One path to cell death in the nervous system

Alteration of NF-kappaB activity leads to mitochondrial apoptosis after infection with pathological prion protein

Nuclear factor kappa B (NF-kappaB) is a key regulator of the immune response, but in almost the same manner it is involved in induction of inflammation, proliferation and regulation of apoptosis. In the central nervous system activated NF-kappaB plays a neuroprotective role. While in some neurodegenerative disorders the role of NF-kappaB is well characterized, there is poor knowledge on the role of NF-kappaB in prion disease. We found binding but no transcriptional activity of the transcription factor in vitro. Characterizing the mechanism of cell death after infection with pathological prion protein increased caspase-9 and caspase-3 activity was detected and the lack of NF-kappaB activity resulted in the inability to activate target genes that usually play an important role in neuroprotection. Additionally, we investigated the role of NF-kappaB after prion infection of Nfkb1(-/-), Nfkb2(-/-) and Bcl3(-/-) mice and central nervous system-specific p65-deleted mice revealing an accelerated prion disease in NF-kappaB2- and Bcl-3-deficient mice, which is in line with a reduced neuroprotective activity in prion infection. Based on our findings, we propose a model whereby the alteration of NF-kappaB activity at the early stages of infection with pathological prion protein leads to neuronal cell death mediated by mitochondrial apoptosis.

Mechanisms of nitric oxide-induced apoptosis in bovine chromaffin cells: Role of mitochondria and apoptotic proteins

The aim of this work was to establish the possible involvement of mitochondria in the apoptotic event triggered by nitric oxide (NO) in chromaffin cells. Using bovine chromaffin cells in primary culture and several NO donors (SNP, SNAP, and GSNO) at apoptotic concentrations (50 microM-1 mM), we have shown that NO induces a time-dependent decrease in the mitochondrial transmembrane potential (DeltaPsi(m)), which correlates with the appearance of hypodiploid cells. Disruption in DeltaPsi(m) is followed by cytochrome c release to the cytosol, which in turn precedes caspase 3 activation. In this mechanism participates the Bcl-2 protein family, because NO donors downregulate the expression of anti-apoptotic members of the family such as Bcl-2 and Bcl-XL, and increase the expression of pro-apoptotic members, Bax and Bcl-Xs, inductors of cytochrome c release to cytosol. Different cell signaling pathways seem to regulate Bax induction and Bcl-2 inhibition because decreased Bcl-2 levels are detected later than enhanced Bax expression. The tumour suppressor protein p53 is also upregulated in a very early phase (30 min) of the NO-induced apoptosis and may be responsible for the further induction of Bax expression. Finally, the translocation of NF-kappaB to the nucleus seems to be another early event in NO-induced apoptosis and it may be involved in the regulation of p53 expression. These results support strongly the participation of mitochondrial mechanisms in NO-induced apoptosis in chromaffin cells and suggest that these cells may be good models for the investigation of molecular basis of neurodegeneration and neuroprotection.

Transcriptional and translational regulation of BACE1 expression--implications for Alzheimer's disease

The proteolytical processing of the amyloid precursor protein (APP) gives rise to beta-amyloid peptides, which accumulate in brains of Alzheimer's disease (AD) patients. Different soluble or insoluble higher molecular weight forms of beta-amyloid peptides have been postulated to trigger a complex pathological cascade that may cause synaptic dysfunction, inflammatory processes, neuronal loss, cognitive impairment, and finally the onset of the disease. The generation of beta-amyloid peptides requires the proteolytical cleavage of APP by an aspartyl protease named beta-site APP-cleaving enzyme 1 (BACE1). The expression and enzymatic activity of BACE1 are increased in brains of AD patients. Here we discuss the importance of a number of recently identified transcription factors as well as post-transcriptional modifications and activation of intracellular signaling molecules for the regulation of BACE1 expression in brain. Importantly, some of these factors are known to be involved in the inflammatory and chronic stress responses of the brain, which are compromised during aging. Moreover, recent evidence indicates that beneficial effects of non-steriodal anti-inflammatory drugs on the progression of AD are mediated--at least in part--by effects on the peroxisome proliferator-activated receptor-gamma response element present in the BACE1 promoter. The identification of the cell type-specific expression and activation of NF-kappaB, Sp1 and YY1 transcription factors may provide a basis to specifically interfere with BACE1 expression and, thereby, to lower the concentrations of beta-amyloid peptides, which may prevent neuronal cell loss and cognitive decline in AD patients.

Altered glucocorticoid receptor signaling cascade in lymphocytes of bipolar disorder patients

Bipolar disorder (BD) is characterized by hypothalamic pituitary adrenal (HPA) axis hyperactivity, glucocorticoid insensitivity and alterations in serotonin and inflammatory mediators. The glucocorticoid receptor (GR), activator protein-1 (AP-1), nuclear factor-kappa B (NF-kappaB) and c-jun N-terminal kinase (JNK) regulate the above mentioned processes; we therefore assessed their role in BD. Fifteen bipolar depressed patients under multiple anti-depressant therapy, 15 bipolar euthymics under lithium monotherapy and 25 matched controls were studied. Whole cell and nuclear extracts from lymphocytes were immunoblotted for GR, c-fos, JNK and NF-kappaB and nuclear aliquots were submitted to electrophoretic mobility shift assay for GR, AP-1 and NF-kappaB. Associations with the anti-depressant therapy and the state of the disease were also sought. Results, expressed as percentage of pooled protein standard sample intergraded optical density (IOD) (mean +/- SD), revealed: (a) depressed patients had significantly higher GR levels than controls in whole cell (82.63 +/- 6.18 versus 76.27 +/- 4.21%, P < 0.01) and nuclear extracts (86.66 +/- 3.81 versus 81.72 +/- 2.71%, P < 0.001) but lower GR-DNA binding (68.75 +/- 7.91 versus 81.84 +/- 4.25%, P < 0.05). Euthymics had normalized whole cell GR content (73.64 +/- 5.95%) and GR-DNA binding activity (76.82 +/- 7.29%) but higher nuclear GR content (86.89+/-3.96%, P<0.01) than controls; (b) nuclear c-fos content and AP-1-DNA-binding were significantly lower in depressed patients than controls (80.49 +/- 2.03 versus 84.82 +/- 3.48%, P < 0.05 and 78.46 +/- 4.17 versus 84.80 +/- 5.79%, P < 0.05, respectively). Euthymics however, showed similar nuclear c-fos and AP-1-DNA-binding to controls (85.48 +/- 2.71 and 87.78 +/- 3.54%, respectively) but lower whole cell c-fos than in controls (81.18 +/- 3.87 versus 87.01 +/- 4.22%, P < 0.001); (c) depressed patients had significantly lower whole cell and nuclear JNK than controls (67.01 +/- 4.29 versus 72.00 +/- 3.68%, P < 0.05 and 80.10 +/- 2.53 versus 86.96 +/- 2.49%, P < 0.001) whereas euthymics showed lower nuclear JNK (83.27 +/- 1.93%, P < 0.01); (d) whole cell NF-kB was higher in the depressed patients than in controls (67.30 +/- 5.00 versus 63.63 +/- 3.3%, P < 0.05). Concluding, intracellular signaling of GR, AP-1 and JNK are altered in BD and may underly disease aetiopathogenesis and/or reflect the effect of the anti-depressants.

Kainate mediates nuclear factor-kappa B activation in hippocampus via phosphatidylinositol-3 kinase and extracellular signal-regulated protein kinase

The transcription factor nuclear factor-kappa B (NF-kappaB) is an inducible regulator of genes that plays a crucial role in the nervous system. Glutamate receptor stimulation is one well-described mechanism for NF-kappaB activation. In the studies presented here we used the glutamate analog, kainate to investigate the signaling mechanisms that couple to NF-kappaB activation in hippocampus. Kainate (250 nM) application to hippocampal slices elicited a time-dependent increase in nuclear NF-kappaB levels in areas CA3 and CA1, but not dentate, compared with controls. Further analysis focused on hippocampal area CA3, revealed increased NF-kappaB DNA binding activity in response to kainate stimulation. Supershift electrophoretic mobility shift assay indicated that the kainate-mediated NF-kappaB complex binding DNA was composed of p65, p50, and c-Rel subunits. Through inhibition studies we found that extracellular signal-regulated protein kinase (ERK) and phosphatidylinositol-3 kinase (PI3K) couple to basal and kainate-mediated NF-kappaB DNA binding activity in area CA3. Kainate elicited decreased total and increased phospho-inhibitor kappa B alpha (IkappaBalpha), suggesting that kainate-mediated activation of NF-kappaB is via the classical IkappaB kinase pathway. Interestingly, inhibition of ERK but not PI3K blocked the kainate-mediated increase in phospho-IkappaBalpha. Thus, our findings support a role for the ERK and PI3K pathways in kainate-mediated NF-kappaB activation in hippocampal area CA3, but these kinases may target the NF-kappaB pathway at different loci.

Bcl‐2 family members make different contributions to cell death in hypoxia and/or hyperoxia in rat cerebral cortex

Hypoxic brain injury during fetal or neonatal development leads to damaged immature neurons and can result in cognitive or behavioral dysfunction. Hyperoxia therapy (treatment with oxygen) is commonly applied to infants with signs of perinatal hypoxia-anoxia. Both hypoxia and hyperoxia have been shown to result in apoptosis in the brains of rats in several animal models. One determinant of cellular commitment to cell death is the differential expression of the Bcl-2 family of proteins in response to trauma. Here, we characterize cell death and the expression of Bcl-2 homologous proteins in 7-day-old neonatal rat cerebral cortex after hypoxia (5% O(2) for 40 min) and/or hyperoxia (>95% O(2) for 2 h after hypoxia). The expression of Bcl-2 and Bcl-X(L), two anti-apoptotic proteins, decreased at 24 h after hypoxia. Bcl-X(L) increased after either hyperoxia or hypoxia+hyperoxia. We did not detect significant changes in the cytoplasmic levels of pro-apoptotic protein Bax after any of these three treatments. Using cell death ELISA and DNA FragEL assays, we observed increased cell death at 24h after hypoxia, hyperoxia or hypoxia+hyperoxia treatments. At 24 h after either hypoxia, hyperoxia or hypoxia+hyperoxia, caspase 3 activity also increased significantly. Our results suggest that both hypoxia and hyperoxia alone can induce cell death. The Bcl-2 --> cytochrome c --> caspase 3 pathway played a role in hypoxia-induced cell death, while other pathways may be involved in hyperoxia-induced cell death.

Caffeic acid phenethyl ester (CAPE) prevents inflammatory stress in organotypic hippocampal slice cultures

Caffeic acid phenethyl ester (CAPE) is an antioxidant component of propolis, a natural product secreted by honeybee. Recent literature shows that CAPE inhibits nuclear factor kappa B (NFkappaB) activation in cell lines. Since NFkappaB was shown to be a crucial factor in neuroinflammation and to be associated with some neuropathologies, CAPE might reduce these disorders in brain too and have therapeutic applications. To test this hypothesis we used a model of endotoxic insult (interferon-gamma, followed by lipopolysaccharide) on rat organotypic hippocampal cultures. Cerebral inflammatory responses were strongly inhibited by CAPE (100 microM): reductions of NFkappaB nuclear activity, tumor necrosis factor alpha and nitric oxide productions were observed. At the dose of maximal effects (100 microM), an increase of cAMP-responsive element binding protein (CREB) activity, which anti-inflammatory role is well known, was seen. We compared CAPE effects with those of other drugs: anti-inflammatory as acetyl-salicylate and dexamethasone (glucocorticoid), antioxidant as pyrrolidine dithiocarbamate, or selective permeant inhibitor of NFkappaB as SN 50 peptide. These studies lead us to conclude that CAPE presents an interesting and original neuropharmacological profile compared to these drugs and might be helpful in the prevention of neurotoxic events due to excessive inflammatory reaction in brain. CAPE interferes with several effectors of neuroinflammation that might have complementary and synergic effects and allows a rather durable control since an acute treatment at the time of endotoxin exposure allows to control inflammatory factors for over 48 h.

Phosphorylation regulates the nucleocytoplasmic distribution of kinase suppressor of Ras

KSR (kinase suppressor of Ras) has been proposed as a molecular scaffold regulating the Raf/MEK/ERK kinase cascade. KSR is phosphorylated on multiple phosphorylation sites by associated kinases. To identify potential mechanisms used by KSR to regulate ERK activation, green fluorescent protein was fused to intact and mutated KSR constructs lacking specific phosphorylation sites, and the subcellular distribution of each construct was observed in live cells. Mutation of a subset of KSR phosphorylation sites caused the redistribution of KSR to the nucleus. To determine whether intact KSR is normally imported to the nucleus, REF-52 fibroblasts expressing KSR were treated with 10 nm leptomycin B, which inhibits Crm1-dependent nuclear export. KSR accumulated in the nucleus within 2 h of treatment with leptomycin B, suggesting that KSR cycles continuously through the nucleus. Nuclear import of KSR was blocked by mutations that inhibit the interaction of KSR with MEK. Coexpression of fluorescent forms of KSR and MEK in cells revealed that each protein promoted the localization of the other in the cytoplasm. These data indicate that the subcellular distribution of KSR is dynamically regulated through phosphorylation and MEK interaction in a manner that may affect signaling through ERK.

Signal transduction for clinicians: why should we care?

Neurons must respond to a bewildering array of external and internal stimuli and must distinguish among them to generate an appropriate response or change in metabolic or electrical activity. Furthermore, the response of a cell to a given stimulus must depend on what else is happening inside and outside the cell at the time of arrival of that stimulus. The process of signal transduction is what gives the cell and organism the flexibility and "knowledge base" to carry out these functions. Conversely, aberrations of signal transduction underlie an increasing array of developmental, genetic, and acquired diseases and conditions of the nervous system. Pharmacological modulation of signal transduction pathways and their effectors holds great promise for the remediation of these neurologic disorders.