Although augmented prostaglandin E2 (PGE2) synthesis and accumulation have been demonstrated in the lesion sites of rodent transient focal ischemia models, the role of PGE 2 in neuronal survival has been controversial, showing both protective and toxic effects. Here we demonstrate the induction of microsomal PGE synthase 1 (mPGES-1), an inducible terminal enzyme for PGE 2 synthesis, in neurons, microglia, and endothelial cells in the cerebral cortex after transient focal ischemia. In mPGES-1 knockout (KO) mice, in which the postischemic PGE 2 production in the cortex was completely absent, the infarction, edema, apoptotic cell death, and caspase-3 activation in the cortex after ischemia were all reduced compared with those in wild-type (WT) mice. Furthermore, the behavioral neurological dysfunctions observed after ischemia in WT mice were significantly ameliorated in KO mice. The ameliorated symptoms observed in KO mice after ischemia were reversed to almost the same severity as WT mice by intracerebroventricular injection of PGE 2 into KO mice. Our observations suggest that mPGES-1 may be a critical determinant of postischemic neurological dysfunctions and a valuable therapeutic target for treatment of human stroke.S troke remains a major cause of death and neuronal disability worldwide. Although effective stroke treatments based on thrombolysis and restoration of blood flow have been developed (1), these therapies are effective only during the first few hours after the onset of the stroke (2). At later times after ischemia, many kinds of gene induction occur (3), some of which are known to be involved in the brain inflammation that is a major factor in the progression of the injury (4, 5).Prostaglandin E 2 (PGE 2 ), one of the most likely candidates for propagation of inflammation, is known to be accumulated at the lesion sites of the postischemic brain (6, 7). PGE 2 is sequentially synthesized from arachidonic acid by cyclooxygenase (COX) and PGE 2 synthase (PGES). Among the COX isoforms, COX-2 is the inducible form; however, it has been immunohistochemically detected in neurons in the normal brain (8). COX-2 has been demonstrated to be up-regulated after transient ischemia in neurons (9-12) and has recently been identified in nonneuronal cells as well at some lesion sites, e.g., in microglia in the brains of patients with multiple sclerosis and chronic cerebral ischemia (13,14). The genetic disruption and chemical inhibition of COX-2 have been shown to ameliorate neuronal death after transient forebrain ischemia, suggesting that the PGE 2 accumulated through COX-2 induction mediates the toxic effects in the brain (7,10,15). Conversely, the genetic disruption of EP2, one of the PGE 2 receptors expressed in the brain, has been shown to exacerbate neuronal death after transient forebrain ischemia, suggesting that PGE 2 has a neuroprotective effect on postischemic injury (16). In in vitro studies, the effect of PGE 2 has also been controversial, with results showing both toxic and protective effects on neuronal ...

Preconditioning (PC) using a preceding sublethal ischemic insult is an attractive strategy for protecting neurons by inducing ischemic tolerance in the brain. Although the underlying molecular mechanisms have been extensively studied, almost all studies have focused on neurons. Here, using a middle cerebral artery occlusion model in mice, we show that astrocytes play an essential role in the induction of brain ischemic tolerance. PC caused activation of glial cells without producing any noticeable brain damage. The spatiotemporal pattern of astrocytic, but not microglial, activation correlated well with that of ischemic tolerance. Interestingly, such activation in astrocytes lasted at least 8 weeks. Importantly, inhibiting astrocytes with fluorocitrate abolished the induction of ischemic tolerance. To investigate the underlying mechanisms, we focused on the P2X7 receptor as a key molecule in astrocyte-mediated ischemic tolerance. P2X7 receptors were dramatically upregulated in activated astrocytes. PC-induced ischemic tolerance was abolished in P2X7 receptor knock-out mice. Moreover, our results suggest that hypoxia-inducible factor-1␣, a well known mediator of ischemic tolerance, is involved in P2X7 receptor-mediated ischemic tolerance. Unlike previous reports focusing on neuron-based mechanisms, our results show that astrocytes play indispensable roles in inducing ischemic tolerance, and that upregulation of P2X7 receptors in astrocytes is essential.

Microsomal prostaglandin E 2 synthase (mPGES)-1 is an inducible protein recently shown to be an important enzyme in inflammatory prostaglandin E 2 (PGE 2 ) production in some peripheral inflammatory lesions. However, in inflammatory sites in the brain, the induction of mPGES-1 is poorly understood. In this study, we demonstrated the expression of mPGES-1 in the brain parenchyma in a lipopolysaccharide (LPS)-induced inflammation model. A local injection of LPS into the rat substantia nigra led to the induction of mPGES-1 in activated microglia. In neuron-glial mixed cultures, mPGES-1 was co-induced with cyclooxygenase-2 (COX-2) specifically in microglia, but not in astrocytes, oligodendrocytes or neurons. In microglia-enriched cultures, the induction of mPGES-1, the activity of PGES and the production of PGE 2 were preceded by the induction of mPGES-1 mRNA and almost completely inhibited by the synthetic glucocorticoid dexamethasone. The induction of mPGES-1 and production of PGE 2 were also either attenuated or absent in microglia treated with mPGES-1 antisense oligonucleotide or microglia from mPGES-1 knockout (KO) mice, respectively, suggesting the necessity of mPGES-1 for microglial PGE 2 production. These results suggest that the activation of microglia contributes to PGE 2 production through the concerted de novo synthesis of mPGES-1 and COX-2 at sites of inflammation of the brain parenchyma.

Abstract-Hepatocyte growth factor (HGF) promotes the survival and migration of immature neurons, but its role in the mature brain has remained elusive. In the hippocampus of juvenile rats, we found that the HGF receptor c-Met was expressed in neurons. Furthermore, it was highly Tyrphosphorylated, more so than in the liver under normal conditions, suggesting that the receptor is activated and that HGF may act continuously in the intact brain. Exogenously applied HGF enhanced synaptic long-term potentiation (LTP) in the CA1 region of hippocampus, but did not affect long-term depression. We further found that HGF augmented N-methyl-D-aspartate receptor-mediated currents in both slices and dissociated neurons. This augmentation is likely to underlie the enhancement of LTP. Considering that the expression of both HGF and c-Met are known to be induced by ischemic stimuli, this modulation would provide a novel understanding of a neuronal regulatory systems shared with pathogenic ischemic states.

Summary Signaling through GABAA receptors controls neural progenitor cell (NPC) development in vitro and is altered in schizophrenic and autistic individuals. However, the in vivo function of GABAA signaling on neural stem cell proliferation, and ultimately neurogenesis, remains unknown. To examine GABAA function in vivo, we electroporated plasmids encoding short-hairpin RNA against the Na-K-2Cl co-transporter NKCC1 in NPCs of the neonatal subventricular zone (SVZ) in mice to reduce GABAA-induced depolarization. Reduced GABAA depolarization identified by a loss of GABAA-induced calcium responses in most electroporated NPCs led to a 70% decrease in the number of proliferative Ki67+ NPCs and a 60% reduction in newborn neuron density. Premature loss of GABAA depolarization in newborn neurons resulted in truncated dendritic arborization at the time of synaptic integration. However, by 6 weeks the dendritic tree had partially recovered and displayed a small, albeit significant, decrease in dendritic complexity but not total dendritic length. To further examine GABAA function on NPCs, we treated animals with a GABAA allosteric agonist, pentobarbital. Enhancement of GABAA activity in NPCs increased the number of proliferative NPCs by 60%. Combining shNKCC1 and pentobarbital prevented the shNKCC1 and the pentobarbital effects on NPC proliferation, suggesting that these manipulations affected NPCs through GABAA receptors. Thus, dysregulation in GABAA depolarizing activity delayed dendritic development and reduced NPC proliferation resulting in decreased neuronal density.