In an in vitro model of cerebral ischemia (oxygen glucose deprivation, OGD) we investigated whether erythropoietin (EPO) plays a critical role in ischemic preconditioning. We found that EPO time and dose-dependently induced protection against OGD in rat primary cortical neurons. Protection was significant at 5 min and reached a maximum at 48 hr after EPO application. Protection was blocked by the coapplication of a soluble Epo receptor (sEpoR) or an antibody against EpoR (anti-EpoR). Medium transfer from OGD-treated astrocytes to untreated neurons induced protection against OGD in neurons, which was attenuated strongly by the application of sEpoR and anti-EpoR. In contrast, medium transfer from OGD-treated neurons to untreated neurons induced protection against OGD that did not involve EPO. In astrocytes the OGD enhanced the nuclear translocation of hypoxia-inducible factor 1 (HIF-1), the major transcription factor regulating EPO expression. Consequently, transcription of EPO-mRNA was increased in astrocytes after OGD. Cultured neurons express EpoR, and the Janus kinase-2 (JAK-2) inhibitor AG490 abolished EPO-induced tolerance against OGD. Furthermore, EPO-induced neuroprotection as well as phosphorylation of the proapoptotic Bcl family member Bad was reduced by the phosphoinositide-3 kinase (PI3K) inhibitor LY294002. The results suggest that astrocytes challenged with OGD provide paracrine protective signals to neurons. We provide evidence for the following signaling cascade: HIF-1 is activated rapidly by hypoxia in astrocytes. After HIF-1 activation the astrocytes express and release EPO. EPO activates the neuronal EPO receptor and, subsequently, JAK-2 and thereby PI3K. PI3K deactivates BAD via Akt-mediated phosphorylation and thus may inhibit hypoxia-induced apoptosis in neurons. Our results establish EPO as an important paracrine neuroprotective mediator of ischemic preconditioning.

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Pneumococcus is the most common and aggressive cause of bacterial meningitis and induces a novel apoptosis-inducing factor–dependent (AIF–dependent) form of brain cell apoptosis. Loss of production of two pneumococcal toxins, pneumolysin and H2O2, eliminated mitochondrial damage and apoptosis. Purified pneumolysin or H2O2 induced microglial and neuronal apoptosis in vitro. Both toxins induced increases of intracellular Ca2+ and triggered the release of AIF from mitochondria. Chelating Ca2+ effectively blocked AIF release and cell death. In experimental pneumococcal meningitis, pneumolysin colocalized with apoptotic neurons of the hippocampus, and infection with pneumococci unable to produce pneumolysin and H2O2 significantly reduced damage. Two bacterial toxins, pneumolysin and, to a lesser extent, H2O2, induce apoptosis by translocation of AIF, suggesting new neuroprotective strategies for pneumococcal meningitis

Innate immunity relies on pattern recognition receptors to detect the presence of infectious pathogens. In the case of Gram-positive bacteria, binding of bacterial lipopeptides to TLR2 is currently regarded as an important mechanism. In the present study, we used the synthetic bacterial lipopeptide Pam3CysSK4, a selective TLR2 agonist, to induce meningeal inflammation in rodents. In a 6-h rat model, intrathecal application of Pam3CysSK4 caused influx of leukocytes into the cerebrospinal fluid (CSF) and induced a marked increase of regional cerebral blood flow and intracranial pressure. In wild-type mice, we observed CSF pleocytosis and an increased number of apoptotic neurons in the dentate gyrus 24 h after intrathecal challenge. Inflammation and associated neuronal loss were absent in TLR2 knockout mice. In purified neurons, cytotoxicity of Pam3CysSK4 itself was not observed. Exposure of microglia to Pam3CysSK4 induced neurotoxic properties in the supernatant of wild-type, but not TLR2-deficient microglia. We conclude that TLR2-mediated signaling is sufficient to induce the host-dependent key features of acute bacterial meningitis. Therefore, synthetic lipopeptides are a highly specific tool to study mechanisms of TLR2-driven neurodegeneration in vivo.

The major goal of this study was to compare mechanisms of the neuroprotective potential of 17 ␤-estradiol in two models for oxidative stress-independent apoptotic neuronal cell death with that in necrotic neuronal cell death in primary neuronal cultures derived from rat hippocampus, septum, or cortex. Neuronal apoptosis was induced either by staurosporine or ethylcholine aziridinium (AF64A), as models for necrotic cell death glutamate exposure or oxygen-glucose deprivation (OGD) were applied. Long-term (20 hr) pretreatment (0.1 M 17 ␤-estradiol) was neuroprotective in apoptotic neuronal cell death induced by AF64A (40 M) only in hippocampal and septal neuronal cultures and not in cortical cultures. The neuroprotective effect was blocked by the estrogen antagonists ICI 182,780 and tamoxifen and the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002. In glutamate and OGD-induced neuronal damage, long-term pretreatment was not effective. In contrast, short-term (1 hr) pretreatment with 17 ␤-estradiol in the dose range of 0.5-1.0 M significantly reduced the release of lactate dehydrogenase and improved morphology of cortical cultures exposed to glutamate or OGD but was not effective in the AF64A model. Staurosporine-induced apoptosis was not prevented by either long-or short-term pretreatment. The strong expression of the estrogen receptor-␣ and the modulation of Bcl proteins by 17 ␤-estradiol in hippocampal and septal but not in cortical cultures indicates that the prevention of apoptotic, but not of necrotic, neuronal cell death by 17 ␤-estradiol possibly depends on the induction of Bcl proteins and the density of estrogen receptor-␣.Key words: apoptosis; necrosis; estradiol; AF64A; staurosporine; oxygen-glucose deprivation; primary neuronal cultures; hippocampus; cortex; septum; Alzheimer' s diseaseThe neuroprotective potential of estrogens has gained increasing attention during the last years. Epidemiological evidence suggests that estrogen replacement therapy for postmenopausal women is associated with an improvement of some measures of cognitive performance, protection against cognitive deterioration, and decreased incidence of Alzheimer's disease