Molecular profile of reactive astrocytes--implications for their role in neurologic disease

Astrocytes express type VIII collagen during the repair process of brain cold injury

We recognized for the first time upregulation of type VIII collagen gene expression during the repair process in the mouse brain cold injury model. Immunohistochemical staining showed that type VIII collagen expression was around the necrotic region, where reactive astrocytes are frequently observed. Cultured astrocytes demonstrated a high expression of type VIII collagen genes. TGF-beta1 enhanced the expression of both alpha1(VIII) and alpha2(VIII) genes by astrocytes in culture. Further, we tested selected biological activities of type VIII collagen, compared with those of type I, IV, and V collagens and fibronectin. Astrocytes adhered to type VIII collagen via receptors requiring metal ions. Astrocyte migration on type VIII collagen was more stimulated than that observed on the other ECM molecules. These data indicate that type VIII collagen plays an important role in glial scar formation during the repair process by astrocytes.

Oltipraz inhibits inducible nitric oxide synthase in vitro and inhibits nitric oxide production in activated microglial cells

The clinically relevant drug oltipraz (OPZ) has previously been shown to inhibit cytochrome P450 enzymes [Chem. Res. Toxicol. 13 (2000) 245]. The current study reveals that OPZ is also able to inhibit *NO formation by purified inducible nitric oxide synthase (iNOS) but not by neuronal nitric oxide synthase in hemoglobin assays. The inhibition of iNOS by OPZ is reversible and competitive with an IC(50) of 5.9 microM and Ki of 0.6 microM. In murine BV-2 microglial cells, an immortalized cell line that produces *NO in response to lipopolysaccharide (LPS), OPZ is able to block the formation of nitrite in LPS treated cells. The inhibitory effect of OPZ on LPS treated cells is not due to cell toxicity. Finally, treatment of cells with OPZ does not induce or suppress expression of iNOS protein as shown by Western blot analysis.

Protective effects of alpinae oxyphyllae fructus (Alpinia oxyphyllaMIQ) water-extracts on neurons from ischemic damage and neuronal cell toxicity

Alpinae Oxyphyllae Fructus (Alpinia oxyphylla MIQ) (Korean name Ik-Jj-In) is a medicinal plant used in Korea for the treatment of various symptoms accompanying hypertension and cerebrovascular disorders. The present study was performed to investigate the effects of Alpinae Oxyphyllae Fructus water-extracts (AOF) on a cultured primary neuron cell system, cell cytotoxicity and lipid peroxidation in Abeta treatment conditions. Cell killing was significantly enhanced by addition of increasing concentrations of Abeta. Pretreatment of AOF attenuated in cell killing enhanced by increasing concentrations of Abeta. Abeta-induced cell death was protected by the application of water extract of AOF in a dose-dependent manner, and concentrations of 50 to 100 micro g/ml had a significant effect compared to exposure of Abeta only. AOF has been shown to protect primary cultured neurons from N-methyl-D-aspartate (NMDA) receptor-mediated glutamate toxicity. The evidence indicated that AOF protects neurons against ischemia-induced cell death. Oral administration of AOF into mice prevented ischemia-induced learning disability and rescued hippocampal CA1 neurons from lethal ischemic damage. The neuroprotective action of exogenous AOF was also confirmed by counting synapses in the hippocampal CA1 region. The presence of AOF in neuron cultures did not repress a NMDA receptor-mediated increase in intracellular Ca(2+), but rescued the neurons from NO-induced death. AOF may exert its neuroprotective effect by reducing the NO-mediated formation of free radicals or antagonizing their toxicity.

Glial activation and TNFR-I upregulation precedes motor dysfunction in the spinal cord of mnd mice

Mice homozygous for the spontaneous motor neuron degeneration mutation (mnd) show at the age of 8 months a marked impairment of the motor function and accumulation of lipofuscin granules in the cytoplasm of almost all neurons of the central nervous system. We previously reported a significant increase in GFAP protein levels in the lumbar spinal cord homogenates by western blot analysis and upregulation of TNF, a proinflammatory cytokine, in the motor neurons of lumbar spinal cord of mnd mice, already in a presymptomatic stage (4 months of age). In the present study, using immunohistochemical analysis, we performed a time course in mnd mice (1, 4 and 9 months of age) evaluating the expression and the distribution of astroglial and microglial cells and the expression of both TNF receptors, TNFR-I and TNFR-II. We observed a marked increase in astroglial and microglial cells and in TNFR-I immunoreactivity already at the 4th month. Since motor neuron dysfunction occurs in mnd mice in the absence of evident loss of spinal motor neurons, the present results indicate that the activation of microglial cells and astrocytes is independent from neuronal degeneration. The role of TNF and TNFR-I on motor neurons is still to be demonstrated.

Astrocyte apoptosis: implications for neuroprotection

Astrocytes, the most abundant glial cell types in the brain, provide metabolic and trophic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions can critically influence neuronal survival. Recent studies show that astrocyte apoptosis may contribute to pathogenesis of many acute and chronic neurodegenerative disorders, such as cerebral ischemia, Alzheimer's disease and Parkinson's disease. We found that incubation of cultured rat astrocytes in a Ca(2+)-containing medium after exposure to a Ca(2+)-free medium causes an increase in intracellular Ca(2+) concentration followed by apoptosis, and that NF-kappa B, reactive oxygen species, and enzymes such as calpain, xanthine oxidase, calcineurin and caspase-3 are involved in reperfusion-induced apoptosis. Furthermore, we demonstrated that heat shock protein, mitogen-activated protein/extracellular signal-regulated kinase, phosphatidylinositol-3 kinase and cyclic GMP phosphodiesterase are target molecules for anti-apoptotic drugs. This review summarizes (1) astrocytic functions in neuroprotection, (2) current evidence of astrocyte apoptosis in both in vitro and in vivo studies including its molecular pathways such as Ca(2+) overload, oxidative stress, NF-kappa B activation, mitochondrial dysfunction, endoplasmic reticulum stress, and protease activation, and (3) several drugs preventing astrocyte apoptosis. As a whole, this article provides new insights into the potential role of astrocytes as targets for neuroprotection. In addition, the advance in the knowledge of molecular mechanisms of astrocyte apoptosis may lead to the development of novel therapeutic strategies for neurodegenerative disorders.

TGF-beta1 induction of the adenine nucleotide translocator 1 in astrocytes occurs through Smads and Sp1 transcription factors

Background

The adenine nucleotide translocator 1 (Ant1) is an inner mitochondrial membrane protein involved with energy mobilization during oxidative phosphorylation. We recently showed that rodent Ant1 is upregulated by transforming growth factor-beta (TGF-β) in reactive astrocytes following CNS injury. In the present study, we describe the molecular mechanisms by which TGF-β1 regulates Ant1 gene expression in cultured primary rodent astrocytes.

Results

Transcription reporter analysis verified that TGF-β1 regulates transcription of the mouse Ant1 gene, but not the gene encoding the closely related Ant2 isoform. A 69 basepair TGF-β1 responsive element of the Ant1 promoter was also identified. Electrophoretic mobility shift assays demonstrated that astrocyte nuclear proteins bind to this response element and TGF-β1 treatment recruits additional nuclear protein binding to this element. Antibody supershift and promoter deletion analyses demonstrated that Sp1 consensus binding sites in the RE are important for TGF-β1 regulation of Ant1 in astrocytes. Additionally, we demonstrate that Smad 2, 3 and 4 transcription factors are expressed in injured cerebral cortex and in primary astrocyte cultures. TGF-β1 activated Smad transcription factors also contribute to Ant1 regulation since transcription reporter assays in the presence of dominant negative (DN)-Smads 3 and 4 significantly reduced induction of Ant1 by TGF-β1.

Conclusion

The specific regulation of Ant1 by TGF-β1 in astrocytes involves a cooperative interaction of both Smad and Sp1 binding elements located immediately upstream of the transcriptional start site. The first report of expression of Smads 2, 3 and 4 in astrocytes provided here is consistent with a regulation of Ant1 gene expression by these transcription factors in reactive astrocytes. Given the similarity in TGF-β1 regulation of Ant1 with other genes that are thought to promote neuronal survival, this interaction may represent a general mechanism that underlies the neuroprotective effects of TGF-β1.

Responses of Cultured Astrocytes, C6 Glioma and 1321NI Astrocytoma Cells to Amyloid beta-Peptide Fragments

The effect of amyloid beta-peptide (betaAP), which can have both neurotrophic or neurotoxic effects on neurons and has been implicated in the pathogenesis of Alzheimer's disease (AD), was studied on astrocytes using primary cultures and astrocyte cell lines (rat C6 glioma, human 1321NI astrocytoma cells). The cultures were exposed to 0.0005-50 mug/ml) betaAP fragments 1-40, 25-35, 31-35, or 40-41 (control) for 24 hr. Some of the fragments were maintained at 37 degrees C for 48 hr to induce aggregation and some of the cell cultures were pretreated with the differentiating agent dBcAMP before the experiments. The astrocyte responses were evaluated for lysosome activity (neutral red assay) and levels of structural proteins, glial fibrillary acidic protein, vimentin, and S-100, which are altered in the dystrophic plaques with associated astrogliosis in AD. The cells frequently responded with biphasic responses, with initial (low-dose) activation-type responses (i.e., increases of indicator compared to controls), before reductions with altered morphology (increased branching of cells) at higher concentrations. However, cell death (with EC(50) values) was not observed, even at the maximum concentrations of betaAP fragments. The findings suggest that the astrocytes have a relatively high resistance against the betaAP toxicity.

Nitric oxide from inflammatory-activated glia synergizes with hypoxia to induce neuronal death

Inflammatory-activated glia are seen in numerous central nervous system (CNS) pathologies and can kill nearby neurons through the release of cytotoxic mediators. Glia, when activated, can express the inducible isoform of nitric oxide synthase (iNOS) producing high levels of nitric oxide (NO), which can kill neurons in certain conditions. We show, however, that inflammatory activation of glia in a mature culture of cerebellar granule neurons and glia causes little or no neuronal death under normal (21%) oxygen conditions. Similarly, hypoxia (2% oxygen) or low levels of an NO donor (100 microM DETA/NO) caused little or no neuronal death in nonactivated cultures. If inflammatory activation of glia or addition of NO donor was combined with hypoxia, however, extensive neuronal death occurred. Death in both cases was prevented by the N-methyl-D-aspartate (NMDA) receptor blocker MK-801, implying that death was mediated by the glutamate receptor. Low levels of NO were found to increase the apparent K(M) of cellular oxygen consumption for oxygen, probably due to NO-induced inhibition of mitochondrial respiration, in competition with oxygen, at cytochrome oxidase. Necrotic death, induced by hypoxia plus DETA/NO, was increased further by deoxyglucose, an inhibitor of glycolysis, suggesting that necrosis was mediated by energy depletion. Hypoxia was found to be a potent stimulator of microglia proliferation, but this proliferation was not significant in inflammatory-activated cultures. These results suggest that low levels of NO can induce neuronal death under hypoxic conditions, mediated by glutamate after NO inhibition of respiration in competition with oxygen. Brain inflammation can thus sensitize to hypoxia-induced death, which may be important in pathologies such as stroke, neurodegeneration, and brain aging.

Responses and signaling pathways in human optic nerve head astrocytes exposed to hydrostatic pressure in vitro

In this study, we examined the effects of mechanical stress induced by elevated hydrostatic pressure (HP) on the migration of human optic nerve head (ONH) astrocytes, using an in vitro model that follows repopulation of a cell-free area (CFA) created on a monolayer of cultured astrocytes. alpha-Tubulin staining detected phenotypic changes in astrocytes exposed to HP. The influence of proliferation in closure of the CFA was determined by incorporation of BrdU under 1.5-cm H2O, control pressure (CP), and 10-cm H2O HP with or without 5-fluorouracil. Under control and experimental conditions, closure of the CFA occurred mostly by migration and less by proliferation. Exposure to 10-cm H2O HP induced faster closure of the CFA at 1, 3, and 5 days. The signaling pathways involved in responses to HP were determined using genistein, tyrphostin A25, AG1478, and AG1295, inhibitors of receptor tyrosine kinases; wortmannin and LY294002, inhibitors of phosphatidyl inositol 3-kinase (PI-3K); and SC58236, an inhibitor of inducible cyclooxygenase-2 (COX2). Genistein and tyrphostin A25 blocked HP-induced migration at 1, 3, and 5 days, but did not affect closure of the CFA under CP. AG1478 and AG1295 blocked HP-induced migration and partially inhibit closure of the CFA under CP. LY294002 blocked HP-induced migration. SC58236 markedly inhibited closure of the CFA under CP by inhibiting COX2 activity. Exposure to HP, a physical stress, induced faster closure of the CFA via activation of members of the epidermal growth factor receptor (EGFR) family and PI-3K pathways. Under CP, closure of the CFA in response to denudation, a form of injury, is due to activation of COX2 in ONH astrocytes.

Reduced progesterone metabolites protect rat hippocampal neurones from kainic acid excitotoxicity in vivo

The ovarian hormone progesterone is neuroprotective in some animal models of neurodegeneration. Progesterone actions in the brain may partly be mediated by the locally produced metabolites 5alpha-dihydroprogesterone and 3alpha,5alpha-tetrahydroprogesterone. The neuroprotective effects of these two metabolites of progesterone were assessed in this study. Ovariectomized Wistar rats were injected with kainic acid, to induce excitotoxic neuronal death in the hippocampus, and with different doses of 5alpha-dihydroprogesterone and 3alpha,5alpha-tetrahydroprogesterone. The number of surviving neurones in the hilus of the dentate gyrus of the hippocampus was assessed with the optical disector method. The administration of kainic acid resulted in a significant decrease in the number of hilar neurones and in the induction of vimentin expression in reactive astrocytes, a sign of neural damage. Low doses of 5alpha-dihydroprogesterone (0.25 and 0.5 mg/kg body weight, b.w.) prevented the loss of hilar neurones and the appearance of vimentin immunoreactivity in astrocytes. Higher doses (1-2 mg/kg b.w.) were not neuroprotective. By contrast, low doses of 3alpha,5alpha-tetrahydroprogesterone (0.25-1 mg/kg b.w.) were unable to protect the hilus from kainic acid while higher doses (2-4 mg/kg b.w.) were protective. The different optimal neuroprotective doses of 5alpha-dihydroprogesterone and 3alpha,5alpha-tetrahydroprogesterone suggest that these two steroids may protect neurones using different mechanisms. The neuroprotective effects of 3alpha,5alpha-tetrahydroprogesterone may be exerted by the inhibition of neuronal activity via the GABAA receptor. This latter possibility is supported by the observation that 3beta,5alpha-tetrahydroprogesterone, an isomer of 3alpha,5alpha-tetrahydroprogesterone that does not bind to GABAA receptor, was not neuroprotective. In summary, our findings suggest that progesterone neuroprotective effects may be, at least in part, mediated by its reduced metabolites 5alpha-dihydroprogesterone and 3alpha,5alpha-tetrahydroprogesterone.

The expression of PEA-15 (phosphoprotein enriched in astrocytes of 15 kDa) defines subpopulations of astrocytes and neurons throughout the adult mouse brain

Phosphoprotein enriched in astrocytes of 15 kDa (PEA-15) is an abundant phosphoprotein in primary cultures of mouse brain astrocytes. Its capability to interact with members of the apoptotic and mitogen activated protein (MAP) kinase cascades endows PEA-15 with anti-apoptotic and anti-proliferative properties. We analyzed the in vivo cellular sources of PEA-15 in the normal adult mouse brain using a novel polyclonal antibody. Immunohistochemical assays revealed numerous PEA-15-immunoreactive cells throughout the brain of wild-type adult mice while no immunoreactive signal was observed in the brain of PEA-15 -/- mice. Cell morphology and double immunofluorescent staining showed that both astrocytes and neurons could be cellular sources of PEA-15. Closer examination revealed that in a given area only part of the astrocytes expressed the protein. The hippocampus was the most striking example of this heterogeneity, a spatial segregation restricting PEA-15 positive astrocytes to the CA1 and CA3 regions. A PEA-15 immunoreactive signal was also observed in a few cells within the subventricular zone and the rostral migratory stream. In vivo analysis of an eventual PEA-15 regulation in astrocytes was performed using a model of astrogliosis occurring along motor neurons degeneration, the transgenic mouse expressing the mutant G93A human superoxyde-dismutase-1, a model of amyotrophic lateral sclerosis. We observed a marked up-regulation of PEA-15 in reactive astrocytes that had developed throughout the ventral horn of the lumbar spinal cord of the transgenic mice. The heterogeneous cellular expression of the protein and its increased expression in pathological situations, combined with the known properties of PEA-15, suggest that PEA-15 expression is associated with a particular metabolic status of cells challenged with potentially apoptotic and/or proliferative signals.

Discovery of a new class of synthetic protein kinase inhibitors that suppress selective aspects of glial activation and protect against beta-amyloid induced injury: a foundation for future medicinal chemistry efforts focused on targeting Alzheimer's disease progression

A prevailing hypothesis in Alzheimer's disease (AD) research is that chronically activated glia may contribute to neuronal dysfunction, through generation of a detrimental state of neuroinflammation. This raises the possibility in drug discovery research of targeting the cycle of untoward glial activation and neuronal dysfunction that characterizes neuroinflammation. Success over the past century with effective anti-inflammatory drug development, in which the molecular targets are intracellular enzymes involved in signal transduction events and cellular homeostasis, demands that a similar approach be tried with neuroinflammation. Suggestive clinical correlations between inflammation markers and AD contribute to the urgency in addressing the hypothesis that targeting selective glial activation processes might be a therapeutic approach complementary to existing drugs and discovery efforts. An academic collaboratorium initiated a rapid inhibitor discovery effort 2 yr ago, focused on development of novel compounds with new mechanisms of action in AD-relevant cellular processes, in order to obtain the small-molecule compounds required to address the neuroinflammation hypothesis and provide a proof of concept for future medicinal chemistry efforts. We summarize here our progress toward this goal in which novel pyridazine-based inhibitors of gene-regulating protein kinases have been discovered. Feasibility studies indicate their potential utility in current medicinal chemistry efforts focused on improvement in molecular properties and the longer term targeting of AD-related pathogenic processes.

Neurotrophic effects of vascular endothelial growth factor on organotypic cortical explants and primary cortical neurons

Vascular endothelial growth factor (VEGF) is well known to play an important regulatory role in vascular growth and development. Because gene knock-outs of VEGF and its receptors flk-1 and flt-1 result in early embryonic lethality, determining roles for VEGF in CNS development has been particularly difficult. Recent studies have shown that VEGF is upregulated after various injuries to the adult brain and that the cytokine affords protection to cultured neurons affected by oxidative or excitotoxic stress. The present study demonstrates, for the first time, that VEGF is directly neurotrophic to CNS neurons in culture. We applied VEGF to normoxic fetal organotypic cortical explants as a model of CNS neuropil, in addition to primary cortical neurons, to assess direct growth effects absent vascular or astroglial activity. We found that VEGF provided a significant dose-responsive increase in the neuronal microtubule markers TUJ1 and MAP-2, as well as mRNA for MAP-2 and flk-1. Antisense oligodeoxynucleotides to flk-1, but not flt-1, inhibited neuritic outgrowth, whereas inhibitors of the signaling pathways MEK1 and P13-AKT both abrogated VEGF-induced growth. VEGF applied to primary cortical neurons produced significant increases in neuronal cell body diameter and the number of emerging neurites mediated by flk-1. Possibly, VEGF achieves its effects by acting on the neuronal microtubular content, which is involved with growth, stability and maturation. Several studies have now shown that VEGF is neurotrophic and neuroprotective independent of a vascular component; we suggest that VEGF plays seminal pleiotrophic roles in CNS development and repair.

Development of novel selective cell ablation in the mammary gland and brain to study cell-cell interactions and chemoprevention

We have generated transgenic mice which express the gene encoding Escherichia coli nitroreductase (NTR) specifically in the luminal epithelial cells of the mammary gland and the glial cells of the brain. The enzyme activates an antitumour drug CB 1954, to produce a cross-linking agent that kills all cells expressing the enzyme. We have shown that administration of the antitumour drug CB 1954 rapidly and selectively kills these cells. Original experiments demonstrated the ability to ablate the luminal cells in the mammary gland with no apparent bystander effect. Subsequently, astrocytes expressing nitroreductase under the targeting of the GFAP promoter were selectively ablated following administration of the prodrug CB 1954 produces a degeneration of granular neurones due to changes in glutamate levels. Recent experiments demonstrated inhibition of myc-dependent mammary tumours using the same enzyme (nitroreductase)-prodrug (CB 1954), combination. Owing to the ease of control of NTR-mediated cell ablation, we anticipate that this system will supersede herpes simplex virus type 1 thymidine kinase. There are widespread potential applications for this approach in the dissection of complex cellular interactions during development and in the adult organism. The present transgenic models also have important applications for the study in vivo of novel prodrugs that can be selected for variable degrees of bystander effects. Such studies will have particular significance for those groups advocating the use of NTR as an appropriate enzyme for gene-directed enzyme prodrug therapy by providing models of a wide range of human disease for mechanistic and therapeutic experimentation. The results clearly demonstrate that the model has potential to study chemoprevention and fundamental questions on cell-cell interactions in cell biology.

Defined inflammatory states in astrocyte cultures: correlation with susceptibility towards CD95-driven apoptosis

A complete cytokine mix (CCM) or its individual components tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) were used to switch resting murine astrocytes to reactive states. The transformation process was characterized by differential up-regulation of interleukin-6 (IL-6), cyclooxygenase-2 (COX-2) and inducible nitric oxide synthetase (iNOS) mRNA and protein and a subsequent release of prostaglandin E2, nitric oxide (NO) and IL-6. Both CD95L and anti-CD95 antibodies triggered caspase activation followed by apoptotic death in fully pro-inflammatory astrocytes, whereas resting cells were totally resistant. Two other death-inducing ligands, TNF and TNF-related apoptosis-inducing ligand (TRAIL) did not induce apoptosis in reactive astrocytes. The switch in astrocyte sensitivity was accompanied by up-regulation of caspase-8 and CD95 as well as the capacity to recruit Fas-associated death domain (FADD) to the activated death receptor complex. Neither CD95-mediated death, nor other inflammatory parameters were affected by inhibition of iNOS or COX, respectively. Accordingly, IFN-gamma was absolutely essential for up-regulation of iNOS, but not for the switch in apoptosis sensitivity. In contrast, p38 kinase activity was identified as an important controller of both the inflammatory reaction and apoptosis both in astrocytes stimulated with CCM and in glia exposed to TNF and IL-1 only.

Caveolin-1 knockdown by small interfering RNA suppresses responses to the chemokine monocyte chemoattractant protein-1 by human astrocytes

Astrocytes regulate the integrity of the blood-brain barrier and influence inflammatory processes in the central nervous system. The pro-inflammatory chemokine monocyte chemoattractant protein-1 (MCP-1), which is both released by and stimulates astrocytes, is thought to play a crucial role in both these activities. Because astrocytes have been shown to possess caveolae, vesicular structures that participate in intracellular transport and signal transduction events, we reasoned that expression of the major structural protein of these organelles, caveolin-1, might feature critically in the cellular responses to MCP-1. To test this hypothesis, caveolin-1 level was "knocked down" in human astrocyte cultures by using a small interfering RNA approach. This method resulted in efficient (>90% loss) and specific knockdown of caveolin-1 expression while sparring glial fibrillary acidic protein as well as several other proteins involved in endocytosis. Astrocytes suffering caveolin-1 loss showed diminished ability to down-modulate and internalize the MCP-1 receptor (CCR2) in response to exposure to this chemokine and also demonstrated significantly reduced capacity to undergo chemotaxis and calcium flux when MCP-1-stimulated. The results highlight a potentially prominent role for caveolae and/or caveolin-1 in mediating astrocyte responses to MCP-1, a feature that might significantly dictate the progression of inflammatory events at the blood-brain barrier.

Neurotrophic effects of vascular endothelial growth factor on organotypic cortical explants and primary cortical neurons

Vascular endothelial growth factor (VEGF) is well known to play an important regulatory role in vascular growth and development. Because gene knock-outs of VEGF and its receptors flk-1 and flt-1 result in early embryonic lethality, determining roles for VEGF in CNS development has been particularly difficult. Recent studies have shown that VEGF is upregulated after various injuries to the adult brain and that the cytokine affords protection to cultured neurons affected by oxidative or excitotoxic stress. The present study demonstrates, for the first time, that VEGF is directly neurotrophic to CNS neurons in culture. We applied VEGF to normoxic fetal organotypic cortical explants as a model of CNS neuropil, in addition to primary cortical neurons, to assess direct growth effects absent vascular or astroglial activity. We found that VEGF provided a significant dose-responsive increase in the neuronal microtubule markers TUJ1 and MAP-2, as well as mRNA for MAP-2 and flk-1. Antisense oligodeoxynucleotides to flk-1, but not flt-1, inhibited neuritic outgrowth, whereas inhibitors of the signaling pathways MEK1 and P13-AKT both abrogated VEGF-induced growth. VEGF applied to primary cortical neurons produced significant increases in neuronal cell body diameter and the number of emerging neurites mediated by flk-1. Possibly, VEGF achieves its effects by acting on the neuronal microtubular content, which is involved with growth, stability and maturation. Several studies have now shown that VEGF is neurotrophic and neuroprotective independent of a vascular component; we suggest that VEGF plays seminal pleiotrophic roles in CNS development and repair.

Secretion of tenascin-C by cultured astrocytes: regulation of cell proliferation and process elongation

Tenascin-C (TNC), an extracellular matrix glycoprotein, is involved in tissue morphogenesis like embryogenesis, wound healing or tumorigenesis. Quiescent astroglia in long-term primary cultures are known to show rapid morphological changes after subculture and serum deprivation/re-addition (SSDR). To elucidate roles of TNC in the morphogenetic processes of cultured astrocytes, we have revealed morphological changes in association with soluble TNC contents in the medium and expression of TNC mRNA, TNC, glial fibrillary acidic protein (GFAP) and integrin beta1, one of its cell surface receptors, in glial cells after SSDR. Soluble TNC in the medium rapidly increased in amount at 4 h when GFAP-positive cells expressed TNC mRNA, TNC and integrin beta1. Cellular proliferation and growth occurred in colonies expressing TNC mRNA, TNC and integrin beta1 during the first 24 h. During the next 24 h, process elongation and cell migration occurred in association with increased GFAP expression and re-elevation of soluble TNC in the medium. Cell bodies became flat and larger with increased GFAP and reduced TNC expression at 72 h, while cultures became confluent with reduced GFAP and TNC expression at 96 h after SSDR. Functional blocking with anti-TNC antibody reduced cell proliferation and induced morphological change from a process-bearing slender shape to a flat and wide shape presumably due to increased cell adhesion. These findings strongly support the idea that endogenous TNC produced and released by astrocytes in response to serum stimulation induces their proliferation and process elongation through a paracrine/autocrine mechanism.

Directed nerve outgrowth is enhanced by engineered glial substrates

In the present study, the influence of astrocyte alignment on the direction and length of regenerating neurites was examined in vitro. Astrocytes were experimentally manipulated by different approaches to create longitudinally aligned monolayers. When cultured on the aligned monolayers, dorsal root ganglion neurites grew parallel to the long axis of the aligned astrocytes and were significantly longer than controls. Engineered monolayers expressed linear arrays of fibronectin, laminin, neural cell adhesion molecule, and chondroitin sulfate proteoglycan that were organized parallel to one another, suggesting that a particular spatial arrangement of these molecules on the astrocyte surface may be necessary to direct nerve regeneration in vivo. In contrast, no bias in directional outgrowth was observed for neurites growing on unorganized monolayers. The results suggest that altering the organization of astrocytes and their scar-associated matrix at the lesion site may be used to influence the direction and the length of adjacent regenerating axons in the damaged brain and spinal cord.

‘Neuro’-peptides in glia: Focus on NPY and galanin

The term neuropeptide was advanced by de Wied and collaborators in the early seventies. At that time, they defined neuropeptides as endogenous substances synthesized in nerve cells and involved in nervous system functions. Since then, several studies have revealed that the very same 'neuropeptides' are also expressed in non-neuronal cells. It is therefore generally accepted that the original definition of these peptides was too limited and, consequently, it has recently been revised. Among the non-neuronal cells that synthesize neuropeptides are several glial cell types.

Astroglial regulation of apolipoprotein E expression in neuronal cells. Implications for Alzheimer's disease

Although apolipoprotein (apo) E is synthesized in the brain primarily by astrocytes, neurons in the central nervous system express apoE, albeit at lower levels than astrocytes, in response to various physiological and pathological conditions, including excitotoxic stress. To investigate how apoE expression is regulated in neurons, we transfected Neuro-2a cells with a 17-kilobase human apoE genomic DNA construct encoding apoE3 or apoE4 along with upstream and downstream regulatory elements. The baseline expression of apoE was low. However, conditioned medium from an astrocytic cell line (C6) or from apoE-null mouse primary astrocytes increased the expression of both isoforms by 3-4-fold at the mRNA level and by 4-10-fold at the protein level. These findings suggest that astrocytes secrete a factor or factors that regulate apoE expression in neuronal cells. The increased expression of apoE was almost completely abolished by incubating neurons with U0126, an inhibitor of extracellular signal-regulated kinase (Erk), suggesting that the Erk pathway controls astroglial regulation of apoE expression in neuronal cells. Human neuronal precursor NT2/D1 cells expressed apoE constitutively; however, after treatment of these cells with retinoic acid to induce differentiation, apoE expression diminished. Cultured mouse primary cortical and hippocampal neurons also expressed low levels of apoE. Astrocyte-conditioned medium rapidly up-regulated apoE expression in fully differentiated NT2 neurons and in cultured mouse primary cortical and hippocampal neurons. Thus, neuronal expression of apoE is regulated by a diffusible factor or factors released from astrocytes, and this regulation depends on the activity of the Erk kinase pathway in neurons.

Differential activation of astrocytes and microglia during post-natal development of dopaminergic neuronal death in the weaver mouse

In order to understand the relationship between astrocytes, microglia and injured neurons, we studied the weaver mutant mouse. One of the main characteristics of this mutant is the progressive degeneration of the dopaminergic (DA) nigrostriatal pathway that starts around postnatal day 15 (P15), in the substantia nigra pars compacta (SNpc) and progresses until adult age (P60). In the present paper, we analysed the relationship between astroglial and microglial cells within DA neurons in the nigrostriatal system of homozygous weaver mice, at different postnatal ages corresponding to specific stages of the DA neuronal loss. The activation of astrocytes was found to be an early event in weaver DA denervation, appearing massively at the onset of DA neuronal loss in the SNpc at P15. Astrocytes remained activated in the adult brain even after the slowing down of the neuronal death process. Interestingly, in the ventral tegmental area, where no DA neuronal death could be detected, a profound, permanent astrogliosis was also observed in adult animals. In contrast, an activation of microglial cells was transiently observed in the SNpc but only at the postnatal age when maximal neuronal death was observed (P30). Lastly, in the striatum, where there was a massive loss of DA nerve terminals, neither astrogliosis nor microglial activation was detected. Hence, the reaction of astrocytes and microglial cells to progressive and spontaneous DA neuronal death showed different temporal kinetics, suggesting a different role for these two cell types in the DA neurodegenerative process in the weaver mouse.

Inhibiting cell proliferation during formation of the glial scar: effects on axon regeneration in the CNS

Following a CNS lesion many glial cell types proliferate and/or migrate to the lesion site, forming the glial scar. The majority of these cells express chondroitin sulphate proteoglycans (CS-PGs), previously shown to inhibit axonal growth. In this study, in an attempt to diminish glial scar formation and improve axonal regeneration, proliferating cells were eliminated from the lesion site. Adult rats received a continuous infusion of 2% cytosine-D-arabinofuranoside (araC) or saline for 7 days over the lesion site, immediately following a unilateral transection of the right medial forebrain bundle. Additional groups of rats that received subdural infusions prior to the lesion, and lesioned rats which received no infusion, were also compared in the analyses. Animals were killed at 4, 7, 12 or 18 days post-lesion (dpl) and immunohistochemistry was used to determine the effects of these treatments on tyrosine hydroxylase (TH)-lesioned axons, and on the injury response of glial cells. Almost complete elimination of NG2 oligodendrocyte progenitor cells from the lesion site was seen up to 7 dpl in araC-infused animals; reduced numbers of reactive CD11b microglia were also seen but no effects were seen on the injury response of GFAP astrocytes. Significantly more TH axons were seen distal to the lesion in araC-treated brains, but these numbers dwindled by 18 dpl.

Spontaneous regeneration of the corticospinal tract after transection in young rats: collagen type IV deposition and astrocytic scar in the lesion site are not the cause but the effect of failure of regeneration

In young rats the corticospinal tract regenerated after a single transection of the spinal cord with a sharp blade, but regeneration failed if the transection was repeated to make a more traumatic injury. To identify cells and associated molecules that promote or impede regeneration, we compared expression of collagen type IV, glial fibrillary acidic protein (GFAP), and vimentin immunoreactivity (IR) at the lesion sites in combination with anterograde axonal tracing between animals with two types of transection. Axonal regeneration occurred as early as 18 hours after transection; regenerating axons penetrated vessel-like structures with collagen type IV-IR at the lesion site, while reactive astrocytes coexpressing GFAP- and vimentin-IR appeared in the lesioned white matter. In contrast, when regeneration failed astrocytes were absent near the lesion. By 7 days sheet-like structures with collagen type IV-IR and astrocytic scar appeared in the lesioned white matter and persisted until the end of the observation period (31 days). On the basis of their spatiotemporal appearance, collagen type IV-IR sheet-like structures and the astrocytic scar follow, rather than cause, the failure of regeneration. The major sign, and perhaps cause, of failure of axonal regeneration is likely the prolonged disappearance of astrocytes around the lesion site in the early postinjury period.

Sleep deprivation prior to transient global cerebral ischemia attenuates glial reaction in the rat hippocampal formation

This study was aimed to ascertain the effect of sleep deprivation on subsequent cerebral ischemia in the rat hippocampal formation. Seven days after transient global cerebral ischemia induced by four-vessel occlusion method, most of the pyramidal cells in the hippocampal CA1 subfield underwent disruption and pyknosis as detected by cresyl violet staining. With OX-42, OX-18, OX-6 and ED1 immunohistochemistry, robust microglia/macrophage reactions were observed in the CA1 and dentate hilus. The majority of reactive microglia was rod-shaped, bushy or amoeboidic cells bearing hypertrophic processes. Astrocytes also displayed hypertrophic processes, whose immunostaining for glial fibrillary acidic protein was markedly enhanced. The ischemia-induced neuronal damage and glial reactions, however, were noticeably attenuated in rats subjected to pretreatment with sleep deprivation for five consecutive days. The most drastic effect was the diminution of OX-18, OX-6 and ED1 immunoreactivities, suggesting that the immune potentiality and/or phagocytosis of these cells was suppressed by prolonged sleep deprivation prior to ischemic insult. It is postulated that sleep deprivation may have a preconditioning influence on subsequent lethal cerebral ischemia. Hence, sleep deprivation may be considered as a therapeutic strategy in brain ischemic damage.

Estudo da imunorreatividade astrocitária para GFAP e vimentina no tronco encefálico de ratos Wistar submetidos ao modelo gliotóxico do brometo de etídio

INTRODUÇÃO: O brometo de etídio (BE) é reconhecido como um agente gliotóxico que causa desaparecimento focal astrocitário e oligodendroglial. OBJETIVO: Investigou-se a imunorreatividade astrocitária à proteína glial fibrilar ácida (GFAP) e à vimentina (VIM) após injeção do BE. MÉTODO: Ratos Wistar adultos foram tomados como controles histológicos (grupo H) ou injetados na cisterna basal com BE a 0,1% (grupo E) ou salina a 0,9% (grupo C). Fragmentos do tronco encefálico foram colhidos das 24 horas aos 31 dias pós-injeção para estudo imuno-histoquímico da GFAP e VIM pelo método da avidina-biotina. RESULTADOS: No grupo E, foram observadas extensas lesões na ponte e no mesencéfalo, com desaparecimento astrocitário da área central 24 horas pós-BE, bem como infiltração macrofágica e astrogliose periférica a partir do 3º dia. Os astrócitos marginais apresentaram imunorreatividade aumentada à GFAP e reexpressão de VIM, esta confinada às bordas imediatas do sítio lesional. No grupo C, foram visualizadas lesões pontinas discretas, com preservação astrocitária central e marcação menos intensa para GFAP nos bordos em relação ao grupo E. Nenhuma imunorreatividade para VIM foi notada em tais astrócitos. CONCLUSÃO: Os astrócitos das margens das lesões induzidas pelo BE apresentaram imunorreatividade aumentada para GFAP e reexpressão de VIM.

Evidence for astrocyte heterogeneity: a distinct subpopulation of protoplasmic-like glial cells is detected in transgenic mice expressing Lmo1-lacZ

The adult mammalian central nervous system (CNS) contains a large number of different cell types, which arise from the ventricular (VZ) and subventricular zones during embryonic development. In this study, we used a transgenic mouse expressing Lmo1-LacZ from a randomly inserted promoter/reporter gene construct to identify a glial subpopulation. LMO1 is an LIM domain-containing protein, thought to act in protein-protein interactions. We found first that in the adult transgenic CNS, beta-galactosidase (beta-gal) was expressed in a specific subpopulation of protoplasmic-like cells, which did not express detectable levels of glial fibrilary acidic protein unless a lesion was produced. Secondly, during development, beta-gal(+) cells were found arising from discrete regions of the VZ. Taken together, these results identify a subpopulation of protoplasmic glial cells in the adult CNS and suggest that they arise from a restricted VZ region during CNS development.

The effect of leukaemia inhibitory factor on SOD1 G93A murine amyotrophic lateral sclerosis

Before potential therapeutic strategies for the treatment of amyotrophic lateral sclerosis (ALS) can be advanced to human clinical trials, there is a need to assess them in an animal model that best resembles the disease process. SOD1 G93A mice have close resemblance to familial ALS (fALS) and have been used in this study to evaluate the therapeutic potential of leukaemia inhibitory factor (LIF). LIF action was investigated by assessing three delivery methods: (1) daily subcutaneous injection; (2) through LIF rods placed adjacent to hind limb skeletal muscle and (3) continuous intrathecal infusion. The effect on disease progression was assessed by semi-quantitative and quantitative functional measurements, and histologically on the survival of motor neurons and number of reactive astrocytes. The results show that LIF had no beneficial effects when administered using the three methods of drug delivery. These results suggest that further evaluation of LIF in this transgenic model is required to fully characterize its' therapeutic potential.

Intracerebroventricular administration of an endothelin ETB receptor agonist increases expressions of GDNF and BDNF in rat brain

Endothelins (ETs) are suggested to be involved in functional alterations of astrocytes after brain injury, including proliferation, hypertrophy and production of neurotrophic factors. In this study, effects of Ala1,3,11,15-endothelin-1 (Ala1,3,11,15-ET-1), an ETB receptor selective agonist, on neurotrophic factor production were examined in rat brain. A continuous intracerebroventricular administration of Ala1,3,11,15-ET-1 (500 pmol/day for 7 days) increased the numbers of GFAP- and vimentin-positive astrocytes in the hippocampus, caudate putamen and cerebrum. Ala1,3,11,15-ET-1 did not induce neuronal degeneration and activation of microglia/macrophage in these brain regions. The intracerebroventricular administration of Ala1,3,11,15-ET-1 for 7 days caused two- to three-fold increases in glial cell line-derived neurotrophic factors (GDNF) mRNA in the hippocampus and cerebrum. The mRNA levels of brain-derived neurotrophic factors (BDNF) in caudate putamen were increased by Ala1,3,11,15-ET-1. Expressions of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) mRNA in these regions were not largely affected by Ala1,3,11,15-ET-1, except cerebral NGF mRNA level was increased. The Ala1,3,11,15-ET-1-induced increases in GDNF and BDNF mRNA levels were accompanied by increases in immunoreactive GDNF and BDNF. Immunohistochemical observations showed that GFAP-positive astrocytes expressed GDNF and BDNF in the brain regions of Ala1,3,11,15-ET-1-infused rats. In cultured rat astrocytes, Ala1,3,11,15-ET-1 (100 nm) increased mRNA levels of GDNF and BDNF. These results suggest that activation of brain ETB receptors induced GDNF and BDNF expression in astrocytes.

Relationship of Nef-positive and GFAP-reactive astrocytes to drug use in early and late HIV infection

Reactive astrocytosis is a well-documented feature of HIV encephalitis (HIVE), but it is unclear whether restricted infection of astrocytes contributes to this phenomenon. In addition, the part played by reactive and/or infected astrocytes in AIDS-related dementia is not fully understood. In this study of patients at different stages of the human immunodeficiency virus (HIV) infection, who had been treated at most with one antiretroviral drug, reactive astrocytes were identified by immunopositivity for glial fibrillary acidic protein (GFAP) and infected astrocytes by positivity for HIV Nef protein. Results were compared for drug-using AIDS patients with (n=9) and without (n=7) HIVE, for presymptomatic HIV-positive drug users (n=12) and for control HIV-negative subjects (n=20), including a group who used drugs (n=10). GFAP-reactive astrocytes in both grey and white matter were significantly more numerous in HIVE subjects than in each of the other groups but did not correlate with viral load. Nef-positive astrocytes were confined to HIVE cases and to white matter, but were numerous in only one subject who was treatment-naive. Nef-positive microglia were identified in all HIVE cases and in occasional AIDS and presymptomatic subjects who did not have HIVE. The results suggest that astrocytes may form an additional viral reservoir in late HIV infection and may contribute to HIVE. However, the number of GFAP-positive astrocytes was neither increased in pre-AIDS nor in drug abuse, in contrast with microglia which we have shown previously to be up-regulated in both states.

Temporal expression of immunoreactivity for heat shock protein 25 (Hsp25) in the rat periodontal ligament following transection of the inferior alveolar nerve

The present study examined the immunohistochemical localization of heat shock protein 25 (Hsp25) during the regeneration of nerve fibers and Schwann cells in the periodontal ligament of the rat lower incisor following transection of the inferior alveolar nerve. In the untreated control group, the periodontal ligament of rat incisor did not contain any Hsp25-immunoreaction. On postoperative day 3 (PO 3d), a small number of Schwann cells with slender cytoplasmic processes exhibited Hsp25-immunoreactivity. From PO 5d to PO 21d, Hsp25-positive nerve fibers and Schwann cells drastically increased in number in the alveolar half of the ligament. Although the axons of some regenerating Ruffini-like endings also showed Hsp25-immunoreactions, the migrated Schwann cells were devoid of Hsp25-immunoreaction. Thereafter, Hsp25-positive structures decreased in number gradually to disappear from the periodontal ligament by PO 56d. This temporal expression of Hsp25 in the periodontal ligament well-reflected the regeneration process of the nerve fibers. Hsp25 in the regenerating nerve fibers and denervated Schwann cells most likely serves in modulating actin dynamics and as a cellular inhibitor of apoptosis, respectively.

GFAP: functional implications gleaned from studies of genetically engineered mice

GFAP is the major intermediate filament of mature astrocytes, and its relatively specific expression in these cells suggests an important function. To study the role of the GFAP gene, mice have been created carrying null alleles (no protein), modified alleles (altered protein), or added wild type alleles (elevated protein). Surprisingly, absence of GFAP has relatively subtle effects on development. On the other hand, over-expression can be lethal, and led to the discovery that GFAP coding mutations are responsible for most cases of Alexander disease, a devastating neurodegenerative disorder. Here we review what the various GFAP mouse models reveal about GFAP and astrocyte function.

Treatment with metallothionein prevents demyelination and axonal damage and increases oligodendrocyte precursors and tissue repair during experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is an animal model for the human demyelinating disease multiple sclerosis (MS). EAE and MS are characterized by significant inflammation, demyelination, neuroglial damage, and cell death. Metallothionein-I and -II (MT-I + II) are antiinflammatory and neuroprotective proteins that are expressed during EAE and MS. We have shown recently that exogenous administration of Zn-MT-II to Lewis rats with EAE significantly reduced clinical symptoms and the inflammatory response, oxidative stress, and apoptosis of the infiltrated central nervous system areas. We show for the first time that Zn-MT-II treatment during EAE significantly prevents demyelination and axonal damage and transection, and stimulates oligodendroglial regeneration from precursor cells, as well as the expression of the growth factors basic fibroblast growth factor (bFGF), transforming growth factor (TGF)beta, neurotrophin-3 (NT-3), NT-4/5, and nerve growth factor (NGF). These beneficial effects of Zn-MT-II treatment could not be attributable to its zinc content per se. The present results support further the use of Zn-MT-II as a safe and successful therapy for multiple sclerosis.

Inflammatory neurodegeneration mediated by nitric oxide, glutamate, and mitochondria

In inflammatory, infectious, ischemic, and neurodegenerative pathologies of the central nervous system (CNS) glia become "activated" by inflammatory mediators, and express new proteins such as the inducible isoform of nitric oxide synthase (iNOS). Although these activated glia have benefi- cial roles, in vitro they potently kill cocultured neurons, and there is increasing evidence that they contribute to pathology in vivo. Nitric oxide (NO) from iNOS appears to be a key mediator of such glial-induced neuronal death. The high sensitivity of neurons to NO is partly due to NO causing inhibition of respiration, rapid glutamate release from both astrocytes and neurons, and subsequent excitotoxic death of the neurons. NO is a potent inhibitor of mitochondrial respiration, due to reversible binding of NO to cytochrome oxidase in competition with oxygen, resulting in inhibition of energy production and sensitization to hypoxia. Activated astrocytes or microglia cause a potent inhibition of respiration in cocultured neurons due to glial NO inhibiting cytochrome oxidase within the neurons, resulting in ATP depletion and glutamate release. In some conditions, glutamate- induced neuronal death can itself be mediated by N-methyl-D-aspartate (NMDA)-receptor activation of the neuronal isoform of NO synthase (nNOS) causing mitochondrial damage. In addition NO can be converted to a number of reactive derivatives such as peroxynitrite, NO2, N2O3, and S-nitrosothiols that can kill cells in part by inhibiting mitochondrial respiration or activation of mitochondrial permeability transition, triggering neuronal apoptosis or necrosis.

Inhibition of endogenous VEGF impedes revascularization and astroglial proliferation: roles for VEGF in brain repair

Vascular endothelial growth factor (VEGF) is upregulated following injury to the CNS. Our previous work has shown that exogenous application of VEGF promotes angiogenesis, blood-brain barrier permeability, and astroglial mitogenicity in the traumatized brain. To develop a model that could link endogenously secreted VEGF to brain tissue repair, a specific neutralizing antibody to VEGF was infused by osmotic minipump directly into the neocortex and striatum for up to 1 week. Tissues adjacent to the infusion/wound site were analyzed for specific vascular and astroglial protein markers and proliferation, necrosis/apoptosis (via TUNEL staining), VEGF, the VEGF receptors flt-1 and flk-1, and bFGF expression using immunohistochemistry and semi-quantitative RT-PCR. Neutralization of native VEGF caused significant decreases in angiogenic activity, astroglial proliferation, and nestin immunoexpression, while vascular and astroglial degeneration was substantially increased, resulting in much larger wound cavities when compared to controls. The hindrance of brain tissue repair occurred despite an increase in bFGF expression at the wound sites. VEGF appears to be an integral factor in CNS wound healing that is essential for vascular endothelial proliferation and survival and may also be necessary for astroglial proliferation and maintenance during the repair of brain injury.

Role of metallothionein-III following central nervous system damage

We evaluated the physiological relevance of metallothionein-III (MT-III) in the central nervous system following damage caused by a focal cryolesion onto the cortex by studying Mt3-null mice. In normal mice, dramatic astrogliosis and microgliosis and T-cell infiltration were observed in the area surrounding the lesioned tissue, along with signs of increased oxidative stress and apoptosis. There was also significant upregulation of cytokines/growth factors such as tumor necrosis factor-alpha, interleukin (IL)-1 alpha/beta, and IL-6 as measured by ribonuclease protection assay. Mt3-null mice did not differ from control mice in these responses, in sharp contrast to results obtained in Mt1- Mt2-null mice. In contrast, Mt3-null mice showed increased expression of several neurotrophins as well as of the neuronal sprouting factor GAP-43. Thus, unlike MT-I and MT-II, MT-III does not affect the inflammatory response elicited in the central nervous system by a cryoinjury, nor does it serve an important antioxidant role, but it may influence neuronal regeneration during the recovery process.

Bombycis corpus extract (BCE) protects hippocampal neurons against excitatory amino acid-induced neurotoxicity

Bombycis corpus (BC) or Bombyx Batryticatus, a batryticated silkworm and white-stiff silkworm, is a drug consisting of the dried larva of silkworm, Mobyz mori L., dead and stiffened due to the infection of Beauveria (Bals.) Vuill. In a previous paper (Kim et al., Pharmacol. Res., 43, 12-16, 2001), BC was shown to protect amyloid-beta-induced cytotoxicity. In the present study, we have found that BCE can prevent or reduce the neurotoxic actions in the hippocampus of the glutamate agonists N-methyl-D-aspartic acid (NMDA) in vitro or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid in vitro. Pre-treatment with BCE (0, 1, 2, 5, and 10 microg/ml for 6-8 h) protected primary hippocampal cultures from embryonic day 18 (E18) embryos against NMDA-induced toxicity (0.1, 1, 10, and 50 nM/ml). BCE added either with NMDA (1 mM) or 1 h later had lesser, but still significant, protective actions. BCE also reduced NMDA-induced toxicity (1 mM). BCE (10 microg/ml) protected cultured neurons against the neurotoxic actions of either AMPA (25 microM) or kainic acid (1 mM) as well. Because the release of glutamate has been implicated in the neural damage after cerebral ischemia and other neural insults, these results suggest that BCE may contribute significantly to protect human brain to such damage.

Neuropathologies in transgenic mice expressing human immunodeficiency virus type 1 Tat protein under the regulation of the astrocyte-specific glial fibrillary acidic protein promoter and doxycycline

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a key pathogenic factor in a variety of acquired immune deficiency syndrome (AIDS)-associated disorders. A number of studies have documented the neurotoxic property of Tat protein, and Tat has therefore been proposed to contribute to AIDS-associated neurological diseases. Nevertheless, the bulk of these studies are performed in in vitro neuronal cultures without taking into account the intricate cell-cell interaction in the brain, or by injection of recombinant Tat protein into the brain, which may cause secondary stress or damage to the brain. To gain a better understanding of the roles of Tat protein in HIV-1 neuropathogenesis, we attempted to establish a transgenic mouse model in which Tat expression was regulated by both the astrocyte-specific glial fibrillary acidic protein promoter and a doxycycline (Dox)-inducible promoter. In the present study, we characterized the phenotypic and neuropathogenic features of these mice. Both in vitro and in vivo assays confirmed that Tat expression occurred exclusively in astrocytes and was Dox-dependent. Tat expression in the brain caused failure to thrive, hunched gesture, tremor, ataxia, and slow cognitive and motor movement, seizures, and premature death. Neuropathologies of these mice were characterized by breakdown of cerebellum and cortex, brain edema, astrocytosis, degeneration of neuronal dendrites, neuronal apoptosis, and increased infiltration of activated monocytes and T lymphocytes. These results together demonstrate that Tat expression in the absence of HIV-1 infection is sufficient to cause neuropathologies similar to most of those noted in the brain of AIDS patients, and provide the first evidence in the context of a whole organism to support a critical role of Tat protein in HIV-1 neuropathogenesis. More importantly, our data suggest that the Dox inducible, brain-targeted Tat transgenic mice offer an in vivo model for delineating the molecular mechanisms of Tat neurotoxicity and for developing therapeutic strategies for treating HIV-associated neurological disorders.

Endothelin-1 stimulates glial cell line-derived neurotrophic factor expression in cultured rat astrocytes

Effects of endothelin-1 (ET-1) on glial cell line-derived neurotrophic factor (GDNF) production in cultured astrocytes were examined. Treatment of cultured astrocytes with ET-1 (100 nM) increased mRNA levels of GDNF in 1-6h. The effect of ET-1 was inhibited by BQ788, an ET(B) receptor antagonist, but not by FR139317, an ET(A) receptor antagonist. ET-1 stimulated release of GDNF into culture medium. Dexamethasone (1 microM) and pyrrolidine dithiocarbamate (PDTC, 100 microM), which inhibit activation of NFkappaB, prevented the increases in GDNF mRNA by H(2)O(2). In contrast, the effect of ET-1 was not affected by dexamethasone and PDTC. The increase of astrocytic GDNF mRNA by ET-1 was inhibited by BAPTA/AM (30 microM) and PD98059 (50 microM), but not by calphostin C, staurosporine, and cyclosporine A. These results suggest that ET-1 stimulated expression of astrocytic GDNF through ET(B) receptor-mediated increases in cytosolic Ca(2+) and ERK activation.

Activation of extracellular signal-regulated kinase by stretch-induced injury in astrocytes involves extracellular ATP and P2 purinergic receptors

Gliosis is characterized by hypertrophic and hyperplastic responses of astrocytes to brain injury. To determine whether injury of astrocytes produced by an in vitro model of brain trauma activates extracellular signal-regulated protein kinase (ERK), a key regulator of cellular proliferation and differentiation, astrocytes cultured on deformable SILASTIC membranes were subjected to rapid, reversible strain (stretch)-induced injury. Activation of ERK was observed 1 min after injury, was maximal from 10 to 30 min, and remained elevated for 3 hr. Activation of ERK was dependent on the rate and magnitude of injury; maximum ERK activation was observed after a 20-60 msec, 7.5 mm membrane displacement. ERK activation was blocked by inhibiting MEK, the upstream activator of ERK. Activation of ERK was reduced when calcium influx was diminished. When extracellular ATP was hydrolyzed by apyrase or ATP/P2 receptors were blocked, injury-induced ERK activation was significantly reduced. P2 receptor antagonist studies indicated a role for P2X2 and P2Y1, but not P2X1, P2X3, or P2X7, receptors in injury-induced ERK activation. These findings demonstrate for the first time that ATP released by mechanical injury is one of the signals that triggers ERK activation and suggest a role for extracellular ATP, P2 purinergic receptors, and calcium-dependent ERK signaling in the astrocytic response to brain trauma.

Effect of reactive cell density on net [2-14C]acetate uptake into rat brain: labeling of clusters containing GFAP+- and lectin+-immunoreactive cells

Astrocytic proliferation is a hallmark of brain injury, but the biological functions and metabolic activities of reactive astrocytes in vivo are poorly understood. [2-14C]Acetate, which is preferentially transported into and, therefore, metabolized by astrocytes, was used to assess injury- and trophic factor-induced changes in astrocyte metabolic activity. Local rates of net [2-14C]acetate uptake and glucose utilization (CMR(glc)), determined with [14C]deoxyglucose to assay overall metabolic activity of all brain cells, were assayed 7 days after a cannula placement; adjacent brain sections were immunostained to identify glial fibrillary acidic protein-positive (GFAP(+)) astrocytes and microglia plus macrophages (lectin-positive cells). GFAP(+) cells were abundant in tissue surrounding the cannula compared to the contralateral hemisphere, whereas lectin(+) cells were restricted to the wound boundary. CMR(glc) fell 25% in regions enriched in reactive astrocytes compared to the homologous contralateral hemisphere, whereas [14C]acetate uptake increased slightly (6%) but statistically significantly; metabolism of both tracers in 13 other brain structures was unchanged. Injection of basic fibroblast growth factor (b-FGF) into cerebral cortex or superior colliculus produced fiber-rich cell clusters containing both GFAP(+) and lectin(+) cells that had a 37% increase in [14C]acetate uptake; GFAP(+)-cell density rose in the nearby neuropil but the corresponding change in [14C]acetate uptake was small (6-8%). Sensory stimulation did not alter [14C]acetate uptake into the clusters. Thus, [14C]acetate uptake was relatively stable with respect to changes in the density of reactive astrocytes that are dispersed throughout the neuropil and to changes in cellular activity arising from sensory stimulation. In contrast, b-FGF-induced cell clusters that contain mixed cell types and numerous fibers accumulated higher levels of [14C]acetate, raising the possibility that increased uptake might be due to high numbers of activated astrocytes and, perhaps, acetate metabolism by other cell types.

Upregulation of neuronal nitric oxide synthase in in vitro stellate astrocytes and in vivo reactive astrocytes after electrically induced status epilepticus

Neuronal nitric oxide synthase (nNOS) is a constitutively expressed and calcium-dependent enzyme. Despite predominantly expressed in neurons, nNOS has been also found in astrocytes, although at lower expression levels. We have studied the regulation of nNOS expression in cultured rat astrocytes from cortex and spinal cord by Western blotting and immunocytochemistry. nNOS was not detectable in cultured astrocytes grown in serum-containing medium (SCM), but was highly expressed after serum deprivation. Accordingly, calcium-dependent NOS activity and both intracellular nitrite levels and nitrotyrosine immunoreactivity after glutamate stimulation were higher in serum-deprived astrocytes than in cells grown in SCM. Serum deprivation induced a modification of astrocytes morphology, from flat to stellate. nNOS up-regulation was also observed in reactive astrocytes of rat hippocampi after electrically induced status epilepticus, as demonstrated by double-labeling experiments. Thus, nNOS upregulation occurs in both in vitro stellate and in vivo reactive astrocytes, suggesting a possible involvement of glial nNOS in neurological diseases characterized by reactive gliosis.

Activation of extracellular signal-regulated kinase by stretch-induced injury in astrocytes involves extracellular ATP and P2 purinergic receptors

Gliosis is characterized by hypertrophic and hyperplastic responses of astrocytes to brain injury. To determine whether injury of astrocytes produced by an in vitro model of brain trauma activates extracellular signal-regulated protein kinase (ERK), a key regulator of cellular proliferation and differentiation, astrocytes cultured on deformable SILASTIC membranes were subjected to rapid, reversible strain (stretch)-induced injury. Activation of ERK was observed 1 min after injury, was maximal from 10 to 30 min, and remained elevated for 3 hr. Activation of ERK was dependent on the rate and magnitude of injury; maximum ERK activation was observed after a 20-60 msec, 7.5 mm membrane displacement. ERK activation was blocked by inhibiting MEK, the upstream activator of ERK. Activation of ERK was reduced when calcium influx was diminished. When extracellular ATP was hydrolyzed by apyrase or ATP/P2 receptors were blocked, injury-induced ERK activation was significantly reduced. P2 receptor antagonist studies indicated a role for P2X2 and P2Y1, but not P2X1, P2X3, or P2X7, receptors in injury-induced ERK activation. These findings demonstrate for the first time that ATP released by mechanical injury is one of the signals that triggers ERK activation and suggest a role for extracellular ATP, P2 purinergic receptors, and calcium-dependent ERK signaling in the astrocytic response to brain trauma.

Up-regulation of proteinase-activated receptor 1 expression in astrocytes during HIV encephalitis

Proteinase-activated receptor 1 (PAR-1) is a G protein-coupled receptor that is activated by thrombin and is implicated in the pathogenesis of inflammation. Although PAR-1 is expressed on immunocompetent cells within the brain such as astrocytes, little is known about its role in the pathogenesis of inflammatory brain diseases. Herein, we investigated PAR-1 regulation of brain inflammation by stimulating human astrocytic cells with thrombin or the selective PAR-1-activating peptide. Activated cells expressed significantly increased levels of IL-1 beta, inducible NO synthase, and PAR-1 mRNA. Moreover, supernatants of these same cells were neurotoxic, which was inhibited by an N-methyl-D-aspartate receptor antagonist. Striatal implantation of the PAR-1-activating peptide significantly induced brain inflammation and neurobehavioral deficits in mice compared with mice implanted with the control peptide or saline. Since HIV-related neurological disease is predicated on brain inflammation and neuronal injury, the expression of PAR-1 in HIV encephalitis (HIVE) was investigated. Immunohistochemical analysis revealed that PAR-1 and (pro)-thrombin protein expression was low in control brains, but intense immunoreactivity was observed on astrocytes in HIVE brains. Similarly, PAR-1 and thrombin mRNA levels were significantly increased in HIVE brains compared with control and multiple sclerosis brains. These data indicated that activation and up-regulation of PAR-1 probably contribute to brain inflammation and neuronal damage during HIV-1 infection, thus providing new therapeutic targets for the treatment of HIV-related neurodegeneration.

Metallothionein-I overexpression decreases brain pathology in transgenic mice with astrocyte-targeted expression of interleukin-6

Transgenic expression of interleukin-6 (IL-6) in the CNS under the control of the glial fibrillary acidic protein (GFAP) gene promoter (GFAP-IL6 mice) causes significant damage and alters the expression of many genes, including a dramatic upregulation of metallothionein-I (MT-I). The findings in this report support the idea that the upregulation of MT-I observed in GFAP-IL6 mice is an important mechanism for coping with brain damage. Thus, GFAP-IL6 mice that were crossed with TgMTI transgenic mice (GFAP-IL6xTgMTI) and overexpressed MT-I in the brain showed a decreased upregulation of cytokines such as IL-6 and a diminished recruitment and activation of macrophages and T cells throughout the CNS but mainly in the cerebellum. The GFAP-IL6 mice showed clear evidence of increased oxidative stress, which was significantly decreased by MT-I overexpression. Interestingly, MT-I overexpression increased angiogenesis in GFAP-IL6 mice but not in control littermates. Overall, the results strongly suggest that MT-I+II proteins are valuable factors that protect against cytokine-induced CNS injury.

Forebrain astroglial plasticity is induced following withdrawal from repeated cocaine administration

Astrocytes actively participate in synaptic plasticity and respond to insult or metabotropic glutamate receptor activation with increased expression of the intermediate filament glial fibrillary acidic protein (GFAP). Extended withdrawal from repeated cocaine administration induces many forms of neuroplasticity. The present study with rats utilized a 3-week withdrawal period from daily cocaine administration (i.p.; 7 days) to investigate whether astrocytes participate in cocaine-mediated plasticity observed in brain nuclei associated with addiction. Following the 3-week withdrawal period, immunoblotting revealed increased GFAP expression in the prefrontal cortex (PFC) and in the shell and core compartments of the nucleus accumbens (NAshell and NAcore). Upregulation of GFAP did not occur in the striatum or in any brain region tested following shorter withdrawal times from repeated cocaine (24 h or 1 week) or following 2-h withdrawal from an acute cocaine injection (30 mg/kg i.p.). However, GFAP expression increased following a 3-week withdrawal from a single cocaine injection selectively in the NAshell. Cell counts revealed that astrocyte cell number increased only in the NAcore while immunoblots of a marker for immature or reactive astrocytes, vimentin, showed an increase only in the PFC following the 3-week withdrawal. Taken together, these results suggest that altered intermediate filament expression within forebrain astrocytes may be a significant part of the plasticity occurring during withdrawal from repeated cocaine. Furthermore, the increase in GFAP may arise from regionally distinct mechanisms, with the NAcore relying more on cell proliferation while the PFC relies on a larger reactive astrocyte population.