Leptin is induced in the ischemic cerebral cortex and exerts neuroprotection through NF-kappaB/c-Rel-dependent transcription.

BACKGROUND AND PURPOSE: Leptin is an adipose hormone endowed with angiopoietic, neurotrophic, and neuroprotective properties. We tested the hypothesis that leptin might act as an endogenous mediator of recovery after ischemic stroke and investigated whether nuclear transcription factors kappaB activation is involved in leptin-mediated neuroprotection. METHODS: The antiapoptotic effects of leptin were evaluated in cultured mouse cortical neurons from wild-type or NF-kappaB/c-Rel(-/-) mice exposed to oxygen-glucose deprivation. Wild-type, c-Rel(-/-) and leptin-deficient ob/ob mice were subjected to permanent middle cerebral artery occlusion. Leptin production was measured in brains from wild-type mice with quantitative reverse transcriptase-polymerase chain reaction and immunostaining. Mice received a leptin bolus (20 microg/g) intraperitoneally at the onset of ischemia. RESULTS: Leptin treatment activated the nuclear translocation of nuclear transcription factors kappaB dimers containing the c-Rel subunit, induced the expression of the antiapoptotic c-Rel target gene Bcl-xL in both control and oxygen-glucose deprivation conditions, and counteracted the oxygen-glucose deprivation-mediated apoptotic death of cultured cortical neurons. Leptin-mediated Bcl-xL induction and neuroprotection against oxygen-glucose deprivation were hampered in cortical neurons from c-Rel(-/-) mice. Leptin mRNA was induced and the protein was detectable in microglia/macrophage cells from the ischemic penumbra of wild-type mice subjected to permanent middle cerebral artery occlusion. Ob/ob mice were more susceptible than wild-type mice to the permanent middle cerebral artery occlusion injury. Leptin injection significantly reduced the permanent middle cerebral artery occlusion-mediated cortical damage in wild-type and ob/ob mice, but not in c-Rel(-/-) mice. CONCLUSIONS: Leptin acts as an endogenous mediator of neuroprotection during cerebral ischemia. Exogenous leptin administration protects against ischemic neuronal injury in vitro and in vivo in a c-Rel-dependent manner.

NF-kappaB p50/RelA and c-Rel-containing dimers: opposite regulators of neuron vulnerability to ischaemia.

Diverse nuclear factor-kappaB subunits mediate opposite effects of extracellular signals on neuron survival. While RelA is activated by neurotoxic agents, c-Rel drives neuroprotective effects. In brain ischaemia RelA and p50 factors rapidly activate, but how they associate with c-Rel to form active dimers and contribute to the changes in diverse dimer activation for neuron susceptibility is unknown. We show that in both cortical neurons exposed to oxygen glucose deprivation (OGD) and mice subjected to brain ischaemia, activation of p50/RelA was associated with inhibition of c-Rel/RelA dimer and no change p50/c-Rel. Targeting c-Rel and RelA expression revealed that c-Rel dimers reduced while p50/RelA enhanced neuronal susceptibility to anoxia. Activation of p50/RelA complex is known to induce the pro-apoptotic Bim and Noxa genes. We now show that c-Rel-containing dimers, p50/c-Rel and RelA/c-Rel, but not p50/RelA, promoted Bcl-xL transcription. Accordingly, the OGD exposure induced Bim, but reduced Bcl-xL promoter activity and decreased the content of endogenous Bcl-xL protein. These findings demonstrate that within the same neuronal cell, the balance between activation of p50/RelA and c-Rel-containing complexes fine tunes the threshold of neuron vulnerability to the ischaemic insult. Selective targeting of different dimers will unravel new approaches to limit ischaemia-associated apoptosis.

Regulation of nuclear factor kappaB in the hippocampus by group I metabotropic glutamate receptors.

An increasing amount of evidence suggests that the family of nuclear factor kappaB (NF-kappaB) transcription factors plays an important role in synaptic plasticity and long-term memory formation. The present study investigated the regulation of NF-kappaB family members p50, p65/RelA, and c-Rel in the hippocampus in response to metabotropic glutamate receptor (mGluR) signaling. Activation of group I metabotropic glutamate receptors (GpI-mGluRs) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) resulted in a time-dependent increase in DNA binding activity of p50, p65, and c-Rel in area CA1 of the hippocampus. An antagonist of mGluR5, 2-Methyl-6-(phenylethynyl)pyridine, inhibited the DHPG-induced activation of NF-kappaB, whereas an antagonist of mGluR1, (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid, did not. Using a series of inhibitors, we investigated the signaling pathways necessary for DHPG-induced activation of NF-kappaB and found that they included the phosphatidyl inositol 3-kinase, protein kinase C, mitogen-activated protein kinase kinase, and p38-mitogen-activated protein kinase pathways. To determine the functional significance of mGluR-induced regulation of NF-kappaB, we measured long-term depression (LTD) of Schaffer-collateral synapses in the hippocampus of c-Rel knock-out mice. Early phase LTD was normal in c-rel(-/-) mice. However, late-phase LTD (>90 min) was impaired in c-rel(-/-) mice. The observations of this deficit in hippocampal synaptic plasticity prompted us to further investigate long-term memory formation in c-rel(-/-) mice. c-rel(-/-) mice exhibited impaired performance in a long-term passive avoidance task, providing additional evidence for c-Rel in long-term memory formation. These results demonstrate that the NF-kappaB transcription factor family is regulated by GpI-mGluRs in the hippocampus and that the c-Rel transcription factor is necessary for long-term maintenance of LTD and formation of long-term memory.

Glutamatergic reinnervation and assembly of glutamatergic synapses in adult rat skeletal muscle occurs at cholinergic endplates.

After denervation of adult rat abdominal muscles, the postsynaptic apparatus of neuromuscular junctions (NMJs) retains its original architecture and clustering of acetylcholine receptors (AChRs). When descending fibers of the spinal cord are surgically diverted to this muscle by a nerve grafting procedure, supraspinal glutamatergic neurons can innervate muscle fibers and restore motor function; the newly formed NMJs switch from a cholinergic to a glutamatergic-type synapse. We show here that regenerating nerve endings contact the fibers in an area occupied by cholinergic endplates. These NMJs are morphologically indistinguishable from those in controls, but they differ in the subunit composition of AChRs. Moreover, by immunofluorescence and immunoelectron microscopy, new NMJs express glutamatergic synapse markers. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluR1 partially colocalizes with AChRs, and vesicular glutamate transporter 2 is localized in the presynaptic compartment. Immunoprecipitation analysis of membranes from reinnervated muscle showed that AMPA receptor subunits GluR1 and GluR2 coimmunoprecipitate with rapsyn, the AChR-anchoring protein at the NMJ. Taken together, these results indicate that cholinergic endplates can be targeted by new glutamatergic projections and that the clustering of AMPA receptors occurs there.

NF-kappaB pathway: a target for preventing beta-amyloid (Abeta)-induced neuronal damage and Abeta42 production.

Beta-amyloid (Abeta) peptides are key proteins in the pathophysiology of Alzheimer's disease (AD). While Abeta42 aggregates very rapidly to form early diffuse plaques, supplemental Abeta40 deposition is required to form mature neuritic plaques. We here investigated the role of nuclear factor-kappaB (NF-kappaB) pathway in Abeta40-mediated neuronal damage and amyloid pathology. In rat primary neurons and human postmitotic neuronal cells, the Abeta peptide induced a dose-dependent neuronal death, reduced the levels of the anti-apoptotic protein Bcl-XL, enhanced the cytosolic release of cytochrome c, and elicited the intracellular accumulation and secretion of Abeta42 oligomers. Moreover, Abeta40 activated the NF-kappaB pathway by selectively inducing the nuclear translocation of p65 and p50 subunits, and promoted an apoptotic profile of gene expression. As inhibitors of the NF-kappaB pathway, we tested the capability of a double-stranded kappaB decoy oligonucleotide, the anti-inflammatory drug aspirin and the selective IkappaB kinase 2 inhibitor, AS602868, to modify the Abeta40-mediated effects. These treatments, transiently applied before Abeta exposure, completely inhibited p50/p65 nuclear translocation and neuronal damage. The kappaB decoy also inhibited the Abeta-induced release of cytochrome c, restored the levels of Bcl-XL, and prevented intraneuronal accumulation and secretion of Abeta42. These results open up interesting perspectives on the development of novel strategies targeting out NF-kappaB p50/p65 dimers for pharmacological intervention in AD.

Expression of functional NR1/NR2B-type NMDA receptors in neuronally differentiated SK-N-SH human cell line.

The present study demonstrates that human SK-N-SH neuroblastoma cells, differentiated by retinoic acid (RA), express functional NMDA receptors and become vulnerable to glutamate toxicity. During exposure to RA, SK-N-SH cells switched from non-neuronal to neuronal phenotype by showing antigenic changes typical of postmitotic neurons together with markers specific for cholinergic cells. Neuronally differentiated cells displayed positive immunoreactivity to the vesicular acetylcholine transporter and active acetylcholine release in response to depolarizing stimuli. The differentiation correlated with the expression of NMDA receptors. RT-PCR and immunoblotting analysis identified NMDA receptor subunits NR1 and NR2B, in RA-differentiated cultures. The NR1 protein immunolocalized to the neuronal cell population and assembled with the NR2B subunit to form functional N-methyl-D-aspartate (NMDA) receptors. Glutamate or NMDA application, concentration-dependently increased the intracellular Ca2+ levels and acetylcholine release in differentiated cultures, but not in undifferentiated SK-N-SH cells. Moreover, differentiated cultures became vulnerable to NMDA receptor-mediated excitotoxicity. The glutamate effects were enhanced by glycine application and were prevented by the NMDA receptor blocker MK 801, as well as by the NR2B selective antagonist ifenprodil. These data suggest that SK-N-SH cells differentiated by brief treatment with RA may represent an unlimited source of neuron-like cells suitable for studying molecular events associated with activation of human NR1/NR2B receptors.

Prevention of neuron and oligodendrocyte degeneration by interleukin-6 (IL-6) and IL-6 receptor/IL-6 fusion protein in organotypic hippocampal slices.

We investigated the effects of IL-6 and a chimeric derivative of IL-6 and soluble IL-6 receptor (IL6RIL6 chimera) on excitotoxic injury in rat organotypic hippocampal slices. Brief application of N-methyl-d-aspartate (NMDA) induced astrocyte reactivity, neuron cell death, and oligodendrocyte degeneration, the latter caused by secondary activation of AMPA/kainate receptors. Both these cytokines rescued neurons and oligodendrocytes, albeit the chimeric compound was much more potent and efficient than IL-6. No change was produced on reactive astrocytosis. The cytokines preserved myelin basic protein (MBP) production in slices exposed to excitotoxic insult, and when applied singularly for a week, they also enhanced both MBP and proteolipid protein expression. These effects occurred through activating the signal transducer gp130 and were associated with stimulation of transcription factors STAT1 and STAT3. Our results suggest that IL-6 and IL6RIL6 may prove to be valuable in treating neurodegenerative and demyelinating diseases.

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.