Tumor necrosis factor alpha increases neuronal vulnerability to excitotoxic necrosis by inducing expression of the AMPA-glutamate receptor subunit GluR1 via an acid sphingomyelinase- and NF-kappaB-dependent mechanism

Astroglial nuclear factor-kappaB regulates learning and memory and synaptic plasticity in female mice

Astrocytes play a pivotal role in regulating synaptic plasticity and synapse formation. The nuclear factor-kappa B (NF-kappaB) family of transcription factors has recently been demonstrated to be an important modulator of synaptic plasticity and learning/memory. In this study, we investigated the role of astroglial NF-kappaB in synaptic plasticity and learning/memory using transgenic mice over-expressing an N-terminal truncated form of inhibitor of NF-kappaB alpha (IkappaBalpha) in astrocytes (GFAP-IkappaBetaalpha-dn). We demonstrated that female transgenic mice, but not males, have robust deficits in hippocampal and extra-hippocampal-dependent learning and memory. We also determined that there are significant deficits in LTP and expression of metabotropic glutamate receptor 5 and post-synaptic density protein 95 (PSD95) in female transgenic mice. These findings indicate that astroglial NF-kappaB is an important regulator of learning/memory and synaptic plasticity.

Interleukin-1β but not tumor necrosis factor-α potentiates neuronal damage by quinolinic acid: Protection by an adenosine A2A receptor antagonist

Quinolinic acid is an agonist at glutamate receptors sensitive to N-methyl-D-aspartate (NMDA). It has been implicated in neural dysfunction associated with infections, trauma, and ischemia, although its neurotoxic potency is relatively low. This study was designed to examine the effects of a combination of quinolinic acid and the proinflammatory cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha). Compounds were administered to the hippocampus of anesthetized male rats, animals being allowed to recover for 7 days before histological analysis of the hippocampus for neuronal damage estimated by counting of intact, healthy neurons. A low dose of quinolinic acid or IL-1beta produced no damage by itself, but the two together induced a significant loss of pyramidal neurons in the hippocampus. Higher doses produced almost total loss of pyramidal cells. Intrahippocampal TNF-alpha produced no effect alone but significantly reduced the neuronal loss produced by quinolinic acid. The adenosine A(2A) receptor antagonist ZM241385 reduced neuronal loss produced by the combinations of quinolinic acid and IL-1beta. The results suggest that simultaneous quinolinic acid and IL-1beta, both being induced by cerebral infection or injury, are synergistic in the production of neuronal damage and could together contribute substantially to traumatic, infective, or ischemic cerebral damage. Antagonism of adenosine A(2A) receptors protects neurons against the combination of quinolinic acid and IL-1beta.

Differential regulation of BACE1 promoter activity by nuclear factor-κB in neurons and glia upon exposure to β-amyloid peptides

The brains of Alzheimer's disease (AD) patients display cerebrovascular and parenchymal deposits of beta-amyloid (A beta) peptides, which are derived by proteolytic processing by the beta-site APP-cleaving enzyme 1 (BACE1) of the amyloid precursor protein (APP). The rat BACE1 promoter has a nuclear factor-kappaB (NF-kappaB) binding site. Deletion studies with a BACE1 promoter/luciferase reporter suggest that the NF-kappaB binding DNA consensus sequence plays a suppressor role, when occupied by NF-kappaB, in the regulation of neuronal brain BACE1 expression. Here we characterize a signal transduction pathway that may be responsible for the increases in A beta associated with AD. We propose that the transcription factor NF-kappaB acts as a repressor in neurons but as an activator of BACE1 transcription in activated astrocytes present in the CNS under chronic stress, a feature present in the AD brain. The activated astrocytic stimulation of BACE1 may in part account for increased BACE1 transcription and subsequent processing of Ab eta in a cell-specific manner in the aged and AD brain. As measured by reporter gene promoter constructs and endogenous BACE1 protein expression, a functional NF-kappaB site was stimulatory in activated astrocytes and A beta-exposed neuronal cells and repressive in neuronal and nonactivated astrocytic cells. Given the evidence for increased levels of activated astrocytes in the aged brain, the age- and AD-associated increases in NF-kappaB in brain may be significant contributors to increases in A beta, acting as a positive feedback loop of chronic inflammation, astrocyte activation, increased p65/p50 activation of BACE1 transcription, and further inflammation.

NF-kappaB/Rel regulates inhibitory and excitatory neuronal function and synaptic plasticity

Changes in synaptic plasticity required for memory formation are dynamically regulated through opposing excitatory and inhibitory neurotransmissions. To explore the potential contribution of NF-kappaB/Rel to these processes, we generated transgenic mice conditionally expressing a potent NF-kappaB/Rel inhibitor termed IkappaBalpha superrepressor (IkappaBalpha-SR). Using the prion promoter-enhancer, IkappaBalpha-SR is robustly expressed in inhibitory GABAergic interneurons and, at lower levels, in excitatory neurons but not in glia. This neuronal pattern of IkappaBalpha-SR expression leads to decreased expression of glutamate decarboxylase 65 (GAD65), the enzyme required for synthesis of the major inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) in GABAergic interneurons. IkappaBalpha-SR expression also results in diminished basal GluR1 levels and impaired synaptic strength (input/output function), both of which are fully restored following activity-based task learning. Consistent with diminished GAD65-derived inhibitory tone and enhanced excitatory firing, IkappaBalpha-SR+ mice exhibit increased late-phase long-term potentiation, hyperactivity, seizures, increased exploratory activity, and enhanced spatial learning and memory. IkappaBalpha-SR+ neurons also express higher levels of the activity-regulated, cytoskeleton-associated (Arc) protein, consistent with neuronal hyperexcitability. These findings suggest that NF-kappaB/Rel transcription factors act as pivotal regulators of activity-dependent inhibitory and excitatory neuronal function regulating synaptic plasticity and memory.

Complement-dependent apoptosis and inflammatory gene changes in murine lupus cerebritis

The role of complement activation in the brains of MRL/lpr lupus mice was determined using the potent C3 convertase inhibitor, CR1-related y (Crry), administered both as an overexpressing Crry transgene and as Crry-Ig. Prominent deposition of complement proteins C3 and C9 in brains of MRL/lpr mice was indicative of complement activation and was significantly reduced by Crry. Apoptosis was determined in brain using different independent measures of apoptosis, including TUNEL staining, DNA laddering, and caspase-3 activity, all of which were markedly increased in lupus mice and could be blocked by inhibiting complement with Crry. Complement activation releases inflammatory mediators that can induce apoptosis. The mRNA for potentially proinflammatory proteins such as TNFR1, inducible NO synthase, and ICAM-1 were up-regulated in brains of lupus mice. Crry prevented the increased expression of these inflammatory molecules, indicating that the changes were complement dependent. Furthermore, microarray analysis revealed complement-dependent up-regulation of glutamate receptor (AMPA-GluR) expression in lupus brains, which was also validated for AMPA-GluR1 mRNA and protein. Our results clearly demonstrate that apoptosis is a prominent feature in lupus brains. Complement activation products either directly and/or indirectly through TNFR1, ICAM-1, inducible NO synthase, and AMPA-GluR, all of which were altered in MRL/lpr mouse brains, have the potential to induce such apoptosis. These findings present the exciting possibility that complement inhibition is a therapeutic option for lupus cerebritis.

Modulator effects of interleukin-1beta and tumor necrosis factor-alpha on AMPA-induced excitotoxicity in mouse organotypic hippocampal slice cultures

The inflammatory cytokines interleukin-1beta and tumor necrosis factor-alpha (TNF-alpha) have been identified as mediators of several forms of neurodegeneration in the brain. However, they can produce either deleterious or beneficial effects on neuronal function. We investigated the effects of these cytokines on neuronal death caused by exposure of mouse organotypic hippocampal slice cultures to toxic concentrations of AMPA. Either potentiation of excitotoxicity or neuroprotection was observed, depending on the concentration of the cytokines and the timing of exposure. A relatively high concentration of mouse recombinant TNF-alpha (10 ng/ml) enhanced excitotoxicity when the cultures were simultaneously exposed to AMPA and to this cytokine. Decreasing the concentration of TNF-alpha to 1 ng/ml resulted in neuroprotection against AMPA-induced neuronal death independently on the application protocol. By using TNF-alpha receptor (TNFR) knock-out mice, we demonstrated that the potentiation of AMPA-induced toxicity by TNF-alpha involves TNF receptor-1, whereas the neuroprotective effect is mediated by TNF receptor-2. AMPA exposure was associated with activation and proliferation of microglia as assessed by macrophage antigen-1 and bromodeoxyuridine immunohistochemistry, suggesting a functional recruitment of cytokine-producing cells at sites of neurodegeneration. Together, these findings are relevant for understanding the role of proinflammatory cytokines and microglia activation in acute and chronic excitotoxic conditions.

Peripheral benzodiazepine receptor ligand PK11195 reduces microglial activation and neuronal death in quinolinic acid-injected rat striatum

The effects of the peripheral benzodiazepine receptor (PBR) ligand, PK11195, were investigated in the rat striatum following the administration of quinolinic acid (QUIN). Intrastriatal QUIN injection caused an increase of PBR expression in the lesioned striatum as demonstrated by immunohistochemical analysis. Double immunofluorescent staining indicated PBR was primarily expressed in ED1-immunoreactive microglia but not in GFAP-immunoreactive astrocytes or NeuN-immunoreactive neurons. PK11195 treatment significantly reduced the level of microglial activation and the expression of pro-inflammatory cytokines and iNOS in QUIN-injected striatum. Oxidative-mediated striatal QUIN damage, characterized by increased expression of markers for lipid peroxidation (4-HNE) and oxidative DNA damage (8-OHdG), was significantly diminished by PK11195 administration. Furthermore, intrastriatal injection of PK11195 with QUIN significantly reduced striatal lesions induced by the excitatory amino acid and diminished QUIN-mediated caspase-3 activation in striatal neurons. These results suggest that inflammatory responses from activated microglia are damaging to striatal neurons and pharmacological targeting of PBR in microglia may be an effective strategy in protecting neurons in neurological disorders such as Huntington's disease.

Glia: novel counter-regulators of opioid analgesia

Development of analgesic tolerance and withdrawal-induced pain enhancement present serious difficulties for the use of opioids for pain control. Although neuronal mechanisms to account for these phenomena have been sought for many decades, their bases remain unresolved. Within the past four years, a novel non-neuronal candidate has been uncovered that opposes acute opioid analgesia and contributes to development of opioid tolerance and tolerance-associated pain enhancement. This novel candidate is spinal cord glia. Glia are important contributors to the creation of enhanced pain states via the release of neuroexcitatory substances. New data suggest that glia also release neuroexcitatory substances in response to morphine, thereby opposing its effects. Controlling glial activation could therefore increase the clinical utility of analgesic drugs.

Inflammation in areas of remote changes following focal brain lesion

Focal brain lesions can lead to metabolic and structural changes in areas distant from but connected to the lesion site. After focal ischemic or excitotoxic lesions of the cortex and/or striatum, secondary changes have been observed in the thalamus, substantia nigra pars reticulata, hippocampus and spinal cord. In all these regions, inflammatory changes characterized by activation of microglia and astrocytes appear. In the thalamus, substantia nigra pars reticulata and hippocampus, an expression of proinflammatory cytokine like tumor necrosis factor-alpha and interleukin-1beta is induced. However, time course of expression and cellular localisation differ between these regions. Neuronal damage has consistently been observed in the thalamus, substantia nigra and spinal cord. It can be present in the hippocampus depending on the procedure of induction of focal cerebral ischemia. This secondary neuronal damage has been linked to antero- and retrograde degeneration. Anterograde degeneration is associated with somewhat later expression of cytokines, which is localised in neurons. In case of retrograde degeneration, the expression of cytokines is earlier and is localised in astrocytes. Pharmacological intervention aiming at reducing expression of tumor necrosis factor-alpha leads to reduction of secondary neuronal damage. These first results suggest that the inflammatory changes in remote areas might be involved in the pathogenesis of secondary neuronal damage.

Tumor necrosis-factor-alpha (TNF-α) induces rapid insertion of Ca2+-permeable α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)/kainate (Ca-A/K) channels in a subset of hippocampal pyramidal neurons

The presence of cell surface Ca2+ permeable alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)/kainate (Ca-A/K) channels on subsets of central neurons influences both normal physiological function and vulnerability to excitotoxicity. Factors that regulate the formation and membrane insertion of Ca-A/K channels, however, are poorly understood. Recently, the cytokine tumor necrosis factor-alpha (TNF-alpha) was shown to increase the cell surface expression of an AMPA receptor (AMPAR) subunit (GluR1) and to potentiate vulnerability to AMPAR-mediated injury. In this study, we examined the possibility that TNF-alpha might also increase numbers of functional Ca-A/K channels. In acute hippocampal slice preparations, TNF-alpha appeared to increase Ca-A/K channel numbers in pyramidal neurons (HPNs), as assessed using a histochemical stain based on kainate-induced uptake of Co2+ ions (Co2+ labeling). In dissociated hippocampal neuronal cultures, TNF-alpha exposure (6 nM, 15 min) induced a rapid increase in cell surface levels not only of GluR1, but also of the AMPAR subunit GluR2, on most neurons, without evident new protein synthesis. Furthermore, consistent with the slice studies, fluorescence Ca2+ imaging techniques revealed an increase in numbers of Ca-A/K channels on what appeared to be a subset of HPNs. These observations are the first to provide evidence for the rapid upregulation of neuronal Ca-A/K channels in response to a cytokine or any other soluble factor, and provide a novel mechanism through which TNF-alpha may modulate both synaptic function and neuronal vulnerability.

TNF alpha potentiates glutamate neurotoxicity by inhibiting glutamate uptake in organotypic brain slice cultures: neuroprotection by NF kappa B inhibition

Glutamate and the proinflammatory cytokine, tumor necrosis factor alpha (TNF alpha), have been suggested to contribute to neurodegenerative diseases. We investigated the interaction of TNF alpha and glutamate on neuronal cell death using fluorescence propidium iodide uptake in rat organotypic hippocampal-entorhinal cortex (HEC) brain slice culture that maintains the cytoarchitecture of the intact brain. Time course and concentration studies indicate that glutamate produced significant neuronal cell death in all four brain areas examined, for example, entorhinal cortex, hippocampal CA1 and CA3 fields, and dentate gyrus. TNF alpha alone at concentration of 20 ng/ml caused little or no detectable neuronal cell death, however, when combined with submaximal glutamate (3.3 mM), TNF alpha significantly increased and accelerated glutamate neurotoxicity. TNF alpha potentiation of glutamate neurotoxicity is blocked by NMDA receptor antagonists but not by AMPA antagonists CNQX and NBQX. Studies directly measuring [14C]-glutamate uptake in HEC slices indicate that TNF alpha dose-dependently inhibited glutamate uptake. Further, inhibitors of glial glutamate transporters potentiated glutamate neurotoxicity similar to TNF alpha. The antioxidant butylated hydroxytoluene (BHT) and the NF kappa B inhibitor PTD-p65 peptide inhibit NF kappa B activation and TNF alpha potentiation of glutamate neurotoxicity. BHT prevented the inhibition of TNFalpha on glutamate transport in HEC slices and also blocked nuclear translocation of NF kappa B subunit p65. These data indicate that TNF alpha and glutamate can act synergistically to induce neuronal cell death. TNF alpha potentiation of glutamate neurotoxicity through the blockade of glutamate transporter activity may represent an important mechanism of neurodegeneration associated with neuroinflammation.

Post-ischemic inflammation: molecular mechanisms and therapeutic implications

Ischemic stroke is characterized by the disruption of cerebral blood flow (CBF). This reduction of CBF results in energy failure and secondary biochemical disturbances, eliciting a robust in situ inflammatory response. Post-ischemic inflammation is a dynamic process involving a complicated set of interactions among various inflammatory cells and molecules. The resident inflammatory brain cells, microglia, are especially activated in response to ischemic insults, many of which are regulated by nuclear transcription factor, kappa B (NF-kappaB). As a result, several inflammatory genes are expressed, leading to local generation of various cytokines, which in turn promulgate inflammatory signals. Meanwhile, endothelial cells lining the local cerebral blood vessels are stimulated to produce adhesion molecules, causing the migration of peripheral circulating leukocytes into the compromised brain tissue, an event that amplifies inflammatory signaling cascades. Post-ischemic inflammation appears to serve multiple purposes, depending on its timing and magnitude, as well as the topographic distribution of various inflammatory molecules. Data from experimental manipulations of some inflammatory molecules are yielding insight into therapeutic strategies for ischemic stroke. This review focuses on some recent advances regarding the regulation of inflammatory signaling pathways, the detrimental effects of post-ischemic inflammation and the potential molecular targets for ischemic stroke therapy.

Differential regulation of AMPA receptor and GABA receptor trafficking by tumor necrosis factor-alpha

The proinflammatory cytokine tumor necrosis factor-alpha (TNFalpha) causes a rapid exocytosis of AMPA receptors in hippocampal pyramidal cells and is constitutively required for the maintenance of normal surface expression of AMPA receptors. Here we demonstrate that TNFalpha acts on neuronal TNFR1 receptors to preferentially exocytose glutamate receptor 2-lacking AMPA receptors through a phosphatidylinositol 3 kinase-dependent process. This increases excitatory synaptic strength while changing the molecular stoichiometry of synaptic AMPA receptors. Conversely, TNFalpha causes an endocytosis of GABA(A) receptors, resulting in fewer surface GABA(A) receptors and a decrease in inhibitory synaptic strength. These results suggest that TNFalpha can regulate neuronal circuit homeostasis in a manner that may exacerbate excitotoxic damage resulting from neuronal insults.

Regulation of cytokine-induced neuron death by ovarian hormones: involvement of antiapoptotic protein expression and c-JUN N-terminal kinase-mediated proapoptotic signaling

Mechanisms underlying the divergent effects of ovarian hormones on neuron death induced by TNFalpha were investigated in differentiated PC12 cells (dPC12). dPC12 cells were exposed to 17beta-estradiol (E, 1.0 nm), progesterone (P, 100 nm), or a combination of both hormones for 0-72 h before treatment with TNFalpha (0-150 ng) to induce cell death. Cells undergoing apoptosis were identified by a terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling assay and fluorescence-activated cell sorting after 18 h. Cell death induced by TNFalpha was decreased 89% after E treatment and increased 2-fold after P treatment compared with cells treated with TNFalpha alone. Treatment with E for 24 h before TNFalpha exposure was required for maximum neuroprotection, whereas P-enhanced death was maximal after a 30-min P treatment. TNFalpha induced a 3-fold increased activity of c-JUN-N-terminal kinase (JNK) 1 in d PC12 cells within 20 min that could be increased 5- to 8-fold by P together with TNFalpha. A peptide inhibitor of JNK1 abrogated P enhancement of TNFalpha-mediated dPC12 death but had only a minimal effect on cell death by TNFalpha alone. Inhibition of caspase-8 activation reduced death induced by TNFalpha alone but was much less effective for P+TNF. P alone did not activate caspase-8. E increased estrogen receptor alpha (ERalpha) and Bcl-xL expression and all but abolished TNFalpha receptor 1 (TNFR1) expression. P decreased ERalpha and Bcl-xL expression and doubled TNFR1 expression. These data suggest that P regulates apoptosis or survival through augmentation of JNK signaling and altered TNFR1 expression, whereas E mainly affects the expression of BCL-xL, TNFR1, and ERalpha.

How fast does the GluR1Qflip channel open?

Opening of a ligand-gated ion channel is the step at which the binding of a neurotransmitter is transduced into the electrical signal by allowing ions to flow through the transmembrane channel, thereby altering the postsynaptic membrane potential. We report the kinetics for the opening of the GluR1Qflip channel, an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit of the ionotropic glutamate receptors. Using a laser-pulse photolysis technique that permits glutamate to be liberated photolytically from gamma-O-(alpha-carboxy-2-nitrobenzyl)glutamate (caged glutamate) with a time constant of approximately 30 micros, we show that, after the binding of glutamate, the channel opened with a rate constant of (2.9 +/- 0.2) x 10(4) s(-1) and closed with a rate constant of (2.1 +/- 0.1) x 10(3) s(-1). The observed shortest rise time (20-80% of the receptor current response), i.e. the fastest time by which the GluR1Qflip channel can open, was predicted to be 35 micros. This value is three times shorter than those previously reported. The minimal kinetic mechanism for channel opening consists of binding of two glutamate molecules, with the channel-opening probability being 0.93 +/- 0.10. These findings identify GluR1Qflip as one of the temporally efficient receptors that transduce the binding of chemical signals (i.e. glutamate) into an electrical impulse.

Tumor necrosis factor-α expression in areas of remote degeneration following middle cerebral artery occlusion of the rat

Remote areas undergoing delayed neuronal degeneration after focal brain ischemia display a preceding glial activation. The expression of proinflammatory cytokines there has not been examined so far. We examined the expression of TNFalpha in the thalamus and the substantia nigra pars reticulata (SNr) 1, 3 and 7 days after transient middle cerebral artery occlusion (MCAO) of the rat. We used antibodies against glial fibrillary acidic protein (GFAP), OX-42, NeuN and tumor necrosis factor-alpha (TNFalpha) for immunohistochemistry/double-immunofluorescent labeling to investigate the time course of glial activation and the cellular localization of TNFalpha. Neuronal degeneration was measured by means of cell counting in Nissl-stained sections. In the ipsilateral thalamus, TNFalpha was upregulated already 1 day after MCAO. Microglia and astroglia were activated after 3 days. A cellular colocalisation of GFAP and TNFalpha was observed. Neuronal degeneration was evident at day 14. In the SNr, TNFalpha expression was enhanced 3 days after MCAO. Microglia was activated after 3 days and astroglia after 7 days. A cellular colocalisation of NeuN and TNFalpha was observed. Neuronal degeneration was evident at day 14. Thus, in both areas, expression of TNFalpha precedes astrogliosis and neuronal degeneration. The different patterns of TNFalpha upregulation of the substantia nigra pars reticulata and the thalamus following middle cerebral artery occlusion may reflect different pathophysiological mechanisms leading to remote neuronal degeneration.