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Intracellular calcium ions regulate the structure and functions of cytoskeletal proteins. On the other hand, recent studies have shown that the cytoskeleton, and actin filaments in particular, can modulate calcium influx through plasma membrane ligand- and voltage-gated channels. We now report that calcium release from inositol trisphosphate (IP3) and ryanodine-sensitive endoplasmic reticulum (ER) stores is modulated by polymerization and depolymerization of actin filaments in cultured hippocampal neurons. Depolymerization of actin filaments with cytochalasin D attenuates calcium release induced by carbamylcholine (CCh; a muscarinic agonist for IP3 pathway), caffeine (a ryanodine receptor agonist) and thapsigargin (an inhibitor of the ER calciumATPase) in both the presence and absence of extracellular calcium. Conversely, the actin polymerizing agent jasplakinolide potentiates calcium release induced by CCh, caffeine and thapsigargin. Cytochalasin D attenuated, while jasplakinolide augmented, thapsigargin-induced JNK activation and neuronal cell death. Our data show that the actin cytoskeleton regulates ER calcium release, suggesting roles for actin in the various physiological and pathological processes that involve calcium release. [ABSTRACT FROM AUTHOR]

Biomarkers in the cerebrospinal fluid (CSF) are currently regarded as indispensable indicators for accurate differential diagnosis of neurodegenerative disorders. Although high levels of astrocyte-secreted glial fibrillar acidic protein (GFAP) in the CSF of patients with Alzheimer's disease (AD) have been reported, the levels of GFAP in the CSF have not been fully investigated in other neurological disorders that cause dementia, such as dementia with Lewy bodies (DLB) and frontotemporal lobar degeneration (FTLD). In this study, we determined the levels of GFAP in the CSF of healthy control subjects and AD, DLB, and FTLD patients to address two questions: (i) Do the levels of GFAP differ among these disorders? and (ii) Can GFAP be used as a biomarker for the differential diagnosis of these neurodegenerative disorders? The levels of GFAP in AD, DLB, and FTLD patients were significantly higher than those in the healthy control subjects. Although the levels of GFAP were not significantly different between AD and DLB patients, a higher level of GFAP was observed in FTLD patients than in AD and DLB patients. It is concluded that representative neurological disorders causing dementia were associated with higher levels of GFAP in the CSF. We propose the following mechanism concerning the amount of glial fibrillar acidic protein (GFAP) in the cerebrospinal fluid (CSF) in Alzheimer's disease (AD), dementia with Lewy bodies (DLB), and frontotemporal lobar degeneration (FTLD). The increase in the release of GFAP into CSF is considered to reflect the sum of degeneration of astrocytes and astrocytosis. The sum of degeneration and astrocytosis or the GFAP release could be in the order of FTLD > DLB > AD > normal condition., (© 2015 International Society for Neurochemistry.)

Gangliosides are considered to be involved in the maintenance and repair of nervous tissues. Recently, novel roles of gangliosides in the regulation of complement system were reported by us. In this study, we compared complement activation, inflammatory reaction and disruption of glycolipid-enriched microdomain (GEM)/rafts among various mutant mice of ganglioside synthases, i.e. GM2/GD2 synthase knockout (KO), GD3 synthase KO, double KO (DKO) of these two enzymes and wild type. Up-regulation of complement-related genes, deposits of C1q, proliferation of astrocytes and infiltration of microglia also showed similar gradual severity depending on the defects in ganglioside compositions. In the expression of inflammatory cytokines such as IL-1β and tumor necrosis factor α, only DKO showed definite up-regulation. Immunoblotting of fractions from sucrose density gradient ultracentrifugation revealed that lipid raft markers such as caveolin-1 and flotillin-1 tended to disperse from the raft fractions with intensities of DKO > GM2/GD2 synthase KO > GD3 synthase KO > wild type. Decay-accelerating factor and neural cell adhesion molecule tended to disappear from the raft fraction. Phospholipids and cholesterol also tended to decrease in GEM/rafts in GM2/GD2 synthase KO and DKO, although total amounts were almost equivalent. These results indicate that destruction of GEM/rafts is caused by ganglioside deficiency with gradual intensity depending on the degree of defects of their compositions., (© 2011 The Authors. Journal of Neurochemistry © 2011 International Society for Neurochemistry.)

GM2/GD2 synthase gene knockout mice lack all complex gangliosides, which are abundantly expressed in the nervous systems of vertebrates. In turn, they have increased precursor structures GM3 and GD3, probably replacing the roles of the depleted complex gangliosides. In this study, we found that 9-O-acetyl GD3 is also highly expressed as one of the major glycosphingolipids accumulating in the nervous tissues of the mutant mice. The identity of the novel component was confirmed by neuraminidase treatment, thin layer chromatography-immunostaining, two-dimensional thin layer chromatography with base treatment, and mass spectrometry. All candidate factors reported to be possible inducer of 9-O- acetylation, such as bitamine D binding protein, acetyl CoA transporter, or O-acetyl ganglioside synthase were not up-regulated. Tis21 which had been reported to be a 9-O-acetylation inducer was partially down-regulated in the null mutants, suggesting that Tis21 is not involved in the induction of 9-O-acetyl-GD3 and that accumulated high amount of GD3 might be the main factor for the dramatic increase of 9-O-acetyl GD3. The ability to acetylate exogenously added GD3 in the normal mouse astrocytes was examined, showing that the wild-type brain might be able to synthesize very low levels of 9-O-acetyl GD3. Increased 9-O-acetyl GD3, in addition to GM3 and GD3, may play an important role in the compensation for deleted complex gangliosides in the mutant mice.

The hippocampus is a brain region critical for learning and memory processes believed to result from long-lasting changes in the function and structure of synapses. Recent findings suggest that ATP functions as a neurotransmitter or neuromodulator in the mammalian brain, where it activates several different types of ionotropic and G protein-coupled ATP receptors that transduce calcium signals. However, the roles of specific ATP receptors in synaptic plasticity have not been established. Here we show that mice lacking the P2X3 ATP receptor (P2X3KO mice) exhibit abnormalities in hippocampal synaptic plasticity that can be restored by pharmacological modification of calcium-sensitive kinase and phosphatase activities. Calcium imaging studies revealed an attenuated calcium response to ATP in hippocampal neurons from P2X3KO mice. Basal synaptic transmission, paired-pulse facilitation and long-term potentiation are normal at synapses in hippocampal slices from P2X3KO. However, long-term depression is severely impaired at CA1, CA3 and dentate gyrus synapses. Long-term depression can be partially rescued in slices treated with a protein phosphatase 1-2 A activator or by postsynaptic inhibition of calcium/calmodulin-dependent protein kinase II. Despite the deficit in hippocampal long-term depression, P2X3KO mice performed normally in water maze tests of spatial learning, suggesting that long-term depression is not critical for this type of hippocampus-dependent learning and memory.

Best known for their pivotal role in a form of programmed cell death called apoptosis, caspases may also function in more subtle physiological processes. Caspases are present in synapses and dendrites of neurons where they can be activated in response to glutamate receptor stimulation and calcium influx. Here we tested the hypothesis that caspase-1 plays a role in modulating long-term potentiation (LTP) at hippocampal synapses. We provide evidence that caspase-1 plays a role in regulating alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated calcium influx and synaptic plasticity in the hippocampus. LTP of excitatory postsynaptic potentials at CA1 synapses was significantly enhanced when hippocampal slices were treated with either a pan-caspase inhibitor or a selective inhibitor of caspase-1, but not by an inhibitor of caspase-6. Inhibition of caspase-1 significantly enhanced the AMPA current-mediated component of LTP without affecting the N-methyl-D-aspartate current-mediated component. Calcium responses to AMPA were enhanced in hippocampal neurons treated with a caspase-1 inhibitor suggesting that caspase-1 normally functions to reduce AMPA receptor-mediated calcium influx. These findings suggest that, by selectively reducing AMPA currents and calcium influx, caspase-1 functions as a negative regulator of LTP at hippocampal synapses.

Mutations in alpha-synuclein cause some cases of familial Parkinson's disease (PD), but the mechanism by which alpha-synuclein promotes degeneration of dopamine-producing neurons is unknown. We report that human neural cells expressing mutant alpha-synuclein (A30P and A53T) have higher plasma membrane ion permeability. The higher ion permeability caused by mutant alpha-synuclein would be because of relatively large pores through which most cations can pass non-selectively. Both the basal level of [Ca2+]i and the Ca2+ response to membrane depolarization are greater in cells expressing mutant alpha-synuclein. The membrane permeable Ca2+ chelator BAPTA-AM significantly protected the cells against oxidative stress, whereas neither L-type (nifedipine) nor N-type (omega-conotoxin-GVIA) Ca2+ channel blockers protected the cells. These findings suggest that the high membrane ion permeability caused by mutant alpha-synuclein may contribute to the degeneration of neurons in PD.

Telomeres are specialized structures at the ends of chromosomes that consist of tandem repeats of the DNA sequence TTAGGG and several proteins that protect the DNA and regulate the plasticity of the telomeres. The telomere-associated protein TRF2 (telomeric repeat binding factor 2) is critical for the control of telomere structure and function; TRF2 dysfunction results in the exposure of the telomere ends and activation of ATM (ataxia telangiectasin mutated)-mediated DNA damage response. Recent findings suggest that telomere attrition can cause senescence or apoptosis of mitotic cells, but the function of telomeres in differentiated neurons is unknown. Here, we examined the impact of telomere dysfunction via TRF2 inhibition in neurons (primary embryonic hippocampal neurons) and mitotic neural cells (astrocytes and neuroblastoma cells). We demonstrate that telomere dysfunction induced by adenovirus-mediated expression of dominant-negative TRF2 (DN-TRF2) triggers a DNA damage response involving the formation of nuclear foci containing phosphorylated histone H2AX and activated ATM in each cell type. In mitotic neural cells DN-TRF2 induced activation of both p53 and p21 and senescence (as indicated by an up-regulation of beta-galactosidase). In contrast, in neurons DN-TRF2 increased p21, but neither p53 nor beta-galactosidase was induced. In addition, TRF2 inhibition enhanced the morphological, molecular and biophysical differentiation of hippocampal neurons. These findings demonstrate divergent molecular and physiological responses to telomere dysfunction in mitotic neural cells and neurons, indicate a role for TRF2 in regulating neuronal differentiation, and suggest a potential therapeutic application of inhibition of TRF2 function in the treatment of neural tumors.

We have characterized several potential stem cell markers and defined the membrane properties of rat fetal (E10.5) neural stem cells (NSC) by immunocytochemistry, electrophysiology and microarray analysis. Immunocytochemical analysis demonstrates specificity of expression of Sox1, ABCG2/Bcrp1, and shows that nucleostemin labels both progenitor and stem cell populations. NSCs, like hematopoietic stem cells, express high levels of aldehyde dehydrogenase (ALDH) as assessed by Aldefluor labeling. Microarray analysis of 96 transporters and channels showed that Glucose transporter 1 (Glut1/Slc2a1) expression is unique to fetal NSCs or other differentiated cells. Electrophysiological examination showed that fetal NSCs respond to acetylcholine and its agonists, such as nicotine and muscarine. NSCs express low levels of tetrodotoxin (TTX) sensitive and insensitive sodium channels and calcium channels while expressing at least three kinds of potassium channels. We find that gap junction communication is mediated by connexin (Cx)43 and Cx45, and is essential for NSC survival and proliferation. Overall, our results show that fetal NSCs exhibit a unique signature that can be used to determine their location and assess their ability to respond to their environment.

Glucagon-like peptide 1 (GLP-1) activates receptors coupled to cAMP production and calcium influx in pancreatic cells, resulting in enhanced glucose sensitivity and insulin secretion. Despite evidence that the GLP-1 receptor is present and active in neurons, little is known of the roles of GLP-1 in neuronal physiology. As GLP-1 modulates calcium homeostasis in pancreatic beta cells, and because calcium plays important roles in neuronal plasticity and neurodegenerative processes, we examined the effects of GLP-1 on calcium regulation in cultured rat hippocampal neurons. When neurons were pre-treated with GLP-1, calcium responses to glutamate and membrane depolarization were attenuated. Whole-cell patch clamp analyses showed that glutamate-induced currents and currents through voltage-dependent calcium channels were significantly decreased in neurons pre-treated with GLP-1. Pre-treatment of neurons with GLP-1 significantly decreased their vulnerability to death induced by glutamate. Acute application of GLP-1 resulted in a transient elevation of intracellular calcium levels, consistent with the established effects of GLP-1 on cAMP production and activation of cAMP response element-binding protein. Collectively, our findings suggest that, by modulating calcium responses to glutamate and membrane depolarization, GLP-1 may play important roles in regulating neuronal plasticity and cell survival.

Oxidized metabolites of dopamine, known as dopamine quinone derivatives, are thought to play a pivotal role in the degeneration of dopaminergic neurons. Although such quinone derivatives are usually produced via the autoxidation of catecholamines, tyrosinase, which is a key enzyme in melanin biosynthesis via the production of DOPA and subsequent molecules, may potentially accelerate the induction of catecholamine quinone derivatives by its oxidase activity. In the present study, we developed neuronal cell lines in which the expression of human tyrosinase was inducible. Overexpression of tyrosinase in cultured cell lines resulted in (i) increased intracellular dopamine content; (ii) induction of oxidase activity not only for DOPA but also for dopamine; (iii) formation of melanin pigments in cell soma; and (iv) increased intracellular reactive oxygen species. Interestingly, the expressed tyrosinase protein was initially distributed in the entire cytoplasm and then accumulated to form catecholamine-positive granular structures by 3 days after the induction. The granular structures consisted of numerous rounded, dark bodies of melanin pigments and were largely coincident with the distribution of lysosomes. This cellular model that exhibits increased dopamine production will provide a useful tool for detailed analyses of the potentially noxious effects of oxidized catecholamine metabolites.

Tau, a microtubule binding protein, is not only a major component of neurofibrillary tangles in Alzheimer's disease, but also a causative gene for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We show here that an FTDP-17 tau mutation (V337M) in SH-SY5Y cells reduces microtubule polymerization, increases voltage-dependent calcium current (ICa) density, and decreases ICa rundown. The reduced rundown of ICa by V337M was significantly inhibited by nifedipine (L-type Ca channel blocker), whereas omega-conotoxin GVIA (N-type Ca channel blocker) showed smaller effects, indicating that tau mutations affect L-type calcium channel activity. The depolarization-induced increase in intracellular calcium was also significantly augmented by the V337M tau mutation. Treatment with a microtubule polymerizing agent (taxol), an adenylyl cyclase inhibitor, or a protein kinase A (PKA) inhibitor, counteracted the effects of mutant tau on ICa. Taxol also attenuated the Ca2+ response to depolarization in cells expressing mutant tau. Apoptosis in SH-SY5Y cells induced by serum deprivation was exacerbated by the V337M mutation, and nifedipine, taxol, and a PKA inhibitor significantly protected cells against apoptosis. Our results indicate that a tau mutation which decreases its microtubule-binding ability augments calcium influx by depolymerizing microtubules and activating adenylyl cyclase and PKA.

Since most brain glycoproteins from beta-1,4-galactosyltransferase (beta-1,4-GalT) I knockout mice were galactosylated without apparent reduction the gene expression of novel beta-1,4-GalTs II and V which are involved in N-linked oligosaccharide biosynthesis in addition to beta-1,4-GalT I was studied during mouse brain development. Isolation and characterization of beta-1,4-GalT II and V cDNAs from mouse brains indicates that they are also functioning in the brain. Northern blot analysis revealed that the beta-1,4-GalT I gene is expressed mainly in mid-embryonic stages, while the expression level of beta-1,4-GalT II transcript remains constant and of beta-1,4-GalT V transcript increases during mouse brain development after birth. In situ hybridization revealed that beta-1,4-GalT II and V signals are present in most neural cells, with a marked difference between them in the hippocampus of adult mouse brain tissue. The differential gene expression of beta-1,4-GalTs I, II and V during mouse brain development could affect the differential galactosylation of brain glycoproteins, as revealed by lectin blot analysis.

Tumor necrosis factor-alpha (TNF alpha) is a cytokine rapidly produced in the brain in response to vigorous neuronal activity and tissue injury. TNF alpha may protect neurons against excitotoxic and oxidative insults by a mechanism involving activation of the transcription factor NF-kappaB. Whole-cell perforated patch clamp recordings in cultured rat hippocampal neurons showed that long-term treatment (24-48 h) with TNF alpha increases Ca2+ current density; pharmacological analysis indicated a major increase in current through L-type voltage-dependent calcium channels. Long-term treatment with TNF alpha caused a decrease in currents induced by glutamate, NMDA, AMPA, and kainate. Shorter exposures to TNF alpha (acute; 2 h) did not alter Ca2+ current or glutamate receptor agonist-induced currents. Ceramide, an intracellular messenger that activates the transcription factor NF-kappaB, mimicked the actions of TNFs on Ca2+ current density and currents induced by glutamate receptor agonists. Cotreatment with kappaB decoy DNA abolished the effects of TNF alpha on Ca2+ current and excitatory amino acid-induced currents, demonstrating a requirement for NF-kappaB activation in the actions of TNF alpha. Neurons pretreated with TNF alpha exhibited increased intracellular Ca2+ concentrations following membrane depolarization but reduced intracellular Ca2+ concentration responses to excitatory amino acids, compared with neurons in untreated control cultures or cultures cotreated with kappaB decoy DNA. These findings suggest important roles for the transcription factor NF-kappaB in modulation of voltage-dependent calcium channels and glutamate receptors and the many physiological and pathophysiological processes in which these ion channels are involved. Such signaling mechanisms may be particularly important in injury settings such as ischemia or trauma, where TNF alpha expression is increased and NF-kappaB is activated.

Many cases of autosomal dominant inherited forms of early-onset Alzheimer's disease are caused by mutations in the genes encoding presenilin-1 (PS-1; chromosome 14) and presenilin-2 (PS-2; chromosome 1). PSs are expressed in neurons throughout the brain wherein they appear to be localized primarily to the endoplasmic reticulum (ER) of cell bodies and dendrites. PS-1 and PS-2 show high homology and are predicted to have eight transmembrane domains with the C terminus, N terminus, and a loop domain all on the cytosolic side of the membrane; an enzymatic cleavage of PSs occurs at a site near the loop domain. The normal function of PSs is unknown, but data suggest roles in membrane trafficking, amyloid precursor protein processing, and regulation of ER calcium homeostasis. Homology of PSs to the C. elegans gene sel-12, which is involved in Notch signaling, and phenotypic similarities of PS-1 and Notch knockout mice suggest a developmental role for PSs in the nervous system. When expressed in cultured cells and transgenic mice, mutant PSs promote increased production of a long form of amyloid beta-peptide (A beta1-42) that may possess enhanced amyloidogenic and neurotoxic properties. PS mutations sensitize cultured neural cells to apoptosis induced by trophic factor withdrawal, metabolic insults, and amyloid beta-peptide. The mechanism responsible for the proapoptotic action of mutant PSs may involve perturbed calcium release from ER stores and increased levels of oxidative stress. Recent studies of apoptosis in many different cell types suggest that ER calcium signaling can modulate apoptosis. The evolving picture of PS roles in neuronal plasticity and Alzheimer's disease is bringing to the forefront the ER, an organelle increasingly recognized as a key regulator of neuronal plasticity and survival.

Lysophosphatidic acid (LPA) is a lipid biomediator enriched in the brain. A novel LPA-induced response in rat hippocampal neurons is described herein, namely, a rapid and sustained elevation in the concentration of free intracellular calcium ([Ca2+]i). This increase is specific, in that the related lipids phosphatidic acid and lysophosphatidylcholine did not induce an alteration in [Ca2+]i. Moreover, consistent with a receptor-mediated process, there was no further increase in [Ca2+]i after a second addition of LPA. The LPA-induced increase in [Ca2+]i required extracellular calcium. However, studies with Cd2+, Ni2+, and nifedipine and nystatin-perforated patch clamp analyses did not indicate involvement of voltage-gated calcium channels in the LPA-induced response. In contrast, glutamate appears to have a significant role in the LPA-induced increase in [Ca2+]i, because this increase was inhibited by NMDA receptor antagonists and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonists. Thus, LPA treatment may result in an increased extracellular glutamate concentration that could stimulate AMPA/kainate receptors and thereby alleviate the Mg2+ block of the NMDA receptors and lead to glutamate stimulation of an influx of calcium via NMDA receptors.

Proteolytic cleavage of beta-amyloid precursor protein (beta APP) by alpha-secretase results in release of one secreted form (sAPP) of APP (sAPP alpha), whereas cleavage by beta-secretase releases a C-terminally truncated sAPP (sAPP beta) plus amyloid beta-peptide (A beta). beta APP mutations linked to some inherited forms of Alzheimer's disease may alter its processing such that levels of sAPP alpha are reduced and levels of sAPP beta increased. sAPP alpha s may play important roles in neuronal plasticity and survival, whereas A beta can be neurotoxic. sAPP alpha was approximately 100-fold more potent than sAPP beta in protecting hippocampal neurons against excitotoxicity, A beta toxicity, and glucose deprivation. Whole-cell patch clamp and calcium imaging analyses showed that sAPP beta was less effective than sAPP alpha in suppressing synaptic activity, activating K+ channels, and attenuating calcium responses to glutamate. Using various truncated sAPP alpha and sAPP beta APP695 products generated by eukaryotic and prokaryotic expression systems, and synthetic sAPP peptides, the activity of sAPP alpha was localized to amino acids 591-612 at the C-terminus. Heparinases greatly reduced the actions of sAPP alpha s, indicating a role for a heparin-binding domain at the C-terminus of sAPP alpha in receptor activation. These findings indicate that alternative processing of beta APP has profound effects on the bioactivity of the resultant sAPP products and suggest that reduced levels of sAPP alpha could contribute to neuronal degeneration in Alzheimer's disease.

Bovine pituitary glycoprotein hormones contain unique N-linked sugar chains with GalNAc beta 1-->4GlcNAc 4GlcNAc structure in their outer chain moieties. In the present study, whether bovine pituitary membrane glycoproteins contain the sugar chains with the disaccharide structure was investigated. Western blot analysis of the membrane glycoproteins using Wistaria floribunda agglutinin (WFA), which binds oligosaccharides terminating with beta-N-acetylgalactosamine residue(s), showed that most protein bands detected with Coomassie Brilliant Blue staining bind to WFA. However, no WFA binding was observed for the bands after treatment of the blotted filter with jack bean beta-N-acetylhexosaminidase or N-Glycanase. The WFA-positive bands were also detected in membrane glycoprotein samples from bovine cerebrum, cerebellum, and medulla oblongata, although their expression levels were low. Structural analysis of the oligosaccharides released by hydrazinolysis from the pituitary membrane glycoproteins by serial lectin column chromatography and sequential exoglycosidase digestion revealed that the major oligosaccharides, which bound to a WFA-agarose column, are of biantennary complex type with one and two GalNAc beta 1-->4GlcNAc groups in their outer chain moieties. These results indicate that the beta-N-acetylgalactosaminylation is not unique to the glycoprotein hormones but occurs to most bovine pituitary glycoproteins.

Gangliosides are synthesized by sequential catalytic reaction of multiple glycosyltransferases. GM2/GD2 synthase and GD3 synthase are key enzymes for ganglioside synthesis, because their relative activities regulate the main profiles of ganglioside expression. Mouse GD3 synthase (EC 2.4.99.8) cDNA was cloned by eukaryotic expression cloning, and its mRNA expression as well as that of GM2/GD2 synthase gene during the development of the mouse CNS was analyzed by using northern blotting, reverse transcription-polymerase chain reaction, and in situ hybridization. When brain tissue was analyzed as a whole mass, a typical pattern corresponding to the reported findings obtained by biochemical analyses was observed, i.e., high expression of GD3 synthase gene in the early stage and gradual increase of GM2/GD2 synthase gene expression in the late stage of the development. However, the results of in situ hybridization of these two genes revealed that the expression kinetics of these two genes were heterogeneous among various sites in the brain under development. These findings suggest that various expression patterns of the two genes reflect differences in the course of the development of individual sites, and also different ganglioside components are required in individual portions of the brain for development and maintenance of the function.

Among various tissues of mouse, beta 1,4-N-acetylgalactosaminyltransferase (GM2/GD2 synthase) gene is expressed predominantly in the brain. Further analysis of the gene expression in the mouse CNS was performed by northern blotting and by enzyme assays using extracts from various parts of the CNS. In situ hybridization was also done to investigate the distribution of cells generating GM2/GD2 synthase. In northern blots, diverse levels of the gene expression were observed, depending on the regions examined. By in situ hybridization, pyramidal cells in the hippocampus, granular cells in dentate gyrus and cerebral cortex, Purkinje cells in cerebellum, and mitral cells in the olfactory bulb expressed high levels of the mRNA; these results corresponded to the results obtained by northern blot. Enzyme levels in these sites were accordingly high. However, enzyme levels in certain areas with low mRNA intensities, such as thalamus and pons medulla, were higher than expected from the results of northern blotting. The significance of the high gene expression in certain areas for brain function and the reason for the discrepancy between mRNA level and enzyme activity in some regions are discussed.

The excitatory neurotransmitter glutamate is believed to play important roles in development, synaptic plasticity, and neurodegenerative conditions. Recent studies have shown that neurotrophic factors can modulate neuronal excitability and survival and neurite outgrowth responses to glutamate, but the mechanisms are unknown. The present study tested the hypothesis that neurotrophic factors modulate responses to glutamate by affecting the expression of specific glutamate-receptor proteins. Exposure of cultured embryonic rat hippocampal cells to basic fibroblast growth factor (bFGF) resulted in a concentration-dependent increase in levels of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor subunit GluR1 protein as determined by western blot, dot-blot, and immunocytochemical analyses. In contrast, bFGF did not alter levels of GluP2/3, GluR4, or the NMDA-receptor subunit NR1. Nerve growth factor did not affect GluR1 levels. Calcium-imaging studies revealed that elevation of [Ca2+]i, resulting from selective AMPA-receptor activation, was enhanced in bFGF-pretreated neurons. On the other hand, [Ca2+]i responses to NMDA-receptor activation were suppressed in bFGF-treated neurons, consistent with previous studies showing that bFGF can protect neurons against NMDA toxicity. Moreover, neurons pretreated with bFGF were relatively resistant to the toxicities of glutamate and AMPA, both of which were shown to be mediated by NMDA receptors. These data suggest that differential regulation of the expression of specific glutamate-receptor subunits may be an important mechanism whereby neurotrophic factors modulate activity-dependent neuronal plasticity and vulnerability to excitotoxicity.

Exposure of cultured rat hippocampal neurons to glutamate resulted in accumulation of cellular peroxides (measured using the dye 2,7-dichlorofluorescein). Peroxide accumulation was prevented by an N-methyl-D-aspartate (NMDA) receptor antagonist and by removal of extracellular Ca2+, indicating the involvement of NMDA receptor-induced Ca2+ influx in peroxide accumulation. Glutamate-induced reactive oxygen species contributed to loss of Ca2+ homeostasis and excitotoxic injury because antioxidants (vitamin E, propyl gallate, and N-tert-butyl-alpha-phenylnitrone) suppressed glutamate-induced elevation of intracellular Ca2+ concentration ([Ca2+]i) and cell death. Basic fibroblast growth factor (bFGF), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF), but not ciliary neurotrophic factor, each suppressed accumulation of peroxides induced by glutamate and protected neurons against excitotoxicity. bFGF, NGF, and BDNF each increased (to varying degrees) activity levels of superoxide dismutases and glutathione reductase. NGF increased catalase activity, and BDNF increased glutathione peroxidase activity. The ability of the neurotrophic factors to suppress glutamate toxicity and glutamate-induced peroxide accumulation was attenuated by the tyrosine kinase inhibitor genistein, indicating the requirement for tyrosine phosphorylation in the neuro-protective signal transduction mechanism. The data suggest that glutamate toxicity involves peroxide production, which contributes to loss of Ca2+ homeostasis, and that induction of antioxidant defense systems is a mechanism underlying the [Ca2+]i-stabilizing and excitoprotective actions of neurotrophic factors.

Increasing data suggest that the amyloid beta-peptide (A beta), which accumulates in the brains of Alzheimer's victims, plays a role in promoting neuronal degeneration. Cell culture studies have shown that A beta can be neurotoxic and recent findings suggest that the mechanism involves destabilization of cellular calcium homeostasis. We now report that cytochalasin D, a compound that depolymerizes actin microfilaments selectively, protects cultured rat hippocampal neurons against A beta neurotoxicity. Cytochalasin D was effective at concentrations that depolymerized actin (10-100 nM). The elevation of [Ca2+]i induced by A beta, and the enhancement of [Ca2+]i responses to glutamate in neurons exposed to A beta, were markedly attenuated in neurons pretreated with cytochalasin D. The protective effect of cytochalasin D appeared to result from a specific effect on actin filaments and reduction in calcium influx, because cytochalasin E, another actin filament-disrupting agent, also protected neurons against A beta toxicity; the microtubule-disrupting agent colchicine was ineffective; cytochalasin D did not protect neurons against the toxicity of hydrogen peroxide. These findings suggest that actin filaments play a role in modulating [Ca2+]i responses to neurotoxic insults and that depolymerization of actin can protect neurons against insults relevant to the pathogenesis of Alzheimer's disease.

We examined the mechanism underlying the ATP-induced increase in the cytosolic Ca2+ concentration ([Ca]in) in acutely isolated chick ciliary ganglion neurons, using fura-2 microfluorometry. The ATP-induced increase in [Ca]in was dependent on external Ca2+, was blocked in a dose-dependent manner by reactive blue 2, and was substantially inhibited by both L- and N-type Ca2+ channel blockers. ATP was effective in increasing [Ca]in in the presence of a desensitizing concentration of nicotine (100 microM), and simultaneous addition of maximal doses of ATP and nicotine caused an additive increase in [Ca]in, suggesting that ATP acts on a site distinct from nicotinic acetylcholine receptors. ATP also increased the cytosolic Na+ concentration as determined by sodium-binding benzofuran isophthalate microfluorometry. These results suggest that ATP increases Na+ influx through P2 purinoceptor-associated channels resulting in membrane depolarization, which in turn increases Ca2+ influx through voltage-dependent Ca2+ channels. However, ATP still caused a small increase in [Ca]in under Na+-free conditions, and this [Ca]in increase was little affected by Ca2+ channel blockers. ATP also increased Mn2+ influx under Na+-free conditions, as indicated by quenching of fura-2 fluorescence. These results suggest that nonselective cationic channels activated by ATP are permeable not only to Ca2+ but also to Mn2+, in addition to monovalent cations.