoptosis, caspase 3. The concentration of HBDDE required for half-maximal cell death was less than 10 microM ( approximately 5-fold less than the reported IC(50) values for PKC alpha and gamma in vitro). Furthermore, HBDDE induced apoptosis even after phorbol-ester-mediated down-regulation of PKC alpha and gamma, indicating that this effect is independent of these isoforms. Consistent with this, 2-[1-(3-dimethylaminopropyl) indol-3-yl]-3-(indol-3-yl)-maleimide (GF 109203X), a general inhibitor of all classical and some novel PKCs, did not interfere with survival. Thus, HBDDE should not be used as an isoform-selective inhibitor of PKC alpha or gamma in intact cells. Nevertheless, identification of its target in granule neurons will provide valuable information about survival pathways.

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

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

Apoptosis and cell death in neuronal cells: where does Ca2+ fit in?

Mounting evidence shows that neuronal death is an important and essential component of brain tissue homeostasis, with major forms of cell death occurring: necrosis and apoptosis. No general consensus exists as to whether these two forms of neuronal death represent separate cellular processes or just two different forms of a common 'death pathway'. One difference between them is the role played by intracellular Ca2+: central and obligatory, in necrosis and possible, but not always necessary in triggering apoptosis. Furthermore, the same assessment of the involvement of Ca2+ signalling could also distinguish between two possible apoptotic states in the nervous system: one, the 'developmental apoptosis', involving immature and developing neurons, in which Ca2+ plays mainly an apoprotector role, and another one, associated mainly with pathological instances and involving fully matured and established neurons, in which Ca2+ plays an apo-inducing role.

Activation of metabotropic glutamate receptors by L-AP4 stimulates survival of rat cerebellar granule cells in culture

The results presented here show that the metabotropic glutamate receptor agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) is capable of markedly stimulating the survival of rat cerebellar granule cells in culture. This is the first demonstration of a neurotrophic role for metabotropic glutamate receptors. The survival promoting action of L-AP4 does not involve a large, rapid rise in [Ca2+]i which is seen with other neurotrophic agents in granule cells such as N-methyl-D-aspartate, ionomycin and high potassium. In addition, the survival-promoting effect of L-AP4 did not appear to be related to changes in cAMP levels. Survival due to L-AP4 was enhanced by pertussis toxin and by forskolin and was unaffected by inhibitors of cAMP-dependent protein kinase. Measurement of cAMP levels after long-term treatment with neurotrophic agents showed no clear relationship between cAMP concentration and granule cell survival. The mechanism of L-AP4 stimulated cell survival is unknown but seems unlikely to involve an acute rise in [Ca2+]i or modulation of cAMP levels. Survival induced by L-AP4 was not blocked by the antagonist (RS)-alpha-methyl-4-carboxyphenylglycine. Similarity in these properties with those of the mGLu7 receptor suggests that granule cell survival was stimulated by an mGlu7-like metabotropic receptor.

Selective stimulation of excitatory amino acid receptor subtypes and the survival of granule cells in culture: effect of quisqualate and AMPA

Differentiating granule cells develop survival requirements in vitro which can be met by treatment with high K+ or excitatory amino acids. Promotion of cell survival by N-methyl-D-aspartate (NMDA) or kainate has already been established and here we report that treatment of the cells with alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) or quisqualate (QA) also leads to cell rescue. In comparison with the effect of NMDA, the influence of AMPA/QA is small, resulting in a 20-30% increase in cell survival, with a peak at a very narrow concentration range (0.5-2.0 microM QA and 5-10 microM AMPA). The effect is exclusive to AMPA receptor stimulation, since stimulation of metabotropic glutamate receptors with (1S3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) has no effect. Furthermore, AMPA/QA rescue of cells is blocked by ionotropic non-NMDA receptor antagonists, 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzoquinoxaline (NBQX). In addition, both nifedipine and dizolcipline (MK-801) interfered with the cell survival promoting effect of AMPA, suggesting that the influence of AMPA is mediated via calcium influx involving both depolarization-activated voltage sensitive calcium channels and NMDA receptors stimulated as a result of AMPA-induced release of glutamate. Possible reasons for the small cell survival promoting effect of AMPA/QA compared with the influence of high K+ or NMDA are discussed.

NMDA receptor-mediated excitoprotection of cultured cerebellar granule neurons fails to alter glutamate-induced expression of c-fos and c-jun mRNAs

Exposure of cultured cerebellar granule neurons to subtoxic concentrations of N-methyl-D-aspartate (NMDA) induces a state of excitoprotection when measured by subsequent exposure to toxic concentrations of glutamate. This NMDA-induced excitoprotective state is prevented by inhibitors of new RNA and protein synthesis. Since the neurotrophic and excitoprotective effects of NMDA in cerebellar granule neurons may involve changes in the expression of the immediate early genes c-fos and c-jun, we measured c-fos and c-jun mRNAs in cerebellar granule neurons after exposure to either toxic concentrations of glutamate or excitoprotective (subtoxic) concentrations of NMDA. Exposure of cerebellar granule neurons to toxic concentrations of glutamate induced a dramatic increase in c-fos and c-jun mRNAs which was not associated with a corresponding increase in c-fos and c-jun proteins as measured immunocytochemically. However, the increase in c-fos and c-jun mRNAs induced by toxic concentrations of glutamate was not altered by preexposing cerebellar granule neurons to NMDA, suggesting that increased expression of c-fos and c-jun mRNAs is not sufficient for glutamate toxicity of these neurons. Preexposure of cerebellar granule neurons to NMDA for 24 h, which induced a maximal excitoprotective state, resulted in a transient increase in c-fos, and to a lesser degree c-jun, mRNAs similar to that induced by toxic concentrations of glutamate. The induction of c-fos, but not that of c-jun, mRNA both by excitoprotective concentrations of NMDA and by neurotoxic concentrations of glutamate was blocked by the non-competitive NMDA receptor antagonist, MK-801.(ABSTRACT TRUNCATED AT 250 WORDS)

Promotion of granule cell survival by high K+ or excitatory amino acid treatment and Ca2+/calmodulin-dependent protein kinase activity

Cerebellar granule cells in culture develop survival requirements which can be met either by chronic membrane depolarization (25 mM K+) or by stimulation of ionotropic excitatory amino acid receptors. We observed previously that this trophic effect is mediated via Ca2+ influx, either through dihydropyridine-sensitive, voltage-dependent calcium channels (activated directly by high K+ or indirectly by kainate) or through N-methyl-D-aspartate receptor-linked ion channels. Steps after Ca2+ entry in the transduction cascade mediating the survival-supporting effect of high K+ and excitatory amino acids have now been examined. Using protein kinase inhibitors (H-7, polymixin B and gangliosides), and modulating protein kinase C activity by treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, we obtained evidence against the involvement of protein kinase C and cyclic nucleotide-dependent protein kinases in the transduction cascade. On the other hand, calmidazolium (employed as a calmodulin inhibitor) counteracted the trophic effect of elevated K+ with high potency (IC50 0.3 microM), which exceeded by approximately 10-fold the potency for the blockade by the drug of voltage-sensitive calcium channels. The potency of calmidazolium in interfering with the N-methyl-D-aspartate rescue of cells was also much higher in comparison with the inhibition of 45Ca2+ influx through N-methyl-D-aspartate receptor-linked channels. Our results indicated that after calmodulin the next step in the trophic effects involves Ca2+/calmodulin-dependent protein kinase II activity. KN-62, a fairly specific antagonist of this enzyme, compromised elevated K+ or excitatory amino acid-supported cell survival with high potency (IC50 2.5 microM). In the relevant concentration range, KN-62 had little or no effect on Ca2+ entry through either voltage- or N-methyl-D-aspartate receptor-gated channels. Combining information on the toxic action of glutamate in "mature" granule cells with the trophic effect of either excitatory amino acids or high K+ treatment on "young" cells, we conclude that after the initial steps involving calcium in both cases the respective transduction pathways diverge. The toxic action of glutamate seems to be mediated through protein kinase C [Favaron et al. (1990) Proc. natn. Acad. Sci. U.S.A. 87, 1983-1987 whereas a Ca2+/calmodulin-dependent protein kinase, which can be inhibited by KN-62 (but is resistant to gangliosides and to inhibitors whose potency is higher for protein kinase C than for Ca2+ calmodulin-dependent protein kinases, such as H-7 and polymixin B), is involved critically in the trophic effect.

Brain-derived neurotrophic factor is a survival factor for cultured rat cerebellar granule neurons and protects them against glutamate-induced neurotoxicity

We have studied the effects of different neurotrophins on the survival and proliferation of rat cerebellar granule cells in culture. These neurons express trkB and trkC, the putative neuronal receptors for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) respectively. Binding studies using iodinated BDNF and NT-3 demonstrated that both BDNF and NT-3 bind to the cerebellar granule neurons with a similar affinity of approximately 2 x 10(-9) M. The number of receptors per granule cell was surprisingly high, approximately 30 x 10(-4) and 2 x 10(5) for BDNF and NT-3, respectively. Both NT-3 and BDNF elevated c-fos mRNA in the granule neurons, but only BDNF up-regulated the mRNA encoding the low-affinity neurotrophin receptor (p75). In contrast to NT-3, BDNF acted as a survival factor for the granule neurons. BDNF also induced sprouting of the granule neurons and significantly protected them against neurotoxicity induced by high (1 mM) glutamate concentrations. Cultured granule neurons also expressed low levels of BDNF mRNA which were increased by kainic acid, a glutamate receptor agonist. Thus, BDNF, but not NT-3, is a survival factor for cultured cerebellar granule neurons and activation of glutamate receptor(s) up-regulates BDNF expression in these cells.

Neurotrophic effects of NMDA receptor activation on developing cerebellar granule cells

Glutamate acting on N-methyl-D-aspartate (NMDA) receptors controls a variety of aspects of neuronal plasticity in the adult and developing brain. This review summarizes its effects on developing cerebellar granule cells. The glutamatergic mossy fibre input to cerebellar granule cells exerts a neurotrophic effect on these cells during development. The investigation of potential neurotrophic agents can be carried out using enriched granule cell cultures. Considerable evidence now indicates that glutamate acting on N-methyl-D-aspartate receptors is an important neurotrophic factor that regulates granule cell development. In culture, neurite growth, differentiation and cell survival are all stimulated by N-methyl-D-aspartate receptor activation. The intracellular pathways involved following Ca2+ entry through the N-methyl-D-aspartate receptor channel are beginning to be elucidated. The cerebellar granule cell culture system may provide an ideal model to investigate the molecular mechanisms involved in long term N-methyl-D-aspartate receptor-mediated changes in neuronal function.

Phosphoproteins of cultured cerebellar granule cells and response to the differentiation-promoting stimuli NMDA, high K+ and ionomycin

In order to investigate signalling pathways involved in the control of granule cell differentiation, survival and other functions by depolarization or activation of NMDA receptors we have characterized protein phosphorylation in cerebellar granule cells. Cultures of cerebellar granule cells were incubated with 32P orthophosphate and then challenged with NMDA, K+ or the Ca2+ ionophore ionomycin, agents which raise [Ca2+]i and stimulate differentiation and survival. Upon separation of labelled phosphoproteins by two-dimensional gel electrophoresis three differences were found in response to all of these agents. These were an increase in acidity of two phosphoproteins of 87 and 48 kDa (p87 and p48) and increased 32P-incorporation into a phosphoprotein of 120 kDa (p120). Treatment with PMA which stimulates neurite outgrowth but not survival affected p87 (increased its acidity) but not p48. The acidic shift of p87, therefore, is not sufficient to stimulate granule cell survival. The identification of p87 as the actin-binding MARCKS protein and the demonstration of its presence in neurites and growth cones of granule cells suggests that it may be involved in NMDA-stimulated neurite outgrowth. The phosphoproteins p120 and p48 may potentially be involved in events linking the rise in [Ca2+]i to increased granule cell survival or other aspects of granule cell differentiation.

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)

N-methyl-D-aspartate responses in rat cerebellar granule cells are modified by chronic depolarisation in culture

Following culture in high (25 mM) K+ conditions cerebellar granule cells only respond with a rise in cytosolic free calcium concentration ([Ca2+]i after removal of external Mg2+. When granule cells are grown in low (5 mM) K+ N-methyl-D-aspartate (NMDA) exerts a neurotrophic effect. We show that at the critical time for this effect NMDA will elicit a rise in [Ca2+]i in 5 mM K+ cultures even in the presence of Mg2+ and that growth in 25 mM K+ induces the rapid appearance of a Mg2+ block of NMDA receptors in granule cells. This suggests firstly, that a rise in [Ca2+]i could be involved in the neurotrophic effect of NMDA and secondly, that the characteristics of the NMDA responses in granule cells are modified as a result of growth under depolarising conditions.