Enhancement of immunoreactivity for NF-kappa B in human cerebral infarctions

Hypoxia signaling to genes: significance in Alzheimer's disease

Aberrations in neural signaling, converging to and diverging from oxidative metabolism and blood supply, contribute to the initiation and maintenance of inflammatory responses, neuronal degeneration, and age-related cognitive decline in Alzheimer's disease (AD). Hypoxia/ischemia triggers phospholipase A2, leading to the accumulation of free arachidonic and docosahexaenoic acids (AA, DHA), as well as that of lysophospholipids. Some of these bioactive lipid messengers in turn give rise to several downstream lipid messengers, such as platelet-activating factor (PAF) and ecosanoids (prostaglandins and leukotrienes). Eicosanoid synthesis is highly regulated in hypoxia and in reperfusion subsequent to ischemia. As one of the consequences, mitochondrial function is disrupted and reactive oxygen species (ROS) both contribute to the expansion of cellular inflammatory responses and reduce the expression of genes required to maintain synaptic structure and function. On the other hand, pro-inflammatory genes are up-regulated. One of these, the inducible cyclooxygenase-2 (COX-2), along with oxygen-starved mitochondria, comprise the major sources of ROS in the brain during hypoxia, ischemia, and reperfusion. One outcome is a sustained metabolic stress that drives progressive dysfunction, apoptosis and/or necrosis, and brain cell death. How hypoxia modulates oxygen-sensitive gene expression is not well understood. Pro-inflammatory gene families that contribute to neurodegeneration are transiently activated in part by the heterodimeric oxygen-sensitive DNA-binding proteins nuclear factor for kappa B (NF-kappaB) and hypoxia-inducible factor-alpha (HIF-1alpha). Here the authors summarize current studies supporting the hypothesis that synaptically-derived lipid messengers play significant roles in ischemic stroke and that hypoxia is an important contributor to the onset and progression of AD neurodegeneration.

Opposing roles for NF-kappa B/Rel factors p65 and c-Rel in the modulation of neuron survival elicited by glutamate and interleukin-1beta

The nuclear transcription factors NF-kappaB/Rel have been shown to function as key regulators of either cell death or survival in neuronal cells. Here, we investigated whether selective activation of diverse NF-kappaB/Rel family members might lead to distinct effects on neuron viability. In both cultured rat cerebellar granule cells and mouse hippocampal slices, we examined NF-kappaB/Rel activation induced by two opposing modulators of cell viability: 1) interleukin-1beta (IL-1beta), which promotes neuron survival and 2) glutamate, which can elicit toxicity. IL-1beta produced a prolonged stimulation of NF-kappaB/Rel factors by inducing both IkappaBalpha and IkappaBbeta degradation. Glutamate produced a delayed and transient activation of NF-kappaB/Rel, which was associated with a brief loss of IkappaBalpha. Moreover, IL-1beta activated the p50, p65, and c-Rel subunits of NF-kappaB/Rel, whereas glutamate activated only the p50 and p65 proteins. The inhibition of NF-kappaB/Rel protein expression by antisense oligonucleotides in cerebellar granule cells showed that p65 was involved in glutamate-mediated cell death, whereas c-Rel was essential for IL-1beta-preserved cell survival. Furthermore, the depletion of c-Rel in cultured neurons as well as in the hippocampus from the c-Rel(-/-) mouse converted the IL-1beta effect into toxicity. These findings suggest that, within a single neuron, the balance between cell death and survival in response to external stimuli may rely on the activation of distinct NF-kappaB/Rel proteins.

Growing old with nuclear factor-kappaB

The transcription factor nuclear factor-kappa B (NF-kappaB) is involved in the regulation of a broad spectrum of genes that play important roles in a myriad of physiological and pathological events ranging from the immune response to carcinogenesis. Interestingly, many processes in which NF-kappa B plays a central role have long been noted for their alteration with age. A number of research groups have reported rather dramatic changes in NF-kappaB activity as humans and animals age, with tissue-specific increases and decreases in NF-kappaB activity being reported. The extent to which changes in NF-kappaB activity drive aging and influence life span in humans and other mammals is not clear. However, given the dramatic impact that NF-kappaB can have on the function of numerous tissues and organs, understanding how NF-kappaB activity changes with age will undoubtedly enhance our understanding of the many diseases associated with growing old.

Triflusal posttreatment inhibits glial nuclear factor-kappaB, downregulates the glial response, and is neuroprotective in an excitotoxic injury model in postnatal brain

BACKGROUND AND PURPOSE

Nuclear factor-kappaB (NF-kappaB) and the signal transducer and activator of transcription 3 (STAT3) are important transcription factors regulating inflammatory mechanisms and the glial response to neural injury, determining lesion outcome. In this study we evaluate the ability of triflusal (2-acetoxy-4-trifluoromethylbenzoic acid), an antiplatelet agent inhibitor of NF-kappaB activation, to improve lesion outcome after excitotoxic damage to the immature brain.

METHODS

Postnatal day 9 rats received an intracortical injection of the excitotoxin N-methyl-D-aspartate (NMDA) and oral administration of triflusal (30 mg/kg) either as 3 doses before NMDA injection (pretreatment) or as a single dose 8 hours after NMDA injection (posttreatment). After survival times of 10 and 24 hours, brains were processed for toluidine blue staining, tomato lectin histochemistry, and glial fibrillary acidic protein, NF-kappaB, and STAT3 immunocytochemistry.

RESULTS

NMDA-lesioned animals that were not treated with triflusal showed activation of NF-kappaB in neuronal cells at first and in glial cells subsequently. Animals that received pretreatment with triflusal showed a strong downregulation of neuronal and glial NF-kappaB but a similar development of the glial response and an equivalent lesion volume compared with nontreated animals. In contrast, animals receiving triflusal posttreatment showed increased early neuronal NF-kappaB but a reduction in the subsequent glial NF-kappaB, accompanied by important downregulation of the microglial and astroglial response and a drastic reduction in the lesion size. STAT3 activation was not affected by triflusal treatment.

CONCLUSIONS

Triflusal posttreatment diminishes glial NF-kappaB, downregulates the glial response, and improves the lesion outcome, suggesting a neuroprotective role of this compound against excitotoxic injury in the immature brain.

Rapid expression of neuronal and inducible nitric oxide synthases during post-ischemic reperfusion in rat brain

OBJECTIVE

To determine whether neuronal and inducible nitric oxide synthase (nNOS and iNOS) isoforms are expressed within cortical neurons during early reperfusion after focal cerebral ischemia.

METHODS

Male spontaneously hypertensive rats underwent occlusion of the left middle cerebral artery for 2 h. Coronal brain sections with normal and ischemic cortex were obtained after 15 min or 1, 6 or 24 h of reperfusion. Immunohistochemical and double-label immunofluorescent techniques were used to confirm cellular identity and localize nNOS and iNOS.

RESULTS

Immunoreactive nNOS was identified within isolated neurons in layer V of normal cortex. However, the number of nNOS-immunoreactive neurons in ischemic cortex rose markedly at 15 min and persisted for 24 h (P< or =0.001 at each time point when compared to normal cortex). Cells that were immunoreactive for nNOS appeared in perivascular clusters within ischemic brain at all sampling times. Immunoreactive iNOS was also expressed within neurons in ischemic cortex, peaking after 15 min of reperfusion (P< or =0.01). Although nNOS-immunoreactive neurons were observed in random numbers within normal tissue throughout reperfusion, iNOS-immunoreactive neurons increased steadily in the same region (P< or =0.05).

CONCLUSIONS

Ischemic neurons become immunoreactive for both nNOS and iNOS during early reperfusion. Expression of iNOS immunoreactivity in unaffected neurons may reflect transcription of immediate early genes in response to stimulatory neurotransmission from ischemic cortex.

Neuroinflammatory signaling upregulation in Alzheimer's disease

Alzheimer's disease (AD) is a progressive, neurodestructive process of the human neocortex, characterized by the deterioration of memory and higher cognitive function. A progressive and irreversible brain disorder, AD is characterized by three major pathogenic episodes involving (a) an aberrant processing and deposition of beta-amyloid precursor protein (betaAPP) to form neurotoxic beta-amyloid (betaA) peptides and an aggregated insoluble polymer of betaA that forms the senile plaque, (b) the establishment of intraneuronal neuritic tau pathology yielding widespread deposits of agyrophilic neurofibrillary tangles (NFT) and (c) the initiation and proliferation of a brain-specific inflammatory response. These three seemingly disperse attributes of AD etiopathogenesis are linked by the fact that proinflammatory microglia, reactive astrocytes and their associated cytokines and chemokines are associated with the biology of the microtubule associated protein tau, betaA speciation and aggregation. Missense mutations in the presenilin genes PS1 and PS2, implicated in early onset familial AD, cause abnormal betaAPP processing with resultant overproduction of betaA42 and related neurotoxic peptides. Specific betaA fragments such as betaA42 can further potentiate proinflammatory mechanisms. Expression of the inducible oxidoreductase cyclooxygenase-2 and cytosolic phospholipase A2 (cPLA2) are strongly activated during cerebral ischemia and trauma, epilepsy and AD, indicating the induction of proinflammatory gene pathways as a response to brain injury. Neurotoxic metals such as aluminum and zinc, both implicated in AD etiopathogenesis, and arachidonic acid, a major metabolite of brain cPLA2 activity, each polymerize hyperphosphorylated tau to form NFT-like bundles. Further, epidemiological and longitudinal studies have identified a reduced risk for AD in patients (<70 yrs) previously treated with non-steroidal anti-inflammatory drugs for non-CNS afflictions that include arthritis. This review will focus on the interrelationships between the mechanisms of PS1, PS2 and betaAPP gene expression, tau and betaA deposition and the induction, regulation and proliferation in AD of the neuroinflammatory response. Novel therapeutic interventions in AD are discussed.

Sequential expression of surface antigens and transcription factor NFkappaB by hippocampal cells in excitotoxicity and experimental epilepsy

Neurodegeneration and gliosis have been extensively described after long-lasting seizures; evidence for cytokine involvement in neuron-glia interactions does exist. We have therefore studied the hippocampal expression of molecules responsible for immune and inflammatory reactions, at different time-points following either experimental status epilepticus (SE) or direct excitotoxic damage. Experiments consisting of immunohistochemical labeling of glial markers, major histocompatibility complex (MHC) and nuclear factor kappaB (NFkappaB), were performed. NFkappaB nuclear translocation was controlled and measured using the electrophoretic mobility shift assay. One day after SE, neurodegeneration was obvious in CA3 pyramidal layers; NFkappaB staining in neurons and its translocation to the nucleus enhanced. From day 4 to at least day 8 post-SE, MHC-positive microglia, NFkappaB over-expression in thickened astrocytes, and increased levels of its activated form could be observed. The excitotoxic model caused more severe lesions, but NFkappaB and MHC expression were similar in both models. These results suggest that during long-lasting seizures: (i) neuronal firing activates NFkappaB expression and translocation; (ii) microglia expresses MHC; (iii) astrocytes, probably stimulated by microglial cytokines, over-express NFkappaB, the activation of which induces a cascade of reactions, particularly the transcription of cytokines and or neuroprotective molecules. Further clarification of the toxic or protective consequences of delayed inflammatory responses may be interesting in therapy of epilepsy.

Beta-amyloid stimulation of inducible nitric-oxide synthase in astrocytes is interleukin-1beta- and tumor necrosis factor-alpha (TNFalpha)-dependent, and involves a TNFalpha receptor-associated factor- and NFkappaB-inducing kinase-dependent signaling mechanism

In Alzheimer's disease, beta-amyloid (Abeta) plaques are surrounded by activated astrocytes and microglia. A growing body of evidence suggests that these activated glia contribute to neurotoxicity through the induction of inflammatory cytokines such as interleukin (IL)-1beta and tumor necrosis factor-alpha (TNFalpha) and the production of neurotoxic free radicals, mediated in part by the expression of inducible nitric-oxide synthase (iNOS). Here, we address the possibility that Abeta-stimulated iNOS expression might result from an initial induction of IL-1beta and TNFalpha. We find that in Abeta-stimulated astrocyte cultures, IL-1beta and TNFalpha production occur before iNOS production, new protein synthesis is required for increased iNOS mRNA levels, and the IL-1 receptor antagonist IL-1ra can inhibit nitrite accumulation. Likewise, dominant-negative mutants of tumor necrosis factor-alpha receptor-associated factor (TRAF) 6, TRAF2, and NFkappaB-inducing kinase (NIK), intracellular proteins involved in IL-1 and TNFalpha receptor signaling cascades, inhibit Abeta-stimulated iNOS promoter activity. Our data suggest that Abeta stimulation of astrocyte iNOS is mediated in part by IL-1beta and TNFalpha, and involves a TRAF6-, TRAF2-, and NIK-dependent signaling mechanism.

Transcription factor nuclear factor-kappa B is activated in neurons after focal cerebral ischemia

Nuclear factor-kappa B (NF-kappaB) is a multisubunit transcription factor that when activated induces the expression of genes encoding acute-phase proteins, cell adhesion molecules, cell surface receptors, and cytokines. NF-kappaB is composed of a variety of protein subunits of which p50-and p65-kDa (RelA) are the most widely studied. Under resting conditions, these subunits reside in the cytoplasm as an inactive complex bound by inhibitor proteins, IkappaB alpha and IkappaB beta. On activation, IkappaB is phosphorylated by IkappaB kinase and ubiquitinated and degraded by the proteasome; simultaneously, the active heterodimer translocates to the nucleus where it can initiate gene transcription. In the periphery, NF-kappaB is involved in inflammation through stimulation of the production of inflammatory mediators. The role of NF-kappaB in the brain is unclear. In vitro, NF-kappaB activation can be either protective or deleterious. The role of NF-kappaB in ischemic neuronal cell death in vivo was investigated. Adult male rats were subjected to 2 hours of focal ischemia induced by middle cerebral artery occlusion (MCAO). At 2, 6, and 12 hours after reperfusion, the expression and transactivation of NF-kappaB in ischemic versus nonischemic cortex and striatum were determined by immunocytochemistry and by electrophoretic mobility gel-shift analysis. At all time points studied, p50 and p65 immunoreactivity was found exclusively in the nuclei of cortical and striatal neurons in the ischemic hemisphere. The contralateral nonischemic hemisphere showed no evidence of nuclear NF-kappaB immunoreactivity. Double immunofluorescence confirmed expression of p50 in nuclei of neurons. Increased NF-kappaB DNA-binding activity in nuclear extracts prepared from the ischemic hemisphere was further substantiated by electrophoretic mobility gel-shift analysis. Because the activation of NF-kappaB by many stimuli can be blocked by antioxidants in vitro, the effect of the antioxidant, LY341122, previously shown to be neuroprotective, on NF-kappaB activation in the MCAO model was evaluated. No significant activation of NF-kappaB was found by electrophoretic mobility gel-shift analysis in animals treated with LY341122. These results demonstrate that transient focal cerebral ischemia results in activation of NF-kappaB in neurons and supports previous observations that neuroprotective antioxidants may inhibit neuronal death by preventing the activation of NF-kappaB.

Cytokine involvement in dynorphin-induced allodynia

Dynorphin A is an endogenous opioid peptide, which has previously been shown to produce a long-lasting allodynia and hyperalgesia in mice, behavioral states consistent with signs of clinically observed neuropathic pain. This dynorphin-induced allodynia was used as a pharmacological, central model of neuropathic pain. In this study, we examined the involvement of the cytokine IL-1beta, the transcription factor nuclear factor kappa B (NF-kappaB), and de novo protein synthesis in the development of allodynia induced by intrathecal (i.t.) administration of dynorphin in male ICR mice. Pretreatment with the protein synthesis inhibitor cycloheximide (0. 3-85nmol), the NF-kappaB inhibitor pyrrolidinedithiocarbamate (PDTC) (0.001-1000pmol), the IL-1 receptor antagonist (IL-1ra) protein (0. 01-100ng), the caspase-1 inhibitor (YVAD) (0.1-300pmol), and the anti-inflammatory cytokine IL-10 (0.1-300ng) all dose-dependently reduced the induction of dynorphin-induced allodynia. Finally, IL-10 administered within the first 24h after the dynorphin insult prevented the development of chronic allodynia. These results demonstrate that the anti-inflammatory cytokines IL-10 and IL-1ra impede the development of dynorphin-induced allodynia. These results also suggest that production of new proteins through NF-kappaB activation is required for the induction of allodynia. We speculate that IL-1ra, IL-10, PDTC and cycloheximide interfere with the central pro-inflammatory cascade. Modulation of cytokine activity in the spinal cord may therefore prove to be an effective therapeutic strategy for the treatment of chronic pain.

STAT3 and NFkappaB activation precedes glial reactivity in the excitotoxically injured young cortex but not in the corresponding distal thalamic nuclei

In this study we evaluated the activation of the cytokine and growth factor responsive transcription factors signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappa B (NFkappaB) after different grades of neural damage in the immature rat brain using double immunocytochemical techniques and electron microscopy. Following neocortical N-methyl-D-aspartate induced excitotoxic cell death, both these transcription factors are mainly activated in astrocytes, although microglia, endothelial cells, and neurons show transient activation at specific times and locations. Interestingly, activation of both transcription factors is only observed in cortical areas affected by severe tissue damage, neuronal degeneration, and blood-brain barrier (BBB) disruption. In contrast, the milder glial response occurring in the distal thalamus is not preceded by immunocytochemically detectable STAT3 and NFkappaB activation, although microglial response, astroglial hypertrophy, and glial fibrillary acidic protein (GFAP) overexpression do occur. In the cortex, astrocytes show STAT3 and NFkappaB activation already at 2 to 4 hours post-lesion, preceding cell hypertrophy and GFAP upregulation, and being maintained in the long-term formed glial scar. STAT3 and NFkappaB activation in microglial cells is protracted and observed at 10 to 24 hours post-lesion. The early activation of both transcription factors in astroglial cells could contribute to the changes in gene expression leading to astrogliosis and the release of signalling molecules which may contribute to the subsequent activation of these transcription factors in microglial cells.

Redox regulation of nuclear factor kappa B: therapeutic potential for attenuating inflammatory responses

Nuclear factor kappa B (NF-kappaB) is a protein transcription factor that is required for maximal transcription of a wide array of pro-inflammatory mediators that are involved in the pathogenesis of stroke. The purpose of this review article is to describe what is known about the molecular biology of NF NF-kappaB and to review current understanding of the interaction between reactive oxygen species (ROS) in NF-kappaB. ROS seem to play a duel role by participating in the NF-kappaB activation cascade and by directly modulating DNA binding affinity. Exogenous and endogenous antioxidants are effective in blocking activation of NF-kappaB and preventing the consequences of pro-inflammatory gene expression. Phase II enzymes either directly or indirectly play a major in vivo role in minimizing oxidative stress by scavenging peroxides, peroxide breakdown products and dicarbonyls and in regeneration of lipid peroxidation chain-breaker, vitamin E. Dietary phase II enzyme inducers have been demonstrated to increase phase II enzyme activities in a variety of tissues. These data, together, suggest that phase II enzyme inducers could have therapeutic value for ameliorating inflammatory conditions.

Expression of cytokine genes and increased nuclear factor-kappa B activity in the brains of scrapie-infected mice

A number of aspects of the pathogenesis of scrapie remain to be elucidated. The cellular and molecular aspects of the neuropathology in scrapie suggest the possibility that the proinflammatory cytokines could act as pathogenic mediators in this neurodegenerative disease. To understand this possibility, we examined the expression of proinflammatory cytokine genes in brains of IM mice-infected with 87V scrapie agent. Additionally, we also analyzed the activity of nuclear factor-kappa B (NF-kappaB), which is the major transcriptional activator for inflammatory cytokines, and formation of reactive oxygen species (ROS) as a common upstream messenger for its activation. The induction of mRNAs of the inflammatory cytokines, IL-1alpha, IL-1beta and TNF-alpha, was detected only in the brains of scrapie-infected mice. The activity of NF-kappaB was significantly increased in the nuclear extracts from brains of the scrapie-infected group and the immunoreactivity of NF-kappaB was increased in the hippocampus and thalamus in the brains of scrapie-infected mice. The NF-kappaB immunoreactivity was observed mainly in GFAP-positive astrocytes and also detected in the PrP-amyloid plaques in the brains of 87V scrapie-infected mice. Gene expression of IL-6 and iNOS, the representative target genes for NF-kappaB activation, were activated only in the infected group. The production of ROS was significantly increased in the brain mitochondrial fractions of scrapie-infected mice. These results suggest that prion accumulation in astrocytes might activate NF-kappaB through the increase of ROS generation, and thus alterations in NF-kappaB-directed gene expression may contribute to both the neurodegeneration and proinflammatory responses which occur in scrapie.

Microglial-neuronal interactions in synaptic damage and recovery

An understanding of the role of microglial cells in synaptic signaling is still elusive, but the neuron-microglia relationship may have important ramifications for brain plasticity and injury. This review summarizes current knowledge and theories concerning microglial-neuronal signaling, both in terms of neuron-to-microglia signals that cause activation and microglia-to-neuron signals that affect neuronal response to injury. Microglial activation in the brain involves a stereotypical pattern of changes including proliferation and migration to sites of neuronal activity or injury, increased or de novo expression of immunomodulators including cytokines and growth factors, and the full transformation into brain-resident phagocytes capable of clearing damaged cells and debris. The factors released from neurons that elicit such phenotypical and functional alterations are not well known but may include cytokines, oxidized lipids, and/or neurotransmitters. Once activated, microglia can promote neuronal injury through the release of low-molecular-weight neurotoxins and support neuronal recovery through the release of growth factors and the isolation/removal of damaged neurons and myelin debris. Because microglia respond quickly to neuronal damage and have robust effects on neurons, astrocytes, and oligodendrocytes, microglial cells could play potentially key roles in orchestrating the multicell cascade that follows synaptic plasticity and damage.

Inhibition of NF-kappaB potentiates amyloid beta-mediated neuronal apoptosis

One mechanism leading to neurodegeneration during Alzheimer's disease (AD) is amyloid beta peptide (Abeta) neurotoxicity. Abeta elicits in cultured central nervous system neurons a biphasic response: a low-dose neurotrophic response and a high-dose neurotoxic response. Previously we reported that NF-kappaB is activated by low doses of Abeta only. Here we show that NF-kappaB activation leads to neuroprotection. In primary neurons we found that a pretreatment with 0.1 microM Abeta-(1-40) protects against neuronal death induced with 10 microM Abeta-(1-40). As a known neuroprotective agent we next analyzed the effect of tumor necrosis factor alpha (TNF-alpha). Maximal activation of NF-kappaB was found with 2 ng/ml TNF-alpha. Pretreatment with TNF-alpha protected cerebellar granule cells from cell death induced by 10 microM Abeta-(1-40). This protection is described by an inverted U-shaped dose response and is maximal with a NF-kappaB-activating dose. The molecular specificity of this protective effect was analyzed by specific blockade of NF-kappaB activation. Overexpression of a transdominant negative IkappaB-alpha blocks NF-kappaB activation and potentiates Abeta-mediated neuronal apoptosis. Our findings show that activation of NF-kappaB is the underlying mechanism of the neuroprotective effect of low-dose Abeta and TNF-alpha. In accordance with these in vitro data we find that nuclear NF-kappaB immunoreactivity around various plaque stages of AD patients is reduced in comparison to age-matched controls. Taken together these data suggest that pharmacological NF-kappaB activation may be a useful approach in the treatment of AD and related neurodegenerative disorders.

Neuronal death and survival in two models of hypoxic-ischemic brain damage

Two unilateral hypoxic-ischemia (HI) models (moderate and severe) in immature rat brain have been used to investigate the role of various transcription factors and related proteins in delayed neuronal death and survival. The moderate HI model results in an apoptotic-like neuronal death in selectively vulnerable regions of the brain while the more severe HI injury consistently produces widespread necrosis resulting in infarction, with some necrosis resistant cell populations showing evidence of an apoptotic type death. In susceptible regions undergoing an apoptotic-like death there was not only a prolonged induction of the immediate early genes, c-jun, c-fos and nur77, but also of possible target genes amyloid precursor protein (APP751) and CPP32. In contrast, increased levels of BDNF, phosphorylated CREB and PGHS-2 were found in cells resistant to the moderate HI insult suggesting that these proteins either alone or in combination may be of importance in the process of neuroprotection. An additional feature of both the moderate and severe brain insults was the rapid activation and/or proliferation of glial cells (microglia and astrocytes) in and around the site of damage. The glial response following HI was associated with an upregulation of both the CCAAT-enhancer binding protein alpha (microglia only) and NFkappaB transcription factors.

Mechanisms of apoptosis in PC12 cells irreversibly differentiated with nerve growth factor and cyclic AMP

PC12 cells treated with cAMP become irreversibly differentiated and die by apoptosis when deprived of trophic support, instead of dedifferentiating and reentering the cell cycle. To approach the molecular mechanism underlying the cAMP-induced switch from differentiation/proliferation to apoptosis, we compared three sequential markers of a candidate apoptogenic signal transduction pathway (ceramide, free radicals and NF-kappaB), after trophic factor withdrawal in PC12 cells before and after irreversible differentiation. Serum withdrawal increased ceramide and free radical production regardless of the state of differentiation of the cells. It was followed by cell death, however, only in the absence of NGF and/or cAMP, and was no longer required for apoptosis in NGF/cAMP-differentiated cells. NGF and cAMP withdrawal sufficed. NF-kappaB was activated by NGF withdrawal in reversibly differentiated PC12 cells during dedifferentiation and reentry into the cell cycle, whereas in NGF/cAMP-differentiated cells, it was activated, at a late stage of the apoptotic process, concomitantly with cell death. These results show that a serum factor inhibits ceramide-dependent apoptosis upstream of ceramide and free radical production, whereas NGF- and cAMP-dependent mechanisms inhibit apoptosis either downstream or parallel to these events. After terminal differentiation by cAMP, apoptosis appears to be initiated from the second site, consistent with the serum independence of these cells and the absence of ceramide and free radical production when the NGF/cAMP-dependent inhibitions are released. The differential regulation of NF-kappaB appears to be an important step in the switch from mitosis to apoptosis that occurs during irreversible differentiation of PC12 cells by cAMP.

Induction of heme oxygenase-1 and major histocompatibility complex antigens in transient forebrain ischemia

Recent studies strongly suggest that oxidative stresses participate in ischemia/reperfusion-induced neurodegeneration. In addition, heme oxygenase (HO) and major histocompatibility complex (MHC) antigens serve as functional molecules against oxidative stress and as self-recognition markers in the immune system, respectively. In this study, we examined the induction of HO and MHC antigens in the rat hippocampus after transient forebrain ischemia. The protein level of HO-1 was significantly enhanced after an episode of ischemia. After ischemia, HO-1 expression was observed early but transiently in the CA1 pyramidal neurons and later but continuously in glial cells. Glial cells expressing HO-1 were predominantly ameboid microglia, but not astrocytes. Ameboid microglia expressing HO-1 were predominantly localized with MHC class II antigens. These results indicate that (1) HO-1 expression in CA1 pyramidal neurons may be harmful, and (2) ischemia induces HO-1 in ameboid microglia that express MHC class II antigens, indicating a very specific microglial stress protein response.

Amyloid beta-peptide stimulates nitric oxide production in astrocytes through an NFkappaB-dependent mechanism

The major pathological features of Alzheimer's disease (AD) include amyloid plaques composed primarily of the beta-amyloid (Abeta) peptide, degenerating neurons and neurofibrillary tangles, and the presence of numerous activated astrocytes and microglia. Although extensive genetic data implicate Abeta in the neurodegenerative cascade of AD, the molecular mechanisms underlying its effects on neurons and glia and the relationship between glial activation and neuronal death are not well defined. Abeta has been shown to induce glial activation, and a growing body of evidence suggests that activated glia contribute to neurotoxicity through generation of inflammatory cytokines and neurotoxic free radicals, such as nitric oxide (NO), potent sources of oxidative stress known to occur in AD. It is therefore crucial to identify specific Abeta-induced molecular pathways mediating these responses in activated glia. We report that Abeta stimulates the activation of the transcription factor NFkappaB in rat astrocytes, that NFkappaB activation occurs selectively from p65 transactivation domain 2, and that Abeta-induced NO synthase expression and NO production occur through an NFkappaB-dependent mechanism. This demonstration of how Abeta couples an intracellular signal transduction pathway involving NFkappaB to a potentially neurotoxic response provides a key mechanistic link between Abeta and the generation of oxidative damage. Our results also suggest possible molecular targets upon which to focus future drug discovery efforts for AD.

Nuclear factor-kappa B activation during cerebral reperfusion: effect of attenuation with N-acetylcysteine treatment

We examined activation of the transcription factor, nuclear factor-kappaB (NF-kappaB), which participates in the upregulation of endothelial cell adhesion proteins, during reperfusion after temporary middle cerebral artery occlusion (TMCAO). We hypothesized that N-acetylcysteine (NAC), an antioxidant which inhibits NF-kappaB activation, would alter events in brain reperfusion injury. We used a rat model of TMCAO. The left sides of the brains were rendered ischemic for 2 h, and then the area was allowed to reperfuse. The animals were treated with NAC (150 mg/kg) or saline placebo, sacrificed, and activated NF-kappaB was assessed in both the left and right hemispheres, all at varying intervals. Cerebral infarction volume was also measured in each of the hemispheres collected from a separate group of animals. Activated NF-kappaB, consisting of p65 and p50 Rel proteins, was significantly increased 15 min after reperfusion in the affected hemisphere. The activation at 15 min was completely abolished with NAC treatment. NAC treatment 1 h prior to the end of occlusion and at 24 h reduced the percentage infarction volume of the affected hemispheres from 35.5+/-2.8% (S.E.) to 18. 1+/-2.1% (p<0.01). NAC treatment at 1 h after the occlusion (after the NF-kappaB peak) and again at 24 h also significantly reduced the percentage infarction volume from 34.8+/-3.8% to 24.6+/-3.8% (p<0. 05). Thus, while NAC inhibited activation of NF-kappaB at 15 min after reperfusion, the drug acted to reduce cerebral infarction by additional, undefined mechanisms. These results bring into question the various roles of NF-kappaB in cerebral infarction followed by reperfusion.

Gene expression induced by cerebral ischemia: an apoptotic perspective

The flow of new information on gene expression related to apoptosis has been relentless in the last several years. This has also been the case with respect to gene expression after cerebral ischemia. Many of genes associated with an apoptotic mode of cell death have now been studied in the context of experimental cerebral ischemia from the immediate early genes through modulating genes such as bcl-2 to genes in the final execution phase such as interleukin-1β converting enzyme (ICE)-related proteases. It was impossible to adequately cite all primary reports on these subjects. However, many excellent reviews have appeared in the last year, which together, cover all these areas of interest. In this review, we have elected to cite only reports published since January 1996 and use an extensive collection of reviews (indicated in italics) to guide the reader to the earlier literature. Our intent is to provide the reader with a timely and useful analysis of the current state of the art. It is hoped that this approach does not cause offense with our colleagues whose contributions before 1996 laid the foundation for much of this work.

Apoptosis, neurotrophic factors and neurodegeneration

Apoptosis is an active process of cell death characterized by distinct morphological features, and is often the end result of a genetic programme of events, i.e. programmed cell death (PCD). There is growing evidence supporting a role for apoptosis in some neurodegenerative diseases. This conclusion is based on DNA fragmentation studies and findings of increased levels of pro-apoptotic genes in human brain and in in vivo and in vitro model systems. Additionally, there is some evidence for a loss of neurotrophin support in neurodegenerative diseases. In Alzheimer's disease, in particular, there is strong evidence from human brain studies, transgenic models and in vitro models to suggest that the mode of nerve cell death is apoptotic. In this review we describe the evidence implicating apoptosis in neurodegenerative diseases with a particular emphasis on Alzheimer's disease.

Age-associated alterations in hippocampal and basal forebrain nuclear factor kappa B activity

Age-related cognitive deficits are often associated with loss of cholinergic activity within the neurotrophin-dependent cholinergic neurons that project from the basal forebrain to the hippocampus. The cause of reduced cholinergic function is unknown, but alterations in transcription factor-signaling pathways causing altered gene expression may cause decreased specific tissue function, resulting in loss of cholinergic activity. We measured transcription factor Nuclear Factor kappa B by electrophoretic mobility shift assay and Western analysis in young and aged rat brain tissues and report that basal levels of Nuclear Factor kappa B DNA-binding activity increase in the hippocampus and basal forebrain with age to significantly higher levels at 30 months of age. This age-associated increase in binding activity is associated with increased translocation of p65 to the nucleus. These data show an age-associated alteration in Nuclear Factor kappa B signal transduction pathways that may contribute to age-associated decreases in specific tissue function.