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Biomedical subjects

S W Barger

Publications and source records attributed to S W Barger.

At least 19 recordsLinked to original sources
PubMed2001

Vitamin E suppression of microglial activation is neuroprotective.

Neurotoxic microglial-neuronal interactions have been implicated in the pathogenesis of various neurodegenerative diseases such as Alzheimer's disease, and vitamin E has been shown to have direct neuroprotective effects. To determine whether vitamin E also has indirect neuroprotective effects through suppression of microglial activation, we used a microglial-neuronal coculture. Lipopolysaccharide (LPS) treatment of a microglial cell line (N9) induced a time-dependent activation of both p38 mitogen-activated protein kinase (p38 MAPK) and nuclear factor-kappaB (NFkappaB), with consequent increases in interleukin-1alpha (IL-1alpha), tumor necrosis factor-alpha (TNF-alpha), and nitric oxide (NO) production. Differentiated neuronal cells (PC12 cells treated with nerve growth factor) exhibited marked loss of processes and decreased survival when cocultured with LPS-activated microglia. Preincubation of microglia with vitamin E diminished this neurotoxic effect, independently of direct effects of the antioxidant on the neuronal cells. Microglial NO production and the induction of IL-1alpha and TNFalpha expression also were attenuated by vitamin E. Such antiinflammatory effects of vitamin E were correlated with suppression of p38 MAPK and NFkappaB activation and were mimicked by an inhibition of either p38 MAPK (by SB203580) or NFkappaB (by decoy oligonucleotides). These results suggest that, in addition to the beneficial effects of providing direct antioxidant protection to neurons reported by others, vitamin E may provide neuroprotection in vivo through suppression of signaling events necessary for microglial activation.

Animals↗
PubMed2001

Activation of microglia by secreted amyloid precursor protein evokes release of glutamate by cystine exchange and attenuates synaptic function.

Microglial activation as part of a chronic inflammatory response is a prominent component of Alzheimer's disease. Secreted forms of the beta-amyloid precursor protein (sAPP) previously were found to activate microglia, elevating their neurotoxic potential. To explore neurotoxic mechanisms, we analyzed microglia-conditioned medium for agents that could activate glutamate receptors. Conditioned medium from primary rat microglia activated by sAPP caused a calcium elevation in hippocampal neurons, whereas medium from untreated microglia did not. This response was sensitive to the NMDA receptor antagonist, aminophosphonovaleric acid. Analysis of microglia-conditioned by HPLC revealed dramatically higher concentrations of glutamate in cultures exposed to sAPP. Indeed, the glutamate levels in sAPP-treated cultures were substantially higher than those in cultures treated with amyloid beta-peptide. This sAPP-evoked glutamate release was completely blocked by inhibition of the cystine-glutamate antiporter by alpha-aminoadipate or use of cystine-free medium. Furthermore, a sublethal concentration of sAPP compromised synaptic density in microglia-neuron cocultures, as evidenced by neuronal connectivity assay. Finally, the neurotoxicity evoked by sAPP in microglia-neuron cocultures was attenuated by inhibitors of either the neuronal nitric oxide synthase (N(G)-propyl-L-arginine) or inducible nitric oxide synthase (1400 W). Together, these data indicate a scenario by which microglia activated by sAPP release excitotoxic levels of glutamate, probably as a consequence of autoprotective antioxidant glutathione production within the microglia, ultimately causing synaptic degeneration and neuronal death.

2-Aminoadipic Acid↗
PubMed2001

Pro-inflammatory and pro-oxidant properties of the HIV protein Tat in a microglial cell line: attenuation by 17 beta-estradiol.

Microglia are activated in humans following infection with human immunodeficiency virus (HIV), and brain inflammation is thought to be involved in neuronal injury and dysfunction during HIV infection. Numerous studies indicate a role for the HIV regulatory protein Tat in HIV-related inflammatory and neurodegenerative processes, although the specific effects of Tat on microglial activation, and the signal transduction mechanisms thereof, have not been elucidated. In the present study, we document the effects of Tat on microglial activation and characterize the signal transduction pathways responsible for Tat's pro-inflammatory effects. Application of Tat to N9 microglial cells increased multiple parameters of microglial activation, including superoxide production, phagocytosis, nitric oxide release and TNF alpha release. Tat also caused activation of both p42/p44 mitogen activated protein kinase (MAPK) and NF kappa B pathways. Inhibitor studies revealed that Tat-induced NF kappa B activation was responsible for increased nitrite release, while MAPK activation mediated superoxide release, TNF alpha release, and phagocytosis. Lastly, pre-treatment of microglial cells with physiological concentrations of 17 beta-estradiol suppressed Tat-mediated microglial activation by interfering with Tat-induced MAPK activation. Together, these data elucidate specific components of the microglial response to Tat and suggest that Tat could contribute to the neuropathology associated with HIV infection through microglial promulgation of oxidative stress.

Animals↗
PubMed2000

Dehydroepiandrosterone inhibits microglial nitric oxide production in a stimulus-specific manner.

Dehydroepiandrosterone (DHEA) is a steroid that circulates in abundance in the form of a sulfated reserve (DHEA-S). The levels of DHEA decline with age and further in age-related neuropathologies, including Alzheimer disease. Because of their reported anti-inflammatory effects, we tested the actions of these compounds on microglia. At concentrations of 3(-9) to 1(-6) M, DHEA and DHEA-S inhibited the production of nitrite and morphological changes stimulated by lipopolysaccharide. DHEA and DHEA-S also inhibited LPS induction of iNOS protein, but neither inhibited LPS-induced iNOS mRNA or the activation of NF-kappaB. These data suggest that the hormone regulates nitrite production through a post-transcriptional mechanism. Interestingly, microglial nitrite production in response to a secreted form of the beta-amyloid precursor protein (sAPP) was unaffected by DHEA. Another Alzheimer-related factor, amyloid beta-peptide, also stimulated microglial nitrite production but in a manner dependent on the co-stimulus interferon-gamma. DHEA was found to inhibit only the interferon-gamma component of the microglial response. These data add to a growing body of evidence for differences in the profiles of mononuclear phagocytes activated by distinct stimuli.

Alzheimer Disease↗
PubMed2000

Neuronal-glial interactions mediated by interleukin-1 enhance neuronal acetylcholinesterase activity and mRNA expression.

Cholinergic dysfunction in Alzheimer's disease has been attributed to stress-induced increases in acetylcholinesterase (AChE) activity. Interleukin-1 (IL-1) is overexpressed in Alzheimer's disease, and stress-related changes in long-term potentiation, an ACh-related cerebral function, are triggered by interleukin-1. Microglial cultures (N9) synthesized and released IL-1 in response to conditioned media obtained from glutamate-treated primary neuron cultures or PC12 cells. This conditioned media contained elevated levels of secreted beta-amyloid precursor protein (sAPP). Naive PC12 cells cocultured with stimulated N9 cultures showed increased AChE activity and mRNA expression. These effects on AChE expression and activity could be blocked by either preincubating the glutamate-treated PC12 supernatants with anti-sAPP antibodies or preincubating naive PC12 cells with IL-1 receptor antagonist. These findings were confirmed in vivo; IL-1-containing pellets implanted into rat cortex also increased AChE mRNA levels. Neuronal stress in Alzheimer's disease may induce increases in AChE expression and activity through a molecular cascade that is mediated by sAPP-induced microglial activation and consequent overexpression of IL-1.

Acetylcholinesterase↗
PubMed2000

S100beta induction of the proinflammatory cytokine interleukin-6 in neurons.

Levels of the neurotrophic cytokine S100beta and the proinflammatory cytokine interleukin-6 (IL-6) are both elevated in Alzheimer's brain, and both have been implicated in beta-amyloid plaque formation and progression. We used RT-PCR and electrophoretic mobility shift assay to assess S100beta induction of IL-6 expression and the role of kappaB-dependent transcription in this induction in neuron-enriched cultures and in neuron-glia mixed cultures from fetal rat cortex. S100beta (10 or 100 ng/ml x 24 h) increased IL-6 mRNA levels two- and fivefold, respectively (p<0.05 in each case), and S100beta (100-1,000 ng/ml) induced increases in medium levels of biologically active IL-6 (30-80%). Combined in situ hybridization and immunohistochemistry preparations localized IL-6 mRNA to neurons in these cultures. S100beta induction of IL-6 expression correlated with an increase in DNA binding activity specific for a KB element and was inhibited (75%) by suppression of kappaB binding with double-stranded "decoy" oligonucleotides. The low levels of S100beta required to induce IL-6 overexpression in neurons, shown here, suggest that overexpression of S100beta induces neuronal expression of IL-6 and of IL-6-induced neurodegenerative cascades in Alzheimer's disease.

Animals↗
PubMed1999

Inhibition of AMPA responses by mutated presenilin 1.

The inheritance of Alzheimer's disease in some families, as well as ablation/rescue genetics in mice, suggest that point mutations in the presenilin-1 (PS1) gene can cause disease through an unknown gain-of-function. While mutations associated with familial Alzheimer's can alter apoptotic rates and beta-amyloid precursor processing, it is possible that other physiological effects contribute to pathogenesis. We have begun to explore effects on neurotransmission by monitoring responses of the neuropotent Ntera-2 cell line expressing wild-type PS1 or a FAD mutant thereof. Although no differences were initially apparent in calcium responses of metabotropic receptors, responses to glutamate were dampened in cells expressing the L286V mutant of PS1. Analysis of ionotropic agonists demonstrated that AMPA receptor alterations were responsible for this effect, whereas NMDA responses were unaltered. These data suggest that PS1 mutation could lead to cognitive deficits through subtoxic physiological effects.

Alzheimer Disease↗
PubMed1999

Characterization of a neuronal kappaB-binding factor distinct from NF-kappaB.

Transcription factors that bind kappaB enhancer elements have begun to garner wide attention in neurobiology. Data suggest that activation of kappaB-binding factors in neurons can be protective against various neurotoxins, but other data have connected NF-kappaB to cell death. In electrophoretic mobility shift assays of kappaB-binding activity, we have found that the predominant activity in rat brain tissue, in primary neurons, and in neuronal cell lines has a mobility inconsistent with that of bona fide NF-kappaB (RelA-p50 heterodimer). We have tentatively termed this activity neuronal kappaB-binding factor (NKBF). Competition assays with various DNA probes distinguished NKBF from NF-kappaB. Probes that efficiently bind the p50 homodimer were able to compete with a conventional NF-kappaB probe for NKBF binding, but NKBF did not react with antibodies to p50 (or any other known Rel family members). Furthermore, UV-crosslinking indicated that NKBF is composed of two polypeptides of 82 kDa and 27 kDa. Although NKBF activity can be elevated in a manner independent of new macromolecular synthesis, it does not appear to be modulated by IkappaB. Finally, no NF-kappaB was induced by glutamate in highly enriched neuronal cultures, although it was induced in neuron-glia cocultures. These data suggest that the primary kappaB-binding transcription factor in neurons is a novel protein complex distinct from NF-kappaB.

Animals↗
PubMed1999

Complex influence of the L-type calcium-channel agonist BayK8644(+/-) on N-methyl-D-aspartate responses and neuronal survival.

Past studies have implicated calcium influx through the N-methyl-D-aspartate class of ionotropic glutamate receptors as a key factor in excitotoxicity. Here, primary cultures of hippocampal neurons were exposed to N-methyl-D-aspartate with or without the L-type calcium channel agonist BayK8644(+/-). Calcium influxes were monitored with Fura-2 microfluorescent imaging and 45Ca measurements, and survival was assayed through cell counts. While 100 microM BayK8644 alone evoked a moderate elevation of intraneuronal calcium concentrations ([Ca2+]i), it dramatically attenuated the larger calcium influxes triggered by 500 microM N-methyl-D-aspartate. This attenuation was non-competitive and reversible; it was not inhibited by charybdotoxin or cyclosporin A. In spite of this attenuation of [Ca2+]i responses, 5-min exposures to BayK8644 produced much greater neurotoxicity 24 h later than did doses of N-methyl-D-aspartate evoking larger [Ca2+]i increases. This neurotoxicity was not observed with potassium-mediated depolarization or cobalt; indeed, both reversed the neurotoxicity of BayK8644. The relevant conclusions are two-fold: BayK8644 inhibits influx of calcium through a ligand-gated glutamate receptor, and BayK8644 exhibits considerable neurotoxicity. The former effect does not appear to depend upon the major metabolic pathways that modulate N-methyl-D-aspartate channels and thus may involve a direct allosteric interaction with the N-methyl-D-aspartate receptor. The toxicity of BayK8644 depends, at least partially, upon its activation of voltage-gated (cobalt-sensitive) calcium channels. However, the reversal of this toxicity by depolarization suggests that depolarization can be beneficial to neuronal survival through mechanisms other than calcium influx through voltage-gated calcium channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1999

Inhibition of the activity of a neuronal kappaB-binding factor by glutamate.

Activation of transcription factors with affinity for kappaB enhancers is generally correlated with enhanced survival of neurons. In an apparent exception, excitotoxic concentrations of glutamate have been reported to elevate the activity of one such factor, nuclear factor-kappaB (NF-kappaB). Our data indicate that the constitutive neuronal kappaB-binding factor (NKBF) is distinct from bona fide NF-kappaB (RelA/p50 heterodimer). Therefore, we analyzed glutamate's effects on KB-binding activity in highly enriched primary neuronal cultures and in mixed neuron/glia cocultures. Electrophoretic mobility shift assays indicated that a 30-60-min exposure to 50-500 microM glutamate reduced NKBF activity by as much as 70%. Subtoxic doses of glutamate had little or no effect on this DNA-binding activity. Selective antagonists of either NMDA or AMPA [(RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate]/kai nat e receptors inhibited the influence of glutamate on NKBF activity. The effect of glutamate was mimicked by calcium ionophore, and it was blocked by lowering extracellular calcium concentrations or by cyclosporin A. Bona fide NF-kappaB was found only in cocultures containing significant numbers of glia, where it could be activated by glutamate. These data suggest that the primary influence of excitatory amino acids on neuronal KB-binding activity is an inhibitory one, strengthening the correlation between this transcriptional parameter and neuronal survival.

Animals↗
PubMed1998

Neuroprotection by dehydroepiandrosterone-sulfate: role of an NFkappaB-like factor.

Levels of dehydroepiandrosterone (DHEA) and its sulfated derivative (DHEA-S) decline during aging and reach even lower levels in Alzheimer's disease (AD). Previously published effects of DHEA and DHEA-S on unchallenged neuronal survival led us to test them in an excitotoxicity paradigm. While DHEA-S protected hippocampal neurons against glutamate, little protection was observed with equivalent doses of DHEA itself. This differential neuroprotection was consistent with the ability of DHEA-S (but not DHEA) to elevate a kappaB-dependent transcription factor activity, a phenomenon we previously have connected with neuroprotection. Furthermore, suppression of kappaB DNA-binding by 'decoy' oligonucleotides blocked the neuroprotective activity of DHEA-S. These findings imply that age-related declines in the availability of DHEA-S could exacerbate neurotoxicity, and the data suggest that therapeutic gains may be obtained with pharmacological manipulation of kappaB-dependent transcription in neurons.

Animals↗
PubMed1998

Why neurons die: cell death in the nervous system.

It is likely that humans are born with all of the nerve cells (neurons) that will serve them throughout life. For all practical purposes, when our neurons die, they are lost forever. During nervous system development, about one-and-a-half times the adult number of neurons are created. These "extra" neurons are then destroyed or commit suicide. This process of programmed cell death occurs through a series of events termed apoptosis and is an appropriate and essential event during brain development. Later in life, inappropriate neuronal cell death may result from pathological causes such as traumatic injury, environmental toxins, cardiovascular disorders, infectious agents, or genetic diseases. In some cases, the death occurs through apoptosis. In other cases, cell death is random, irreversible, and uncontrollable; to distinguish it from the controlled, planned cell death of apoptosis, we call this necrotic cell death. Understanding the difference between apoptotic and necrotic cell death is essential for designing therapies which will prevent or limit inappropriate cell death in the nervous system.

Animals↗
PubMed1998

S100 beta increases levels of beta-amyloid precursor protein and its encoding mRNA in rat neuronal cultures.

S100beta has been implicated in the formation of dystrophic neurites, overexpressing beta-amyloid precursor protein (betaAPP), in the beta-amyloid plaques of Alzheimer's disease. We assessed the effects of S100beta on cell viability of, neurite outgrowth from, and betaAPP expression by neurons in primary cultures from fetal rat cortex. S100beta (1-10 ng/ml) enhanced neuronal viability (as assessed by increased mitrochondrial activity and decreased lactic acid dehydrogenase release) and promoted neurite outgrowth. Higher levels of S100beta (100 ng/ml, but not 1 microg/ml) produced qualitatively similar, but less marked, effects. S100beta also induced increased neuronal expression of the microtubule-associated protein MAP2, an effect that is consistent with trophic effects of S100beta on neurite outgrowth. S100beta (10 and 100 ng/ml) induced graded increases in neuronal expression of betaAPP and of betaAPP mRNA. These results support our previous suggestion that excessive expression of S100beta by activated, plaque-associated astrocytes in Alzheimer's disease contributes to the appearance of dystrophic neurites overexpressing betaAPP in diffuse amyloid deposits, and thus to the conversion of these deposits into the diagnostic neuritic beta-amyloid plaques.

Amyloid beta-Protein Precursor↗
PubMed1997

Microglial activation by Alzheimer amyloid precursor protein and modulation by apolipoprotein E.

A role for beta-amyloid precursor protein (beta-APP) in the development of Alzheimer's disease has been indicated by genetics, and many conditions in which beta-APP is raised have been associated with an increased risk of Alzheimer's disease or an Alzheimer's-like pathology. Inflammatory events may also contribute to Alzheimer's disease. Here we investigate whether a secreted derivative of beta-APP (sAPP-alpha) can induce inflammatory reactions in microglia, which are brain cells of monocytic lineage. We found that treatment with sAPP-alpha increased markers of activation in microglia and enhanced their production of neurotoxins. The ability of sAPP-alpha to activate microglia was blocked by prior incubation of the protein with apolipoprotein E3 but not apolipoprotein E4, a variant associated with an increased risk for Alzheimer's. A product of amyloidogenic beta-APP processing (sAPP-beta) also activated microglia. Because sAPP-beta is deficient in the neuroprotective activity shown by sAPP-alpha, our results indicate that increased amyloidogenic processing could adversely affect the balance of sAPP activities that determine neuronal viability.

Alzheimer Disease↗
PubMed1997

Cellular signaling roles of TGF beta, TNF alpha and beta APP in brain injury responses and Alzheimer's disease.

beta-Amyloid precursor protein (beta APP), transforming growth factor beta (TGF beta), and tumor necrosis factor-alpha (TNF alpha) are remarkably pleiotropic neural cytokines/neurotrophic factors that orchestrate intricate injury-related cellular and molecular interactions. The links between these three factors include: their responses to injury; their interactive effects on astrocytes, microglia and neurons; their ability to induce cytoprotective responses in neurons; and their association with cytopathological alterations in Alzheimer's disease. Astrocytes and microglia each produce and respond to TGF beta and TNF alpha in characteristic ways when the brain is injured. TGF beta, TNF alpha and secreted forms of beta APP (sAPP) can protect neurons against excitotoxic, metabolic and oxidative insults and may thereby serve neuroprotective roles. On the other hand, under certain conditions TNF alpha and the fibrillogenic amyloid beta-peptide (A beta) derivative of beta APP can promote damage of neuronal and glial cells, and may play roles in neurodegenerative disorders. Studies of genetically manipulated mice in which TGF beta, TNF alpha or beta APP ligand or receptor levels are altered suggest important roles for each factor in cellular responses to brain injury and indicate that mediators of neural injury responses also have the potential to enhance amyloidogenesis and/or to interfere with neuroregeneration if expressed at abnormal levels or modified by strategic point mutations. Recent studies have elucidated signal transduction pathways of TGF beta (serine/threonine kinase cascades), TNF alpha (p55 receptor linked to a sphingomyelin-ceramide-NF kappa B pathway), and secreted forms of beta APP (sAPP; receptor guanylate cyclase-cGMP-cGMP-dependent kinase-K+ channel activation). Knowledge of these signaling pathways is revealing novel molecular targets on which to focus neuroprotective therapeutic strategies in disorders ranging from stroke to Alzheimer's disease.

Alzheimer Disease↗
PubMed1997

Isoform-specific modulation by apolipoprotein E of the activities of secreted beta-amyloid precursor protein.

The genes for both the beta-amyloid precursor protein and apolipoprotein E (ApoE) have been linked to Alzheimer's disease. This connection suggests the possibility that these proteins interact physically or functionally. To explore this idea, we focused on the neuroprotective activity of secreted amyloid precursor protein (sAPP) and related signal transduction events. After coincubation with ApoE, sAPP exhibited an enhanced [Ca2+]i-lowering activity and enhanced protection against excitotoxicity in rat primary hippocampal neurons. In contrast, the stimulation of phosphoinositide production by sAPP was inhibited by ApoE. Kinetic analyses and coimmunoprecipitation experiments indicated that these actions result from formation of a heteromeric complex between ApoE and sAPP. Furthermore, the ApoE4 isoform, which seems to accelerate the onset of Alzheimer's disease, was less potent than ApoE3 in modifying each activity of sAPP. These data suggest that sAPP-dependent neuroprotective mechanisms would be compromised in individuals expressing ApoE4, a scenario that may contribute to the development of Alzheimer's disease.

Alleles↗
PubMed1996

Amyloid beta-peptide disrupts carbachol-induced muscarinic cholinergic signal transduction in cortical neurons.

Cholinergic pathways serve important functions in learning and memory processes, and deficits in cholinergic transmission occur in Alzheimer disease (AD). A subset of muscarinic cholinergic receptors are linked to G-proteins that activate phospholipase C, resulting in the liberation of inositol trisphosphate and Ca2+ release from intracellular stores. We now report that amyloid beta-peptide (Abeta), which forms plaques in the brain in AD, impairs muscarinic receptor activation of G proteins in cultured rat cortical neurons. Exposure of rodent fetal cortical neurons to Abeta25-35 and Abeta1-40 resulted in a concentration and time-dependent attenuation of carbachol-induced GTPase activity without affecting muscarinic receptor ligand binding parameters. Downstream events in the signal transduction cascade were similarly attenuated by Abeta. Carbachol-induced accumulation of inositol phosphates (IP, IP2, IP3, and IP4) was decreased and calcium imaging studies revealed that carbachol-induced release of calcium was severely impaired in neurons pretreated with Abeta. Muscarinic cholinergic signal transduction was disrupted with subtoxic levels of exposure to AP. The effects of Abeta on carbachol-induced GTPase activity and calcium release were attenuated by antioxidants, implicating free radicals in the mechanism whereby Abeta induced uncoupling of muscarinic receptors. These data demonstrate that Abeta disrupts muscarinic receptor coupling to G proteins that mediate induction of phosphoinositide accumulation and calcium release, findings that implicate Abeta in the impairment of cholinergic transmission that occurs in AD.

Amyloid beta-Peptides↗