Modulation of Rho GTPase activity alleviates chondroitin sulfate proteoglycan-dependent inhibition of neurite extension

The central nervous system (CNS) fails to regenerate after injury. A glial scar forms at the injury site, contributing to regenerative failure partly resulting from the chondroitin sulfate proteoglycans (CSPGs) in the glial scar. The family of Rho GTPases, which includes Cdc42, Rac1, and RhoA, is involved in growth cone dynamics. Although the response of neural cells to the inactivation of Rho when contacting myelin-related substrates, or CSPG, has been investigated, Rac1's and Cdc42's abilities to modulate CSPG-dependent inhibition have yet to be explored. In this study, a stripe assay was utilized to examine the effects of modulating all three Rho GTPases on neurite extension across inhibitory CSPG lanes. Alternating laminin (LN) and CSPG lanes were created and NG108-15 cells and E9 chick dorsal root ganglia (DRGs) were cultured on the lanes. By using the protein delivery agent Chariot, the neuronal response to exposure of constitutively active (CA) and dominant negative (DN) mutants of the Rho GTPases, along with the bacterial toxin C3, was determined by quantifying the percentage ratio of neurites crossing the CSPG lanes. CA-Cdc42, CA-Rac1, and C3 transferase significantly increased the number of neurites crossing into the CSPG lanes compared with the negative controls for both the NG108-15 cells and the E9 chick DRGs. We also show that these mutant proteins require the delivery vehicle, Chariot, to enter the neurons and affect neurite extension. Therefore, activation of Cdc42 and Rac, as well as inhibition of Rho, helps overcome the CSPG-dependent inhibition of neurite extension.

Signaling through JAK2-STAT5 pathway is essential for IL-3-induced activation of microglia

Microglia, the resident macrophage of the brain, mediates immune and inflammatory responses in the central nervous system (CNS). Activation of microglia and secretion of inflammatory cytokines associate with the pathogenesis of CNS diseases, including multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease, prion disease, and AIDS dementia. Microbial pathogens, cytokines, chemokines, and costimulatory molecules are potent inducers of microglial activation in the CNS. Signaling through its receptor, IL-3 induces the activation of JAK-STAT and MAP kinase pathways in microglial cells. In this study, we found that in vitro treatment of EOC-20 microglial cells with tyrphostin AG490 blocked IL-3-induced tyrosine phosphorylation of JAK2, STAT5A, and STAT5B signaling proteins. Stable transfection of EOC-20 cells with a dominant negative JAK2 mutant also blocked IL-3-induced tyrosine phosphorylation of JAK2, STAT5A, and STAT5B in microglia. The blockade of JAK2-STAT5 pathway resulted in a decrease in IL-3-induced proliferation and expression of CD40 and major histocompatibility complex class II molecules in microglia. These findings highlight the fact that JAK2-STAT5 signaling pathway plays a critical role in mediating IL-3-induced activation of microglia.

Ethanol-induced iNOS and COX-2 expression in cultured astrocytes via NF-κB

The CNS is particularly susceptible to the effects of alcohol and toxicity. Astrocytes are immunoactive cells, and the activation of these cells is associated with several neurodegenerative disorders. By using cultured cortical astrocytes, we show that a short ethanol treatment (100 mM) is able to up-regulate both cyclooxygenase 2 (COX-2) and inducible nitric oxide synthase (iNOS) expression, and that these effects are regulated via nuclear factor kappa B (NF-kappa B) as revealed by the inhibition of NF-kappa B activation with pyrrolidine dithiocarbamate (PDTC) or BAY 11-7082. These results suggest that ethanol is able to induce inflammatory mediators in astrocytes through the NF-kappa B activation.

Zn and Cu alteration in connection with astrocyte metallothionein I/II overexpression in the mouse brain upon physical stress

The distribution of metallothioneins I/II in the mouse brain and their specific area distribution upon physical stress were studied. To induce physical stress, groups of mice were subjected to total darkness for different periods (2 weeks, 1 month, and 2 months). The concentration of metallothioneins, evaluated by immunohistochemistry, as well as area-specific protein expression, were found in the following quantitative order: corpus striatum, cerebellum, mesencephalon, hippocampus with fornix, parts of thalamus, and pons. All other brain areas were marginally affected, or even unaffected, in terms of immunopositive metallothionein reaction. Metallothionein I/II expression was compared with the immunopositivity of glial fibrillary acidic protein (GFAP). It is noteworthy that metallothioneins and GFAP are expressed in different types of astrocytes.

Activated glia induce neuron death via MAP kinase signaling pathways involving JNK and p38

Chronic glial activation in neurodegenerative diseases contributes to neuronal dysfunction and neuron loss through production of neuroinflammatory molecules. However, the molecular mechanisms, particularly the signal transduction pathways involved in glia-dependent neuron death, are poorly understood. As a first step to address this question, we used a neuron-glia co-culture system that allows diffusion of soluble molecules between glia and neurons to test the potential importance of mitogen-activated protein kinase (MAPK) signaling pathways in the glia-induced neuron death. Activation of glia in co-culture by lipopolysaccharide (LPS) induced apoptotic-like neuron death. The MAPKs tested (p38, JNK, ERK1/2) were activated in both glia and neurons following LPS treatment, suggesting their involvement in both glial activation and neuronal response to diffusible, glia-derived neurotoxic molecules. Inhibitors of p38 and JNK partially blocked neuron death in the LPS-treated co-culture, whereas an ERK1/2 pathway inhibitor did not protect neurons. These results show that p38 and JNK MAPKs, but not ERK1/2 MAPK, are important signal transduction pathways contributing to glia-induced neuron death.

Acetylcholinesterase induces the expression of the β-amyloid precursor protein in glia and activates glial cells in culture

Acetylcholinesterase (AChE) activities in CNS physiopathology are increasingly diverse and range from neuritogenesis, through synaptogenesis, to enhancement of amyloid fiber assembly. In Alzheimer's disease, senile plaques and neurodegeneration specially affect regions enriched for cholinergic synapses. In this study we show an effect of AChE that could contribute to the increased deposition of Abeta in certain regions. Affinity-purified AChE induced the expression of amyloid-beta-precursor protein (beta-APP) in glial cells in a concentration-dependent manner up to 5 nM. In glia, AChE also increased inducible nitric oxide synthase (iNOS) assessed by immunocytochemistry and decreased reductive metabolism as evidence of cell activation. AChE could increase the expression of beta-APP in astrocytes and microglia as result of the activation of glial cells. As a whole, we found that AChE has additional effects that could result in an increased synthesis of Abeta, both by increasing beta-APP expression of astrocytes and by further activating glial cells.

Induction of astrocytic cytoplasmic phospholipase A2 and neuronal death after intracerebroventricular carrageenan injection, and neuroprotective effects of quinacrine

Glial reaction is often associated with nervous tissue injury, but thus far, few studies have examined whether it can be a cause of neuronal injury. We now study the effect of intracerebroventricular injection of a carrageenan on cytoplasmic phospholipase A(2) (cPLA(2)) expression and neuronal injury in the hippocampus. The enzyme cPLA(2) hydrolyzes neural membrane glycerophospholipids and generates precursors for proinflammatory mediators. An induction of cPLA(2) in astrocytes and death of neurons in the hippocampus were observed following glial reaction induced by intracerebroventricular injections of carrageenan. cPLA(2) levels and neuronal death were modulated by daily intraperitoneal injections of quinacrine, an inhibitor of phospholipase A(2) that can cross the blood brain barrier. These observations support a role for astrocytic cPLA(2) in mediating neuronal death.

Regulation of tissue inhibitor of metalloproteinase-1 by astrocytes: links to HIV-1 dementia

The neuropathogenesis of HIV-1-associated dementia (HAD) revolves around the secretion of toxic molecules from infected and immune-competent mononuclear phagocytes. Astrocyte activation occurs in parallel but limited insights are available for its role in neurotoxicity and cognitive dysfunction. One means in which astrocytes may affect disease is through their production of tissue inhibitors of metalloproteinases (TIMPs). TIMPs are regulators of matrix metalloproteinases, enzymes that affect blood-brain barrier integrity through altering the extracellular matrix. We hypothesized that in response to injury and inflammation in HAD, astrocytes regulate the production of TIMP-1, the inducible type of TIMP that is important in inflammation. To address astrocyte-mediated TIMP-1 regulation in HAD, we evaluated the responses of primary human to IL-1beta and HIV-1. TIMP-1 levels in plasma, CSF, and brain tissue of control, HIV-1 infected patients without cognitive impairment, and HAD patients were also studied. Our data show that an upregulation of TIMP-1 results from astrocytes acutely activated with IL-1beta. In contrast, CSF and brain tissue samples from HAD patients showed reduced TIMP-1 levels compared to seronegative controls. MMP-2 levels in brains showed the opposite. Consistent with this, prolonged activation of astrocytes led to a reduction in TIMP-1 and MMP-2, but a sustained elevation in MMP-1. Our data suggest that in diseased brain tissue, the ability of astrocytes to counteract the destructive effects of MMP through expression of TIMP-1 is diminished by chronic activation. Our studies reveal new opportunities for repair-based therapeutic strategies in HAD.

Microglial activation and tyrosine hydroxylase immunoreactivity in the substantia nigral region following transient focal ischemia in rats

The temporal profiles of the changes of dopaminergic cells and microglial activation induced by transient cerebral ischemia were investigated in the substantia nigra pars compacta (SNc) located outside ischemic areas of rat brain. Transient cerebral ischemia was induced by intraluminal occlusion of the right middle cerebral artery for 2 h and reperfusion was continued for 1, 2, 3, 4, 7, 10, 14, 28, 60, and 120 days. Dopaminergic cells immunostained with tyrosine hydroxylase (TH)-antibody in the ipsilateral SNc were significantly decreased at 7 days post-ischemia compared with those in the contralateral side (P<0.05). However, at 60 and 120 days, there were no significant differences between ipsilateral and contralateral side of the SNc. Unlike the TH immunoreactivity, activated microglial cells immunostained with OX-42 antibody were significantly increased at 2 and 3 days and then decreased gradually until 10 days post-ischemia. Activated microglial cells were increased at 2 weeks post-ischemia, and this pattern remained until 60 days. These results suggest that the transient changes of TH-immunoreactive cells in the SNc caused by transient focal ischemia are correlated with a biphasic microglial cell activation response.

The selective cyclooxygenase-2 inhibitor rofecoxib suppresses brain inflammation and protects cholinergic neurons from excitotoxic degeneration in vivo

Brain inflammatory processes underlie the pathogenesis of Alzheimer's disease, and non-steroidal anti-inflammatory drugs have a protective effect in the disease. The aim of this work was to study in vivo whether attenuation of brain inflammatory response to excitotoxic insult by the selective cyclooxygenase-2 inhibitor, rofecoxib, may prevent neurodegeneration, as a contribution to a better understanding of the role inflammation plays in the pathology of Alzheimer's disease. We investigated, by immunohistochemical methods, glia reaction, the activation of p38 mitogen-activated protein kinase (p38MAPK) pathway with an antibody selective for the phosphorylated form of the enzyme and the number of choline acetyltransferase-positive neurons and, by in vivo microdialysis, cortical extracellular levels of acetylcholine following the injection of quisqualic acid into the right nucleus basalis of adult rats. Seven days after injection, a marked reduction in the number of choline acetyltransferase-positive neurons was found, along with an intense glia reaction, selective activation of p38MAPK at the injection site and a significant decrease in the extracellular levels of acetylcholine in the cortex ipsilateral to the injection site. The loss of cholinergic neurons persisted for at least up to 28 days. Rofecoxib (3 mg/kg/day, starting 1 h prior to injection of quisqualic acid) treatment for 7 days significantly attenuated glia activation and prevented the loss of choline acetyltransferase-positive cells and a decrease in cortical acetylcholine release. The prevention of cholinergic cell loss by rofecoxib occurred concomitantly with the inhibition of p38MAPK phosphorylation. Our findings suggest an important role of brain inflammatory reaction in cholinergic degeneration and demonstrate a neuroprotective effect of rofecoxib, presumably mediated through the inhibition of p38MAPK phosphorylation.

Hypoxic induction of caspase-11/caspase-1/interleukin-1beta in brain microglia

Caspase-11 is an inducible protease that plays an important role in both inflammation and apoptosis. Inflammatory stimuli induce and activate caspase-11, which is required for the activation of caspase-1 or interleukin-1beta (IL-1beta) converting enzyme (ICE). Caspase-1 in turn mediates the maturation of proinflammatory cytokines such as IL-1beta, which is one of the crucial mediators of neurodegeneration in the central nervous system. Here, we report that hypoxic exposure of cultured brain microglia (BV-2 mouse microglia cells and rat primary microglial cultures) induces expression and activation of caspase-11, which is accompanied by activation of caspase-1 and secretion of mature IL-1beta and IL-18. Hypoxic induction of caspase-11 was observed in both mRNA and protein levels, and was mediated through p38 mitogen-activated protein kinase pathway. Transient global ischemia in rats also induced caspase-11 expression and IL-1beta production in hippocampus supporting our in vitro findings. Caspase-11-expressing cells in hippocampus were morphologically identified as microglia. Taken together, our results indicate that hypoxia induces a sequential event-caspase-11 induction, caspase-1 activation, and IL-1beta release-in brain microglia, and point out the importance of initial caspase-11 induction in hypoxia-induced inflammatory activation of microglia.

Selective modulation of lipopolysaccharide-stimulated cytokine expression and mitogen-activated protein kinase pathways by dibutyryl-cAMP in BV2 microglial cells

Cyclic AMP is a very important regulator in a wide range of biological processes, including inflammatory reactions. To investigate the role of cAMP in microglia, we examined the effect of dibutyryl-cAMP (dbcAMP) on lipopolysaccharide (LPS)-stimulated cytokine expression and signaling pathways in murine BV2 microglial cells. DbcAMP strongly suppressed LPS-induced TNF-alpha expression, without affecting NO, IL-6 or TGF-beta1 expression. In contrast, LPS-induced IL-1beta or IL-10 expressions were dramatically increased by dbcAMP. We further examined the effect of elevated cAMP on signaling molecules such as MAP kinases (p38 MAPK, ERK and JNK), NF-kappaB and AP1, which are involved in the regulation of inflammatory responses. DbcAMP decreased the LPS-induced phosphorylation of p38 MAPK, while it modestly enhanced the ERK activity. JNK phosphorylation was slightly reduced by dbcAMP only at the later time point. Electrophoretic mobility shift assay revealed that the elevated cAMP potentiated AP-1 binding activity by enhancing c-fos binding. On the other hand, dbcAMP repressed NF-kappaB-mediated transcription without affecting NF-kappaB binding. Treatment with H89, a selective inhibitor of protein kinase A, completely reversed cAMP-induced IL-10 and IL-1beta upregulation but only partially reversed the cAMP-induced repression of TNF-alpha. Thus, the effect of dbcAMP in BV2 cells appears to be mediated through both protein kinase A-dependent and -independent pathways. Taken together, our results demonstrate that cAMP modulates microglia activation in a diverse and complex manner.

Up-regulation of inducible nitric oxide synthase in the substantia nigra by lipopolysaccharide causes microglial activation and neurodegeneration

The present study was designed to examine whether expression of iNOS was involved in LPS-induced neurodegeneration in rat substantia nigra (SN) and to study the role of NO in the loss of the SN dopaminergic neurons. In Western blot analysis, iNOS was induced in the SN after injection of LPS in a time- and dose-dependent manner. Immunofluorescence and immunohistochemical analyses revealed that the iNOS is located in a fully activated microglia with the characteristic amoeboid morphology. Furthermore, LPS-induced loss of dopaminergic neurons was significantly inhibited by the administration of L-N(G)-nitroarginine, a selective inhibitor of NOS, and the glucocorticoid dexamethasone. These inhibiting agents for iNOS reduced LPS-induced microglial activation, suggesting that NO has a role in inflammatory-mediated microglial activation. These results demonstrate that LPS induces the expression of iNOS in activated microglia in the SN, and that NO and/or its metabolites may play a crucial role in inflammation-mediated degeneration of dopaminergic neurons.

Astrocytic expression of the Alzheimer's disease beta-secretase (BACE1) is stimulus-dependent

The beta-site APP-cleaving enzyme (BACE1) is a prerequisite for the generation of beta-amyloid peptides, which give rise to cerebrovascular and parenchymal beta-amyloid deposits in the brain of Alzheimer's disease patients. BACE1 is neuronally expressed in the brains of humans and experimental animals such as mice and rats. In addition, we have recently shown that BACE1 protein is expressed by reactive astrocytes in close proximity to beta-amyloid plaques in the brains of aged transgenic Tg2576 mice that overexpress human amyloid precursor protein carrying the double mutation K670N-M671L. To address the question whether astrocytic BACE1 expression is an event specifically triggered by beta-amyloid plaques or whether glial cell activation by other mechanisms also induces BACE1 expression, we used six different experimental strategies to activate brain glial cells acutely or chronically. Brain sections were processed for the expression of BACE1 and glial markers by double immunofluorescence labeling and evaluated by confocal laser scanning microscopy. There was no detectable expression of BACE1 protein by activated microglial cells of the ameboid or ramified phenotype in any of the lesion paradigms studied. In contrast, BACE1 expression by reactive astrocytes was evident in chronic but not in acute models of gliosis. Additionally, we observed BACE1-immunoreactive astrocytes in proximity to beta-amyloid plaques in the brains of aged Tg2576 mice and Alzheimer's disease patients.

Monoamine oxidase: radiotracer development and human studies

Monoamine oxidase (MAO) is an integral protein of outer mitochondrial membranes and occurs in neuronal and nonneuronal cells in the brain and in peripheral organs. It oxidizes amines from both endogenous and exogenous sources, thereby influencing the concentration of neurotransmitter amines as well as many xenobiotics. It occurs in two subtypes, MAO A and MAO B, which are different gene products and have different substrate and inhibitor specificities. Both MAO A and B can be imaged and quantified in the living human brain using positron emission tomography (PET) and radiotracers labeled with carbon-11. PET studies have been carried out to measure the effects of age, MAO inhibitor drugs, tobacco smoke exposure, and other factors on MAO activity in the human brain.

Matrix metalloproteases and their inhibitors are produced by overlapping populations of activated astrocytes

Matrix metalloproteases (MMPs) and tissue inhibitors of metalloproteases (TIMPs) are involved in many cell migration phenomena and produced by many cell types, including neurons and glia. To assess their possible roles in brain injury and regeneration, we investigate their production by glial cells, after brain injury and in tissue culture, and we investigate whether they are capable of digesting known axon-inhibitory proteoglycans. To determine the action of MMPs, we incubated astrocyte conditioned medium with activated MMPs, then did western blots for several chondroitin sulphate proteoglycans. MMP-3 digested all five proteoglycans tested, whereas MMP-2 digested only two and MMP-9 none. To determine whether MMPs or TIMPs are produced by astrocytes in vitro, we tested both primary cultures and astrocyte cell lines by western blotting, and compared them with Schwann cells. All cultures produced at least some MMPs and TIMPs, with no obvious correlation with the ability of axons to grow on those cells. Both MMP-9 and TIMP-3 were regulated by various cytokines. To determine which cells produce MMPs and TIMPs after brain injury, we made lesions of adult rat cortex, and did immunohistochemistry. MMP-2 was seen to be induced in activated astrocytes through the whole thickness of the cortex but not deeper, but MMP-3 was not seen in the injured brain. TIMP-2 and TIMP-3 immunoreactivities were induced in activated astrocytes in deep cortex and the underlying white matter. In situ hybridisation confirmed induction of TIMP-2 in glia as well as neurons, but showed no expression of TIMP-4. These results show that both MMPs and TIMPs are produced by some astrocytes, but TIMP production is particularly strong, especially in deep cortex and white matter which is more inhibitory for axon regeneration. Conversely the MMPs produced may not be adequate to promote migration of cells and axons within the glial scar.

Increased expression of phospholipase D1 in the spinal cords of rats with experimental autoimmune encephalomyelitis

Phospholipase D1 (PLD1) expression was studied in the central nervous system (CNS) under the condition of induced experimental autoimmune encephalomyelitis (EAE) in Lewis rats. After inducing EAE, the expression of PLD1 was analyzed by Western blot and immunohistochemistry. Western blot analysis showed that expression of the isozymes PLD1 significantly increased in the spinal cord at the peak stage of EAE, and declined thereafter. Immunohistochemistry showed that PLD1-positive cells increased in number in EAE lesions, which consisted mainly of ED1-positive macrophages and glial fibrillary acidic protein-positive astrocytes. In contrast, PLD1 was only weakly expressed in some spinal cord astrocytes in control rats. These results suggest that PLD1 is increased in autoimmune CNS inflammation, and possibly involved in the activation of macrophages and astrocytes in EAE lesions.

Expression of endothelial and inducible NOS-isoforms is increased in Alzheimer's disease, in APP23 transgenic mice and after experimental brain lesion in rat: evidence for an induction by amyloid pathology

The nitric oxide-synthesizing enzyme nitric oxide synthase (NOS) is present in the mammalian brain in three different isoforms, two constitutive enzymes (i.e., neuronal, nNOS, and endothelial eNOS) and one inducible enzyme (iNOS). All three isoforms are aberrantly expressed in Alzheimer's disease giving rise to elevated levels of nitric oxide apparently involved in the pathogenesis of this disease by various different mechanisms including oxidative stress and activation of intracellular signalling mechanisms. It still is a matter of debate, however, whether the abnormal expression of NOS isoforms has some primary importance in the pathogenetic chain and might thus be a potential therapeutic target or only reflects a secondary effect that occurs at more advanced stages of the disease process. To tackle this question, we analysed the expression of both eNOS and iNOS in patients with sporadic AD, in transgenic mice expressing human amyloid precursor protein (APP) with the Swedish double mutation under control of the Thy1 promotor (APP23 mice), and after electrolytic cortical lesion in rat, an experimental paradigm associated with elevated expression of APP. In all three conditions, an astrocytosis was induced accompanied by a strong increase of both iNOS and eNOS. Both NOS isoforms were frequently though not always colocalized. Thus, based on the expression pattern of NOS isoforms three types of astrocytes, expressing only one of the two isoforms or both together could be distinguished. In both AD and transgenic mice eNOS-expressing astrocytes exceeded iNOS-expressing astrocytes in number. Astrocytes with elevated levels of iNOS or eNOS were constantly seen in direct association with Abeta-deposits in AD and transgenic mice and were found in the vicinity of the lesion site in the rat cortex. The results of the present study show that expression of both iNOS and eNOS is increased in activated astrocytes under experimental conditions associated with elevated expression of APP (electrolytic brain lesion) or Abeta-deposition (APP23 transgenic mice). Therefore, it is suggested that altered expression of these NOS isoforms being part of AD pathology is secondary to the amyloid pathology and might not be primarily involved in the pathogenetic chain though it might contribute to the maintenance, self-perpetuation and progression of the neurodegenerative process.

Dying neural cells activate glia through the release of a protease product

The close relationship between neurodegeneration and gliosis could play a relevant role in propagating the degenerative event in the brain. Although there is evidence of the neurotoxicity of activated glia, the ability of damaged neurons to modulate glial response remains unexplored. Exposure of primary glial cells to damaged or dead hippocampal neurons was followed by glial release of tumor necrosis factor-alpha (TNF-alpha). This release was reduced by a partial prevention of neural death. By contrast, no TNF-alpha was released when glial cells were exposed to damaged murine fibroblasts. Exposure of glial cells to the cerebrospinal fluid (CSF) of patients with Alzheimer's disease was also followed by TNF-alpha release, while the CSF of subjects with nondegenerative brain disorders evoked no response. These data suggest that damaged neurons both in vitro and in vivo release factor(s) that activate glial response. Heat treatment of sonicated neurons or use of a mixture of protease inhibitors, among them the caspase inhibitors Z-DEVD-FMK and Z-YVAD-FMK, prevented TNF-alpha release from glial cells. We conclude that a primary neurodegenerative event may induce glial response by releasing a neurospecific protein factor via activation of a caspase.

A novel gliotic P2 receptor mediating cyclooxygenase-2 induction in rat and human astrocytes

In astrocytic cultures maintained in vitro, a brief challenge with the ATP analog alpha,beta methyleneATP (alpha,betameATP) results, 3 days later, in marked elongation of astrocytic processes, an event that resembles the astrocytic hypertrophy known to occur in vivo during reactive astrogliosis. alpha,beta meATP-induced effects were observed in primary astrocytes obtained from both rat striatum and cortex (a brain area highly involved in chronic neurodegenerative pathologies), as well as in human astrocytoma cells (ADF cells). Purine-induced gliosis could be reversed by the non-selective P2X/P2Y receptor antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS), but not by oxidized ATP (an antagonist of the P2X(7) receptor), in line with previous studies of our laboratory suggesting the involvement of a P2Y receptor subtype. Induction of reactive gliosis was preceded by increased expression of cyclooxygenase-2 (COX-2), an enzyme whose excessive activation has been implicated in both acute and chronic neurodegenerative diseases. The selective COX-2 inhibitor NS-398 prevented both purine-induced astrogliosis and the associated COX-2 induction, suggesting that inhibition of the transcription of the COX-2 gene may also contribute to the anti-inflammatory properties of this agent. Significant blockade of both alpha,beta meATP-mediated reactive gliosis and COX-2 induction was also observed with PPADS. These data suggest that COX-2 mediates P2Y receptor-induced reactive astrogliosis, and that antagonists selective for this receptor subtype may represent a novel class of anti-inflammatory agents of potential interest in acute and chronic neurological disorders characterized by an inflammatory component and reactive gliosis.

Cyclooxygenase-2 is highly expressed in microglial-like cells in a murine model of prion disease

Prion diseases, or transmissible spongiform encephalopathies, are a relatively rare group of chronic degenerative disorders afflicting both animals and humans, characterized by typical histopathological signs such as amyloid deposition, neuronal loss and spongiform changes. Despite the absence of a typical acute inflammatory response, the consistent microglial activation and astrocytosis, that are found in human pathologies as well as in animal models, suggests the existence of an ongoing inflammatory response in these neurodegenerative diseases. To investigate the role of cyclooxygenase-2 (COX-2) activity in the pathogenesis of chronic neurodegenerative diseases, we studied immunohistochemically the expression of this key enzyme in the formation of prostaglandins during inflammatory responses in a well characterized murine model of prion disease. We found that COX-2 is selectively up-regulated in glial cells presenting the typical morphology of activated microglia and that the number of COX-2-positive cells increases with the progression of the disease. The extensive microglial expression of COX-2, that is likely to be followed by a sustained enzymatic activity leading to the generation of prostaglandins as well as of oxygen free radicals, might have important effects on chronic neurodegeneration. Further functional experiments are required to elucidate the role of COX-2 activity in the pathogenesis of chronic neurodegenerative diseases.

Expression of neuron specific phosphatase, striatal enriched phosphatase (STEP) in reactive astrocytes after transient forebrain ischemia

We studied the distribution and change of striatal enriched phosphatase (STEP) in the gerbil hippocampus after transient forebrain ischemia. STEP was expressed in the perikarya and in neuronal processes; it was not detected in non-neuronal cells of control animals. After 5-min forebrain ischemia, STEP immunoreactivity (STEP-IR) was preserved for 2 days; it disappeared 4 and more days after ischemia with completion of delayed neuronal death (DND) in the CA1 subfield. Furthermore, only in the CA1 after ischemia, STEP was expressed in reactive astrocytes for 4 to 28 days, showing different patterns of glial fibrillary acidic protein (GFAP)-positive reactive astrocytes. After non-or less-than lethal ischemia, STEP expression in reactive astrocytes corresponded with the degree of neuronal degeneration. Immunoblot analysis of the CA1 subfield revealed the expression of three isoforms, STEP45, -56 and -61; their expression patterns changed with time after ischemia. These data suggest that neuronal STEP is preserved until cell degeneration after ischemia and that STEP is expressed in reactive astrocytes only after lethal ischemia, with different expression patterns for its isoforms. Of STEP45, -56 and -61, STEP61 was the most strongly expressed in the reactive astrocytes; both STEP45 and -61 were expressed in neurons and the expression of STEP56 was weak. STEP may play an important role not only in neurons but also in reactive astrocytes after ischemia, depending on neuronal degeneration.

ERK/MAP kinase is chronically activated in human reactive astrocytes

Reactive astrogliosis is the most prominent macroglial response to diverse forms of CNS injury. We assessed a potential role for the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway because it represents a common effector for several major families of transmembrane receptors implicated in astrogliosis. Immunohistochemical detection of activated ERK/MAPK in a series of human neurosurgical specimens utilizing phosphorylation state-dependent antibodies consistently revealed intense immunoreactivity in reactive astrocytes in both subacute and chronic lesions, including infarct, mechanical trauma, chronic epilepsy, and progressive multifocal leukoencephalopathy. Neurons, oligodendroglia, and most inflammatory cells showed little or no detectable activation. These observations suggest a testable hypothesis: activation of the ERK/MAPK pathway is an obligatory step for the triggering and/or persistence of reactive astrogliosis.

Cytosolic phospholipase A2 is induced in reactive glia following different forms of neurodegeneration

Many recent studies have emphasized the deleterious role of inflammation in CNS injury. Increases in free fatty acids, eicosanoids, and products of lipid peroxidation are known to occur in various conditions of acute and chronic CNS injury, including cerebral ischemia, traumatic brain injury, and Alzheimer's disease. Although an inflammatory response can be induced by many different means, phospholipases, such as cytosolic phospholipase A(2) (cPLA(2)), may play an important role in the production of inflammatory mediators and in the production of other potential second messengers. cPLA(2) hydrolyzes membrane phospholipids and its activity liberates free fatty acids leading directly to the production of eicosanoids. We investigated the cellular localization of cytosolic phospholipase A(2) in the CNS following: (1) focal and global cerebral ischemia, (2) facial nerve axotomy, (3) human cases of Alzheimer's disease, (4) transgenic mice overexpressing mutant superoxide dismutase, a mouse model of amyotrophic lateral sclerosis, and (5) transgenic mice overexpressing mutant amyloid precursor protein, which exhibits age-related amyloid deposition characteristic of Alzheimer's disease. We show that in every condition evaluated, cytosolic phospholipase A(2) is present in reactive glial cells within the precise region of neuron loss. In conditions where neurons did not degenerate or are protected from death, cytosolic phospholipase A(2) is not observed. Both astrocytes and microglial cells are immunoreactive for cytosolic phospholipase A(2) following injury, with astrocytes being the most consistent cell type expressing cytosolic phospholipase A(2). The presence of cytosolic phospholipase A(2) does not merely overlap with reactive astroglia, as reactive astrocytes were observed that did not exhibit cytosolic phospholipase A(2) immunoreactivity. In most conditions evaluated, inflammatory processes have been postulated to play a pivotal role and may even participate in neuronal cell death. These results suggest that cytosolic phospholipase A(2) may prove an attractive therapeutic target for neurodegeneration.

Cyclo-oxygenase-2 mediates P2Y receptor-induced reactive astrogliosis

Excessive cyclo-oxygenase-2 (COX-2) induction may play a role in chronic neurological diseases characterized by inflammation and astrogliosis. We have previously identified an astroglial receptor for extracellular nucleotides, a P2Y receptor, whose stimulation leads to arachidonic acid (AA) release, followed, 3 days later, by morphological changes resembling reactive astrogliosis. Since COX-2 may be upregulated by AA metabolites, we assessed a possible role for COX-2 in P2Y receptor-mediated astrogliosis. A brief challenge of rat astrocytes with the ATP analogue alpha,beta-methylene ATP (alpha,beta(me)ATP) resulted, 24 h later, in significantly increased COX-2 expression. The selective COX-2 inhibitor NS-398 completely abolished alpha,beta(me)ATP-induced astrocytic activation. Constitutive astroglial COX-1 or COX-2 did not play any role in purine-induced reactive astrogliosis. PGE2, a main metabolite of COX-2, also induced astrocytic activation. These data suggest that a P2Y receptor mediates reactive astrogliosis via induction of COX-2. Antagonists selective for this receptor may counteract excessive COX-2 activation in both acute and chronic neurological diseases.