Astrocyte activation and reactive gliosis

Attenuated glial reactions and photoreceptor degeneration after retinal detachment in mice deficient in glial fibrillary acidic protein and vimentin

PURPOSE

To characterize the reactions of retinal glial cells (astrocytes and Müller cells) to retinal injury in mice that lack glial fibrillary acidic protein (GFAP) and vimentin (GFAP-/-Vim-/-) and to determine the role of glial cells in retinal detachment (RD)-induced photoreceptor degeneration.

METHODS

RD was induced by subretinal injection of sodium hyaluronate in adult wild-type (WT) and GFAP-/-Vim-/- mice. Astroglial reaction and subsequent monocyte recruitment were quantified by measuring extracellular signal-regulated kinase (Erk) and c-fos activation and the level of expression of chemokine monocyte chemoattractant protein (MCP)-1 and by counting monocytes/microglia in the detached retinas. Immunohistochemistry, immunoblotting, real-time quantitative polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) were used. RD-induced photoreceptor degeneration was assessed by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) and measurement of outer nuclear layer (ONL) thickness.

RESULTS

RD-induced reactive gliosis, characterized by GFAP and vimentin upregulation, Erk and c-fos activation, MCP-1 induction, and increased monocyte recruitment in WT mice. Absence of GFAP and vimentin effectively attenuated reactive responses of retinal glial cells and monocyte infiltration. As a result, detached retinas of GFAP-/-Vim-/- mice exhibited significantly reduced numbers of TUNEL-positive photoreceptor cells and increased ONL thickness compared with those of WT mice.

CONCLUSIONS

The absence of GFAP and vimentin attenuates RD-induced reactive gliosis and, subsequently, limits photoreceptor degeneration. Results of this study indicate that reactive retinal glial cells contribute critically to retinal damage induced by RD and provide a new avenue for limiting photoreceptor degeneration associated with RD and other retinal diseases or damage.

Acute and delayed neuroinflammatory response following experimental penetrating ballistic brain injury in the rat

Background

Neuroinflammation following acute brain trauma is considered to play a prominent role in both the pathological and reconstructive response of the brain to injury. Here we characterize and contrast both an acute and delayed phase of inflammation following experimental penetrating ballistic brain injury (PBBI) in rats out to 7 days post-injury.

Methods

Quantitative real time PCR (QRT-PCR) was used to evaluate changes in inflammatory gene expression from the brain tissue of rats exposed to a unilateral frontal PBBI. Brain histopathology was assessed using hematoxylin and eosin (H&E), silver staining, and immunoreactivity for astrocytes (GFAP), microglia (OX-18) and the inflammatory proteins IL-1β and ICAM-1.

Results

Time course analysis of gene expression levels using QRT-PCR indicated a peak increase during the acute phase of the injury between 3–6 h for the cytokines TNF-α (8–11 fold), IL-1β (11–13 fold), and IL-6 (40–74 fold) as well as the cellular adhesion molecules VCAM (2–3 fold), ICAM-1 (7–15 fold), and E-selectin (11–13 fold). Consistent with the upregulation of pro-inflammatory genes, peripheral blood cell infiltration was a prominent post-injury event with peak levels of infiltrating neutrophils (24 h) and macrophages (72 h) observed throughout the core lesion. In regions of the forebrain immediately surrounding the lesion, strong immunoreactivity for activated astrocytes (GFAP) was observed as early as 6 h post-injury followed by prominent microglial reactivity (OX-18) at 72 h and resolution of both cell types in cortical brain regions by day 7. Delayed thalamic inflammation (remote from the primary lesion) was also observed as indicated by both microglial and astrocyte reactivity (72 h to 7 days) concomitant with the presence of fiber degeneration (silver staining).

Conclusion

In summary, PBBI induces both an acute and delayed neuroinflammatory response occurring in distinct brain regions, which may provide useful diagnostic information for the treatment of this type of brain injury.

Satellite glial cells in sensory ganglia: their possible contribution to inflammatory pain

Neurons in dorsal root ganglia (DRG) are surrounded by an envelope of satellite glial cells (SGCs). Little is known about SGC physiology and their interactions with neurons. In this work, we investigated changes in mouse DRG neurons and SGC following the induction of inflammation in the hind paw by the injection of complete Freund's adjuvant (CFA). The electrophysiological properties of neurons were characterized by intracellular electrodes. Changes in coupling mediated by gap junctions between SGCs were monitored using intracellular injection of the fluorescent dye Lucifer yellow. Pain was assessed with von Frey hairs. We found that two weeks after CFA injection there was a 38% decrease in the threshold for firing an action potential in DRG neurons, consistent with neuronal hyperexcitability. Injection of Lucifer yellow into SGCs revealed that, compared with controls, coupling by gap junctions among SGCs surrounding adjacent neurons increased 2.7-, 3.2-, and 2.5-fold one week, two weeks, and one month, respectively, after CFA injection. In SGCs enveloping neurons that project into the inflamed paw this effect was more enhanced (5.4-fold). Interneuronal coupling was augmented by up to 7% after CFA injection. Pain threshold in the injected paw decreased by 13%, 16%, and 11% compared with controls at one week, two weeks, and one month, respectively, after CFA injection. Intraperitoneal injection of the gap junction blocker carbenoxolone prevented the inflammation-induced decrease in pain threshold. The results show that augmented glial coupling is one of the major events occurring in DRG following inflammation. The elevation in pain threshold after carbenoxolone administration provides indirect support for the idea that augmented intercellular coupling might contribute to chronic pain.

Expression of a functional metabotropic glutamate receptor 5 on enteric glia is altered in states of inflammation

The metabotropic glutamate receptor 5 (mGluR5) is expressed by astrocytes and its expression is modulated by inflammation. Enteric glia have many similarities to astrocytes and are the most numerous cell in the enteric nervous system (ENS). We investigated whether enteric glia express a functional mGluR5 and whether expression of this receptor was altered in colitis. In both enteric plexuses of the ileum and colon of guinea pigs and mice, we observed widespread glial mGluR5 expression. Incubation of isolated segments of the guinea pig ileum with the mGluR5 specific agonist RS-2-chloro-5-hydroxyphenylglycine (CHPG) caused a dose-dependent increase in the glial expression of c-Fos and the phosphorylated form of the extracellular signal-regulated kinase 1/2. Preincubation of tissues with the group I metabotropic glutamate receptor antagonist, S-4-carboxyphenylglycine, abolished the effects of CHPG. We examined mGluR5 expression in the guinea pig trinitrobenzene sulfonic acid and the IL-10 gene-deficient (IL-10(-/-)) mouse models of colitis. In guinea pigs, mGluR5 immunoreactivity became diffusely localized over the colonic myenteric ganglia, suggesting a change in receptor distribution. In contrast, glial mGluR5 expression was significantly reduced in the colonic myenteric plexus of IL-10(-/-) mice, as assessed with both real-time quantitative RT-PCR as well as immunohistochemistry and image analysis. These changes occurred without concomitant changes to enteric ganglia or glial fibrillary acidic protein expression in the IL-10(-/-) mouse. Our data suggest that enteric glia are a functional target of the glutamatergic neurotransmitter system in the ENS and that changes in mGluR5 expression may be of physiological significance during colitis.

Stem cell therapy for autism

Autism spectrum disorders (ASD) are a group of neurodevelopmental conditions whose incidence is reaching epidemic proportions, afflicting approximately 1 in 166 children. Autistic disorder, or autism is the most common form of ASD. Although several neurophysiological alterations have been associated with autism, immune abnormalities and neural hypoperfusion appear to be broadly consistent. These appear to be causative since correlation of altered inflammatory responses, and hypoperfusion with symptology is reported. Mesenchymal stem cells (MSC) are in late phases of clinical development for treatment of graft versus host disease and Crohn's Disease, two conditions of immune dysregulation. Cord blood CD34+ cells are known to be potent angiogenic stimulators, having demonstrated positive effects in not only peripheral ischemia, but also in models of cerebral ischemia. Additionally, anecdotal clinical cases have reported responses in autistic children receiving cord blood CD34+ cells. We propose the combined use of MSC and cord blood CD34+cells may be useful in the treatment of autism.

Extracellular signal-regulated kinase involvement in human astrocyte migration

Glial scar formation occurs after virtually any injury to the brain. The migration of astrocytes into regions of brain injury underlies the formation of the glial scar. The exact role of the glial scar has yet to be elucidated, although it is likely to impair brain recovery. Understanding astrocyte migration is fundamental to understanding the formation of the glial scar. We have used human astrocytes (NT2A cells), derived from human NT2/D1 precursor cells to study astrocyte migration using an in vitro scratch wound model. Time-lapse microscopy and bromodeoxyuridine labeling revealed that the astrocytes migrated rather than proliferated across the scratch. Time course immunocytochemical studies showed that scratching human astrocytes induced the activation (phosphorylation) of ERK 1/2 at 10 min after scratch. The MEK 1/2 inhibitor U0126 inhibited both the ERK 1/2 phosphorylation and the migration of the astrocytes across the wound after scratch. Thus, the migration of human astrocytes after injury is partly initiated by activation of the MEK-ERK signalling pathway.

Neuroprotective effects of estradiol in hippocampal neurons and glia of middle age mice

During aging the hippocampus experiences structural, molecular, and functional alterations. Protection from age-related disorders is provided by several factors, including estrogens. Since aging defects start at middle age, we studied if 17 beta-estradiol (E(2)) protected the hippocampus at this age period. Middle age (10-12 month old) male C57Bl/6 mice were implanted sc with E(2) (15 microg) or cholesterol pellets. Ten days afterwards they received bromodeoxyuridine (BrdU) 4 and 2h before killing to study cell proliferation in the dentate gyrus (DG). A pronounced depletion of BrdU+cells in the DG was found in cholesterol-treated middle age mice, accompanied by astrocytosis, and by neuronal loss in the hilus. Middle age mice receiving E(2) showed increased number of BrdU+cells while the other parameters were remarkably attenuated. When steroid treatment was prolonged for 2 months to study migration of cells in the granular layer of the DG, cell migration was unaffected by E(2). However, E(2)-treated middle age mice presented higher cell density and increased staining for doublecortin, a marker for differentiating neurons. Thus, from the three basic steps of adult neurogenesis (proliferation, migration, and differentiation), E(2) stimulated progenitor proliferation - even after long exposure to E(2) studied by Ki67 immunocytochemistry - and differentiation towards a neuronal lineage. This result, in conjunction with recovery from other aging indicators as increased deposits of the aging pigment lipofuscin in DG cells, loss of hilar neurons and astrocytosis supports a wide range protection of hippocampal function of middle age mice by estrogenic hormones.

Intermediate filaments and stress

Before we can explain why so many closely related intermediate filament genes have evolved in vertebrates, while maintaining such dramatically tissue specific expression, we need to understand their function. The best evidence for intermediate filament function comes from observing the consequences of mutation and mis-expression, primarily in human tissues. Mostly these observations suggest that intermediate filaments are important in allowing individual cells, the tissues and whole organs to cope with various types of stress, in health and disease. Exactly how they do this is unclear and many aspects of cell dysfunction have been associated with intermediate filaments to date. In particular, it is still not clear whether the non-mechanical functions now being attributed to intermediate filaments are primary functions of these structural proteins, or secondary consequences of their function to respond to mechanical stress. We discuss selected situations in which responses to stress are clearly influenced by intermediate filaments.

Time course of changes in pyridoxal 5'-phosphate (vitamin B6 active form) and its neuroprotection in experimental ischemic damage

In the present study, we investigated ischemia-induced changes of pyridoxal 5'-phosphate synthesizing enzyme and degrading enzyme and neuroprotective effects and roles of pyridoxal 5'-phosphate against ischemic damage in the gerbil hippocampal CA1 region. Pyridoxal 5'-phosphate oxidase and pyridoxal phosphate phosphatase immunoreactivities were changed in neurons up to 2 days after ischemia, while 4 days after ischemia their immunoreactivities were expressed in astrocytes. Pyridoxal 5'-phosphate oxidase immunoreactivity and its protein level were highest 12 h after ischemia, while those in pyridoxal phosphate phosphatase were highest 2 days after ischemia. Total activities of these enzymes were changed after ischemia, but specific activities of the enzymes were not altered. Treatment with pyridoxal 5'-phosphate into brains (4 microg/5 microl, i.c.v.) at 30 min before transient ischemia protected about 80% of CA1 pyramidal cells 4 days after ischemia and induced elevation of glutamic acid decarboxylase 67 immunoreactivity in the CA1 region. However, pyridoxal 5'-phosphate treatment into ischemic brains decreased GABA transaminase immunoreactivity in the CA1 region after ischemia. These results indicate that pyridoxal 5'-phosphate may be associated with the inhibitory discharge of GABA in the hippocampal CA1 neurons, and the increased level of GABA may protect hippocampal CA1 pyramidal cells from ischemic damage.

Molecular mechanisms of astrogliosis: new approaches with mouse genetics

Astrocytes are increasingly being recognized as dynamic participants in many aspects of normal central nervous system function. In disease states, reactive astrocytes undergo complex phenotypic changes, generically referred to as astrogliosis. Unraveling the functions of reactive astrocytes and underlying molecular mechanisms is a difficult problem. The use of genetically modified mice is beginning to yield some answers to long-standing questions in the field. What are the functions of reactive astrocytes? What extracellular factors and intracellular signaling mechanisms are responsible for astrocyte activation in various forms of neural injury? In this review we will highlight studies using astrocyte reporter lines for cellular imaging and lineage tracing, as well as gain- and loss-of-function mutations that have begun to shed light on mechanisms of astrogliosis.

Glia: the fulcrum of brain diseases

Neuroglia represented by astrocytes, oligodendrocytes and microglial cells provide for numerous vital functions. Glial cells shape the micro-architecture of the brain matter; they are involved in information transfer by virtue of numerous plasmalemmal receptors and channels; they receive synaptic inputs; they are able to release 'glio'transmitters and produce long-range information exchange; finally they act as pluripotent neural precursors and some of them can even act as stem cells, which provide for adult neurogenesis. Recent advances in gliology emphasised the role of glia in the progression and handling of the insults to the nervous system. The brain pathology, is, to a very great extent, a pathology of glia, which, when falling to function properly, determines the degree of neuronal death, the outcome and the scale of neurological deficit. Glial cells are central in providing for brain homeostasis. As a result glia appears as a brain warden, and as such it is intrinsically endowed with two opposite features: it protects the nervous tissue as long as it can, but it also can rapidly assume the guise of a natural killer, trying to eliminate and seal the damaged area, to save the whole at the expense of the part.

GFAP and its role in Alexander disease

Here we review how GFAP mutations cause Alexander disease. The current data suggest that a combination of events cause the disease. These include: (i) the accumulation of GFAP and the formation of characteristic aggregates, called Rosenthal fibers, (ii) the sequestration of the protein chaperones alpha B-crystallin and HSP27 into Rosenthal fibers, and (iii) the activation of both Jnk and the stress response. These then set in motion events that lead to Alexander disease. We discuss parallels with other intermediate filament diseases and assess potential therapies as part of this review as well as emerging trends in disease diagnosis and other aspects concerning GFAP.

Beta-amyloid toxicity in embryonic rat astrocytes

The senile plaques of Alzheimer's disease contain a high concentration of beta-amyloid (betaA) protein, which may affect the glial population in the septal nucleus, an area of increased risk in AD. BetaA toxicity was measured in septal glia, via a dose-response experiment, by quantifying the effects of three different doses (0.1, 1, and 10 microM) of betaA on cell survival. Astrocytes from embryonic day-16 rats were grown in serum-free media in a single layer culture. Cells were treated on day in vitro (DIV)1 and survival was determined on DIV3 to ascertain which concentration was most toxic. In a separate set of experiments, an attempt was made to protect glial cells from the degenerative effects of betaA, with treatments of growth factors and estrogen. BetaA (10 microM) treatment was administered on DIV1, on DIV2 the cells were treated with estrogen (EST, 10 nM), insulin-like growth factors (IGF1 and IGF2, each 10 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml) or nerve growth factor (NGF, 100 ng/ml), and on DIV3 the cells were visualized and quantified by fluorescence microscopy with DAPI (4,6-diamidino-2-phenylindole). In addition to dose-response and glial protection, experiments were also conducted to determine whether toxic effects were due to apoptosis. Our results suggest that the survival of glial populations is significantly affected in all three concentrations (0.1, 1.0, and 10 microM) of betaA. Glial protection was evident in the presence of NGF, for it showed the significantly highest survival rate relative to the betaA treatment alone. Furthermore, toxic effects of betaA appear to be due primarily to apoptosis. Significant reversal of betaA-induced apoptosis was seen with bFGF and IGF1.

Phosphorylated FTY720 stimulates ERK phosphorylation in astrocytes via S1P receptors

Sphingosine-1-phosphate receptors (S1P1-5) are activated by the endogenous agonist S1P and are expressed in the central nervous system. In astrocytes, activation of S1P receptors leads to phosphorylation of extracellular-signal regulated kinase (ERK), a signaling cascade which plays intimate roles in cell proliferation. Fingolimod (FTY720) is in phase III clinical trials for the treatment of multiple sclerosis and its phosphorylated version (FTY720P) activates S1P receptors. We examined the effects of FTY720P on ERK phosphorylation and determined which S1P receptor subtype(s) mediated this signaling event. FTY720P augmented ERK phosphorylation in cortical cultures prepared from embryonic day 18 rat brains and was blocked by an MEK inhibitor or by pertussis toxin. Co-localisation of phosphorylated ERK occurred in glial fibrillary acidic protein (GFAP) positive astrocytes but not neurons or oligodendrocytes. Furthermore, FTY720P stimulated ERK phosphorylation in highly enriched astrocyte cultures made from postnatal day 2 rat cortices. The effects of FTY720P were mimicked by selective S1P1 receptor agonists and blocked by S1P1 receptor antagonists. Collectively, these results demonstrate that FTY720P mediates ERK phosphorylation in astrocytes via the activation of S1P1 receptors.

The inflammatory response in stroke

Recent works in the area of stroke and brain ischemia has demonstrated the significance of the inflammatory response accompanying necrotic brain injury. Acutely, this response appears to contribute to ischemic pathology, and anti-inflammatory strategies have become popular. This chapter will discuss the current knowledge of the contribution of systemic and local inflammation in experimental stroke. It will review the role of specific cell types including leukocytes, endothelium, glia, microglia, the extracellular matrix and neurons. Intracellular inflammatory signaling pathways such as nuclear factor kappa beta and mitogen-activated protein kinases, and mediators produced by inflammatory cells such as cytokines, chemokines, reactive oxygen species and arachidonic acid metabolites will be reviewed as well as the potential for therapy in stroke and hypoxic-ischemic injury.

Differentiated astrocytes acquire sensitivity to hydrogen sulfide that is diminished by the transformation into reactive astrocytes

Hydrogen sulfide (H2S) enhances the induction of hippocampal long-term potentiation (LTP) and induces calcium waves in astrocytes. Based on these observations, H2S has been proposed to be a synaptic modulator in the brain. Here we show that differentiated astrocytes acquire sensitivity to H2S that is diminished by their transformation into reactive astrocytes. Although sodium hydrosulfide hydrate (NaHS), a donor of H2S, did not increase the intracellular concentration of Ca2+ in progenitors, exposure of progenitors to leukemia inhibitory factor (LIF), which induces differentiation into glial fibrillary acidic protein (GFAP)-positive astrocytes, greatly increased the sensitivity to NaHS. In contrast, epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), dibutyryl cyclic AMP (db cAMP) and interleukin-1beta (IL-1beta) induced the conversion to reactive astrocytes with diminished sensitivity to NaHS. This suppressive effect of EGF on the sensitivity to NaHS was inhibited by cycloheximide, indicating that de novo protein synthesis was required for the suppression of H2S sensitivity.

The scoop on the fly brain: glial engulfment functions in Drosophila

Glial cells provide support and protection for neurons in the embryonic and adult brain, mediated in part through the phagocytic activity of glia. Glial cells engulf apoptotic cells and pruned neurites from the developing nervous system, and also clear degenerating neuronal debris from the adult brain after neural trauma. Studies indicate that Drosophila melanogaster is an ideal model system to elucidate the mechanisms of engulfment by glia. The recent studies reviewed here show that many features of glial engulfment are conserved across species and argue that work in Drosophila will provide valuable cellular and molecular insight into glial engulfment activity in mammals.

Rapamycin is a neuroprotective treatment for traumatic brain injury

The mammalian target of rapamycin, commonly known as mTOR, is a serine/threonine kinase that regulates translation and cell division. mTOR integrates input from multiple upstream signals, including growth factors and nutrients to regulate protein synthesis. Inhibition of mTOR leads to cell cycle arrest, inhibition of cell proliferation, immunosuppression and induction of autophagy. Autophagy, a bulk degradation of sub-cellular constituents, is a process that keeps the balance between protein synthesis and protein degradation and is induced upon amino acids deprivation. Rapamycin, mTOR signaling inhibitor, mimics amino acid and, to some extent, growth factor deprivation. In the present study we examined the effect of rapamycin, on the outcome of mice after brain injury. Our results demonstrate that rapamycin injection 4 h following closed head injury significantly improved functional recovery as manifested by changes in the Neurological Severity Score, a neurobehavioral testing. To verify the activity of the injected rapamycin, we demonstrated that it inhibits p70S6K phosphorylation, reduces microglia/macrophages activation and increases the number of surviving neurons at the site of injury. We therefore suggest that rapamycin is neuroprotective following traumatic brain injury and as a drug used in the clinic for other indications, we propose that further studies on rapamycin should be conducted in order to consider it as a novel therapy for traumatic brain injury.

ROCK inhibition with Y27632 activates astrocytes and increases their expression of neurite growth-inhibitory chondroitin sulfate proteoglycans

Inhibition of Rho-kinase (ROCK) with Y27632 stimulates sprouting by injured corticospinal tract and dorsal column tract axons, and accelerates functional recovery. However, regeneration of these axons across the glial scar was not observed. Here we examined the effects of Y27632 treatment on chondroitin sulfate proteoglycan (CSPG) expression by astrocytes, which are a key component of the reactive gliosis inhibiting axonal regeneration. In vivo, rats underwent a dorsal column transection and were treated with Y27632 via intrathecal pump infusion. Compared with controls, Y27632-treated injury sites displayed exaggerated upregulation of glial fibrillary acid protein and neurocan immunoreactivity along the lesion edge. In vitro, astrocytes assumed a reactive morphology (stellate shape) and increased their expression of CSPGs after Y27632 treatment. Neurite growth by dissociated cortical neurons decreased when cultured on the extracellular matrix (ECM) derived from Y27632-treated astrocytes. This decrease in neurite growth was reversed with chondroitinase-ABC (ChABC) digestion, indicating that the inhibition was due to CSPG depositions within the ECM. Interestingly, conditioned medium (CM) from untreated astrocytes was inhibitory to neurite growth, which was overcome by ChABC digestion. Such inhibitory activity was not found in the CM of Y27632-treated astrocytes. Taken together, these data support a model where ROCK inhibition by Y27632 modifies astrocytic processing of CSPGs, and increases the presence of CSPGs within the ECM while reduces CSPGs in the CM (cerebrospinal fluid in vivo). This increased expression of inhibitory CSPGs in the ECM of the glial scar may counteract the growth promoting effects of ROCK inhibition on axonal growth cones.

Blood–retinal barrier disruption and ultrastructural changes in the hypoxic retina in adult rats: the beneficial effect of melatonin administration

Reactive changes in astrocytes and Müller cells in the retina of adult rats subjected to hypoxia were investigated. Along with this, the integrity of the blood-retinal barrier (BRB) was assessed using fluorescent and electron-dense tracers. In hypoxic rats, mRNA and protein expression of glial fibrillary acidic protein (GFAP) and aquaporin-4 (AQ4) were significantly increased. AQ4 immunoreactive cells were identified as astrocytes and Müller cells by double immunofluorescence labelling. Another alteration in the hypoxic retina was marked reduction in melatonin content compared to controls. In this connection, administration of exogenous melatonin reduced the tissue concentration of vascular endothelial growth factor (VEGF) and nitric oxide (NO); both were elevated in hypoxic rats. A major structural change in the hypoxic retina was swelling of astrocyte and Müller cell processes but this was noticeably attenuated after melatonin administration. Following an intraperitoneal or intravenous injection of rhodamine isothiocyanate (RhIC) or horseradish peroxidase (HRP), leakage of both tracers was observed in the retina in hypoxic rats but not in the controls, indicating that the functional integrity of the BRB is compromised in hypoxia/reoxygenation. It is suggested that enhanced tissue concentration of VEGF and NO production in the hypoxic retina contribute to increased permeability of the retinal blood vessels. The concurrent up-regulation of AQ4, a water-transporting protein, in astrocytes and Müller cells in hypoxia suggests its involvement in oedema formation. Since melatonin effectively reduced the vascular permeability in the retina of hypoxic rats, as evidenced by reduced leakage of RhIC, we suggest that its administration may be of potential benefit in the management of retinal oedema associated with retinal hypoxia.

The cellular response in neuroinflammation: The role of leukocytes, microglia and astrocytes in neuronal death and survival

Neuroinflammation is a complex integration of the responses of all cells present within the CNS, including the neurons, macroglia, microglia and the infiltrating leukocytes. The initiating insult, environmental factors, genetic background and age/past experiences all combine to modulate the integrated response of this complex neuroinflammatory circuit. Here, we explore how these factors interact to lead to either neuroprotective versus neurotoxic inflammatory responses. We specifically focus on microglia and astrocytic regulation of autoreactive T cell responses.

Inflammation in adult and neonatal stroke

This chapter will discuss the current knowledge of the contribution of systemic and local inflammation in acute and sub-chronic stages of experimental stroke in both the adult and neonate. It will review the role of specific cell types and interactions among blood cells, endothelium, glia, microglia, the extracellular matrix and neurons - cumulatively called "neurovascular unit" - in stroke induction and evolution. Intracellular inflammatory signaling pathways such as nuclear factor kappa beta and mitogen-activated protein kinases, and mediators produced by inflammatory cells such as cytokines, chemokines, reactive oxygen species and arachidonic acid metabolites, as well as the modifying role of age on these mechanisms, will be reviewed as well as the potential for therapy in stroke and hypoxic-ischemic injury.

Delayed neurodegeneration and early astrogliosis after excitotoxicity to the aged brain

Excitotoxicity is well recognised as a mechanism underlying neuronal cell death in several brain injuries. To investigate age-dependent differences in neurodegeneration, edema formation and astrogliosis, intrastriatal N-methyl-d-aspartate injections were performed in young (3 months) and aged (22-24 months) male Wistar rats. Animals were sacrificed at different times between 12h and 14 days post-lesion (DPL) and cryostat sections were processed for Toluidine blue, Fluoro-Jade B staining, NeuN and GFAP immunohistochemistry. Our results show that both size of tissue injury and edema were reduced in the old subjects only up to 1DPL, correlating with a slower progression of neurodegeneration with peak numbers of degenerating neurons at 3DPL in the aged, contrasting with maximum neurodegeneration at 1DPL in the young. However, old animals showed an earlier onset of astroglial response, seen at 1DPL, and a larger area of astrogliosis at all time-points studied, including a greater glial scar. In conclusion, after excitotoxic striatal damage, progression of neurodegeneration is delayed in the aged but the astroglial response is earlier and exacerbated. Our results emphasize the importance of using aged animals and several survival times for the study of acute age-related brain insults.

Identification of ganglion cell neurites in human subretinal and epiretinal membranes

AIM

To determine whether neural elements are present in subretinal and epiretinal proliferative vitreoretinopathy (PVR) membranes as well as in diabetic, fibrovascular membranes removed from patients during vitrectomy surgery.

METHODS

Human subretinal and epiretinal membranes of varying durations were immunolabelled with different combinations of antibodies to glial fibrillary acidic protein, vimentin, neurofilament protein and laminin.

RESULTS

Anti-neurofilament-labelled neurites from presumptive ganglion cells were frequently found in epiretinal membranes and occasionally found in subretinal membranes. In addition, the neurites were only observed in regions that also contained glial processes.

CONCLUSIONS

These data demonstrate that neuronal processes are commonly found in human peri-retinal cellular membranes similar to that demonstrated in animal models. These data also suggest that glial cells growing out of the neural retina form a permissive substrate for neurite growth and thus may hold clues to factors that support this growth.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

Brain keratan sulfate and glial scar formation

In response to injury to the central nervous system (CNS), reactive astrocytes appear and accumulate in the wounded area, leading to glial scar formation. Glial scar is the physical barrier to axonal regeneration of injured neurons. Chondroitin sulfate proteoglycans are inhibitory to axon outgrowth and are upregulated in reactive astrocytes upon CNS injury. It is known that keratan sulfate proteoglycans (KSPGs) are also augmented after CNS injury and act as inhibitory cues. We give a brief overview of CNS injury and cover our recent data regarding the relationship between glial scar formation and KS. KS expression in the developing brain is detectable with 5D4, a KS-specific monoclonal antibody. These 5D4 immunoreactivities are eliminated in mice deficient in N-acetylglucosamine 6-O-sulfotransferase-1. In adult mice, brain injury apparently upregulates mRNA expression of N-acetylglucosamine 6-O-sulfotransferase-1 as well as 5D4-reactive KS in the wounded area. Intriguingly, the expression of 5D4-reactive KS and reactive astrocyte accumulation in the wounded area are dramatically diminished in the sulfotransferase-deficient mice. Consequently, the deficient mice exhibit a marked reduction in scar formation and enhancement of neuronal regeneration after brain injury. Thus, N-acetylglucosamine 6-O-sulfotransferase-1 plays indispensable roles in brain KS biosynthesis and glial scar formation after brain injury.

14-3-3γ affects dynamics and integrity of glial filaments by binding to phosphorylated GFAP

Recent findings indicated a protective role of GFAP in ischemic brain, injured spinal cord, and in neurodegenerative disease. We previously demonstrated that 14-3-3γ, once thought to be neuronal specific, was up-regulated by ischemia in astrocytes and may play a specific protective role in astrocytes. Here we report that 14-3-3γ associates with both soluble and filamentous GFAP in a phosphorylation- and cell-cycle-dependent manner in primary cultured astrocytes. The amount of association increases during G2/M phase due to more phosphorylated GFAP. Moreover, this interaction is independent of vimentin, another type III intermediate filament protein in astrocytes which forms glial filaments with GFAP. A series of domain deletion mutants and substitution mutations at phosphorylation sites (from serine to alanine) on GFAP demonstrated that serine 8 in the head domain is essential for the direct association of GFAP to 14-3-3γ. Overexpression of 14-3-3γ destroyed the integrity and affected the movement of GFAP intermediate filaments. This data demonstrates that 14-3-3γ contributes to the regulation of dynamics of GFAP filaments, which may contribute to the stability of the cytoskeleton and the mechanisms of central nervous system neurodegenerative disease.

Cytoskeleton and Vesicle Mobility in Astrocytes

Exocytotic vesicles in astrocytes are increasingly viewed as essential in astrocyte-to-neuron communication in the brain. In neurons and excitable secretory cells, delivery of vesicles to the plasma membrane for exocytosis involves an interaction with the cytoskeleton, in particular microtubules and actin filaments. Whether cytoskeletal elements affect vesicle mobility in astrocytes is unknown. We labeled single vesicles with fluorescent atrial natriuretic peptide and monitored their mobility in rat astrocytes with depolymerized microtubules, actin, and intermediate filaments and in mouse astrocytes deficient in the intermediate filament proteins glial fibrillary acidic protein and vimentin. In astrocytes, as in neurons, microtubules participated in directional vesicle mobility, and actin filaments played an important role in this process. Depolymerization of intermediate filaments strongly affected vesicle trafficking and in their absence the fraction of vesicles with directional mobility was reduced.

Methamphetamine induces long-term changes in GABAA receptor alpha2 subunit and GAD67 expression

The present study investigated whether GABA(A) receptor alpha2 subunit and GAD(67) are involved in chronic high dose methamphetamine (METH)-induced sensitization and neurotoxicity. The METH sensitization was established in rats by 7-day pump infusion plus daily injection (25mg/kg/day) and a subsequent 28-day withdrawal period. Behavioral sensitization was assessed by behavioral ratings after challenge with METH (0.5mg/kg). The neurotoxicity was evaluated by the expression of glial fibrillary acidic protein (GFAP). Western blot assay showed that METH sensitization decreases GABA(A) alpha2 subunit and GAD(67) protein levels in the nucleus accumbens (NAc) core and shell, and conversely, these proteins were increased in the caudate. An upregulation of GFAP expression was observed in the caudate, but not in the NAc core and shell. These data suggest that inhibition of GABA transmission in the NAc is related to METH behavioral sensitization, whereas activation of GABA transmission in the caudate is associated with METH-induced neurotoxicity.

High extracellular K(+) evokes changes in voltage-dependent K(+) and Na (+) currents and volume regulation in astrocytes

[K(+)](e) increase accompanies many pathological states in the CNS and evokes changes in astrocyte morphology and glial fibrillary acidic protein expression, leading to astrogliosis. Changes in the electrophysiological properties and volume regulation of astrocytes during the early stages of astrocytic activation were studied using the patch-clamp technique in spinal cords from 10-day-old rats after incubation in 50 mM K(+). In complex astrocytes, incubation in high K(+) caused depolarization, an input resistance increase, a decrease in membrane capacitance, and an increase in the current densities (CDs) of voltage-dependent K(+) and Na(+) currents. In passive astrocytes, the reversal potential shifted to more positive values and CDs decreased. No changes were observed in astrocyte precursors. Under hypotonic stress, astrocytes in spinal cords pre-exposed to high K(+) revealed a decreased K(+) accumulation around the cell membrane after a depolarizing prepulse, suggesting altered volume regulation. 3D confocal morphometry and the direct visualization of astrocytes in enhanced green fluorescent protein/glial fibrillary acidic protein mice showed a smaller degree of cell swelling in spinal cords pre-exposed to high K(+) compared to controls. We conclude that exposure to high K(+), an early event leading to astrogliosis, caused not only morphological changes in astrocytes but also changes in their membrane properties and cell volume regulation.

Astroglial cells in primary culture: A valid model to study Neospora caninum infection in the CNS

The protozoan Neospora caninum has a veterinary importance because it causes abortion in cattle and neuromuscular alterations in dogs. We infected rat astrocytes, in vitro, with different concentrations of N. caninum. Astrocytes responded to infection by producing the pro-inflammatory cytokine TNF-alpha and the neurotoxic-free radical NO, 24 and 72 h post-infection. These data suggest that astrocytes, which are essential for brain function, are targets for the parasite and this represents a practical and valid model to study the effects of N. caninum on the CNS.

Long-term gliosis around chronically implanted platinum electrodes in the Rhesus macaque motor cortex

Chronically implanted microelectrodes have been an important tool used by neuroscientists for many years and are critical for the development of neural prostheses designed to restore function after traumatic central nervous system (CNS) injury. It is well established that a variety of mammals, including non-human primates (NHP), tolerate noble metal electrodes in the cortex for extended periods of time, but little is known about the long-term effects of electrode implantation at the cellular level. While data from rodents have clearly shown gliosis around such implants, there have been difficulties in demonstrating these reactions in NHP. Glial reactions are to be expected in NHP, since any trauma to the mammalian CNS is believed to result in the formation of a glial scar consisting of reactive astrocytes and microglia around the injury site. Because a glial scar can potentially affect the quality of recordings or stimulations from implanted electrodes, it is important to determine the extent of gliosis around implants in NHP. We studied the response of cortical glial cells to chronic electrode implantation in the motor cortices of Rhesus macaques (Macaca mulatta) after 3 months and 3 years duration. Antibodies specific for astrocytes and microglia were used to detect the presence of glial reactions around electrode implant sites. Reactive glia were found within the cortical neuropil surrounding the chronically implanted noble metal electrodes. Reactive gliosis persisted over the time periods studied and demonstrates the importance of developing strategies to minimize this event, even around noble metal implants.

Inducible gene deletion in astroglia and radial glia--a valuable tool for functional and lineage analysis

Astrocytes are thought to play a variety of key roles in the adult brain, such as their participation in synaptic transmission, in wound healing upon brain injury, and adult neurogenesis. However, to elucidate these functions in vivo has been difficult because of the lack of astrocyte-specific gene targeting. Here we show that the inducible form of Cre (CreERT2) expressed in the locus of the astrocyte-specific glutamate transporter (GLAST) allows precisely timed gene deletion in adult astrocytes as well as radial glial cells at earlier developmental stages. Moreover, postnatal and adult neurogenesis can be targeted at different stages with high efficiency as it originates from astroglial cells. Taken together, this mouse line will allow dissecting the molecular pathways regulating the diverse functions of astrocytes as precursors, support cells, repair cells, and cells involved in neuronal information processing.

Activation of the intrinsic cell death pathway, increased apoptosis and modulation of astrocytes in the cerebellum of diabetic rats

Poorly controlled diabetes mellitus results in structural and functional changes in many brain regions. We demonstrate that in streptozotocin-induced diabetic rats cell death is increased and proliferation decreased in the cerebellum, indicating overall cell loss. Levels of both the proform and cleaved forms of caspases 3, 6 and 9 are increased, with no change in caspases 7, 8 or 12. Colocalization of glial fibrillary acidic protein (GFAP) and cleaved caspase 3 and GFAP in TUNEL-positive cells increased in diabetic rats. Changes in GFAP levels paralleled modifications in proliferating cell nuclear antigen (PCNA), increasing at 1 week of diabetes and decreasing thereafter, and proliferating GFAP-positive cells were decreased in the cerebellum of diabetic rats. These results suggest that astrocytes are dramatically affected in the cerebellum, including an increase in cell death and a decrease in proliferation, and this could play a role in the structural and functional changes in this brain area in diabetes.

Signals for the induction of nitric oxide synthase in astrocytes

Nitric oxide (NO), being a double-edged sword depending on its concentration in the microenvironment, is involved in both physiological and pathological processes of many organ systems including brain and spinal cord. It is now well-documented that once inducible nitric oxide synthase (iNOS) is expressed in CNS in a signal-dependent fashion, NO in excess of physiological thresholds is produced and this excess NO then plays a role in the pathogenesis of stroke, demyelination and other neurodegenerative diseases. Therefore, a keen interest has been generated in recent years in comprehending the regulation of this enzyme in brain cells. The present review summarizes our current understanding of signaling mechanisms leading to transcription of the iNOS gene in activated astrocytes. We attempt this comprehension with a hope to identify potential targets to intervene NO-mediated CNS disorders.

Interactions between Purkinje neurones and Bergmann glia

Throughout the development of the cerebellar cortex, Purkinje neurones interact closely with Bergmann glial cells, a specialized form of astrocyte. This review summarizes the intimate developmental, anatomical and functional relationships between these two cell types, with particular emphasis on recent discoveries regarding glutamate release from climbing and parallel fibres as a pathway for signalling synaptic activity to Bergmann glia.

Unilateral induction of progenitors in the spinal cord of hSOD1G93A transgenic rats correlates with an asymmetrical hind limb paralysis

Transgenic rats expressing a mutated form of the human Cu/Zn superoxide dismutase (hSOD1(G93A)) develop an amyotrophic lateral sclerosis (ALS)-like phenotype, including motor neurone degeneration and reactive gliosis in the spinal cord. This study aimed at examining the presence of endogenous neural progenitors in the lumbar spinal cord of these rats at the end-stage of the disease. Immunohistochemical data clearly demonstrated the induced expression of the stem cell factor reported as a chemoattractant and survival factor for neural stem cells as well as nestin (neuro-epithelial stem cell intermediate filament) in the spinal cord sections. While the stem cell factor immunolabelling appeared diffuse throughout the gray matter, nestin labelling was restricted to clusters within the ventral horn. Interestingly, as paralysis regularly develops asymmetrically, induction of nestin was only detected on the ipsilateral side of the predominant symptoms. Finally, immunohistochemical detection of the stem cell factor receptor (c-Kit) revealed its specific induction which coincided with nestin immunolabelling. Together, these results are indicative of endogenous recruitment of neural progenitors within lesioned tissues and could support the development of treatments involving endogenous or exogenous stem cells.

Gene expression profiling reveals the profound upregulation of hypoxia-responsive genes in primary human astrocytes

Oxygen is vital for the development and survival of mammals. In response to hypoxia, the brain initiates numerous adaptive responses at the organ level as well as at the molecular and cellular levels, including the alteration of gene expression. Astrocytes play critical roles in the proper functioning of the brain; thus the manner in which astrocytes respond to hypoxia is likely important in determining the outcome of brain hypoxia. Here, we used microarray gene expression profiling and data-analysis algorithms to identify and analyze hypoxia-responsive genes in primary human astrocytes. We also compared gene expression patterns in astrocytes with those in human HeLa cells and pulmonary artery endothelial cells (ECs). Remarkably, in astrocytes, five times as many genes were induced as suppressed, whereas in HeLa and pulmonary ECs, as many as or more genes were suppressed than induced. More genes encoding hypoxia-inducible functions, such as glycolytic enzymes and angiogenic growth factors, were strongly induced in astrocytes compared with HeLa cells. Furthermore, gene ontology and computational algorithms revealed that many target genes of the EGF and insulin signaling pathways and the transcriptional regulators Myc, Jun, and p53 were selectively altered by hypoxia in astrocytes. Indeed, Western blot analysis confirmed that two major signal transducers mediating insulin and EGF action, Akt and MEK1/2, were activated by hypoxia in astrocytes. These results provide a global view of the signaling and regulatory network mediating oxygen regulation in human astrocytes.

Minocycline treatment prevents cavitation in rats after a cortical devascularizing lesion

Minocycline, a second-generation tetracycline, has been shown to possess neuroprotective effects in animal models of stroke. Pial vessel disruption in adult Wistar rats leads to a cone-shaped cortical lesion and turns into a fluid-filled cavity surrounded by a GFAP+ glia limitans 21 days after injury. This mimics the clinical situation in lacunar infarcts. Minocycline was given intraperitoneally at a dose of 45 mg/kg 1 and 12 h after lesioning, followed by 22.5 mg/kg twice daily until 6 days after lesioning. Control rats received intraperitoneal injections of equivalent volumes of saline. Cavitation was prevented in five out of six minocycline-treated animals and the glia limitans did not appear as the space was filled with GFAP+ reactive astrocytes. However, the number of activated microglia showed no difference between minocycline-treated and -untreated groups. Minocycline did not reduce the number of infiltrating leukocytes, predominately polymorphonuclear neutrophils (PMNs) determined by myeloperoxidase immunoreactivity, or infiltration of CD3+ lymphocytes. The pial vessel occlusion induced a significant upregulation of IL-1beta expression; however, minocycline treatment did not significantly alter this upregulation of IL-1beta. In this study, we found minocycline facilitated the repopulation of the lesion by reactive astrocytes and therefore prevented cavitation; however, we could not identify the molecular signal.

Effects of lipopolysaccharide on glial phenotype and activity of glutamate transporters: Evidence for delayed up-regulation and redistribution of GLT-1

Excitatory amino acid transporters (EAATs) are responsible for homeostasis of extracellular L-glutamate, and the glial transporters are functionally dominant. EAAT expression or function is altered in acute and chronic neurological conditions, but little is known about the regulation of EAATs in reactive astroglia found in such neuropathologies. These studies examined the effects of the bacterial endotoxin lipopolysaccharide (LPS) on glial EAATs in vitro. The effects of LPS (1 microg/ml, 24-72 h) on EAAT activity and expression were examined in primary cultures of mouse astrocytes. [(3)H]D-aspartate uptake increased to 129% of control by 72 h treatment with LPS. Saturation analysis revealed that apparent K(m) was unchanged whilst V(max) was significantly increased to 172% of control by 72 h LPS treatment. Biotinylation and Western blotting indicated that cell-surface expression of GLT-1 was significantly elevated (146% control) by LPS treatment whereas GLAST expression was unchanged. Confocal analyses revealed that LPS treatment resulted in cytoskeletal changes and stellation of astrocytes, with rearrangement of F-actin (as shown by phalloidin labelling). Immunocytochemistry revealed clustering of GLAST, and increased expression and redistribution of GLT-1 to the cell-surface following treatment with LPS. Similar experiments were conducted in microglia, where LPS (50 ng/ml) was found to up-regulate expression of GLT-1 at 24 and 72 h in concert with cytoskeletal changes accompanying activation. These findings suggest an association of cytoskeletal changes in glia with EAAT activity, with the predominant adaptation involving up-regulation and redistribution of GLT-1.

Sustained astrocytic clusterin expression improves remodeling after brain ischemia

Clusterin is a glycoprotein highly expressed in response to tissue injury. Using clusterin-deficient (Clu-/-) mice, we investigated the role of clusterin after permanent middle cerebral artery occlusion (MCAO). In wild-type (WT) mice, clusterin mRNA displayed a sustained increase in the peri-infarct area from 14 to 30 days post-MCAO. Clusterin transcript was still present up to 90 days post-ischemia in astrocytes surrounding the core infarct. Western blot analysis also revealed an increase of clusterin in the ischemic hemisphere of WT mice, which culminates up to 30 days post-MCAO. Concomitantly, a worse structural restoration and higher number of GFAP-reactive astrocytes in the vicinity of the infarct scar were observed in Clu-/- as compared to WT mice. These findings go beyond previous data supporting a neuroprotective role of clusterin in early ischemic events in that they demonstrate that this glycoprotein plays a central role in the remodeling of ischemic damage.

ADAMTS-1 and -4 are up-regulated following transient middle cerebral artery occlusion in the rat and their expression is modulated by TNF in cultured astrocytes

ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) enzymes are a recently described group of metalloproteinases. The substrates degraded by ADAMTS-1, -4 and -5 suggest that they play a role in turnover of extracellular matrix in the central nervous system (CNS). ADAMTS-1 is also known to exhibit anti-angiogenic activity. Their main endogenous inhibitor is tissue inhibitor of metalloproteinases (TIMP)-3. The present study was designed to investigate ADAMTS-1, -4 and -5 and TIMP-3 expression after experimental cerebral ischaemia and to examine whether cytokines known to be up-regulated in stroke could alter their expression by astrocytes in vitro. Focal cerebral ischaemia was induced by transient middle cerebral artery occlusion in the rat using the filament method. Our results demonstrate a significant increase in expression of ADAMTS-1 and -4 in the occluded hemisphere but no significant change in TIMP-3. This was accompanied by an increase in mRNA levels for interleukin (IL)-1beta, IL-1 receptor antagonist (IL-1ra) and tumour necrosis factor (TNF). ADAMTS-4 mRNA and protein were up-regulated by TNF in primary human astrocyte cultures. The increased ADAMTS-1 and -4 in experimental stroke, together with no change in TIMP-3, may promote ECM breakdown after stroke, enabling infiltration of inflammatory cells and contributing to brain injury. In vitro studies suggest that the in vivo modulation of ADAMTS-1 and -4 may be controlled in part by TNF.

Caspase-mediated cleavage of glial fibrillary acidic protein within degenerating astrocytes of the Alzheimer's disease brain

Recent studies demonstrate roles for activation of caspases and cleavage of cellular proteins within neurons of the Alzheimer's disease (AD) brain. To determine whether a similar role for caspases also occurs within glial cells in AD, we designed a site-directed caspase-cleavage antibody specific to glial fibrillary acidic protein (GFAP), a cytoskeleton protein specifically expressed in astrocytes. In vitro characterization of this antibody using both a cell-free system and a cell model system of apoptosis demonstrated that the antibody (termed GFAP caspase-cleavage product antibody or GFAP-CCP Ab) immunolabeled the predicted caspase-cleavage fragment, but not full-length GFAP, by Western blot analysis. To determine whether caspases cleave GFAP in vivo, tissue sections from control and AD brains were examined by immunohistochemistry using the GFAP-CCP Ab. Two prominent features of staining were evident: immunolabeling of degenerating astrocytes in proximity to blood vessels and staining within plaque-rich regions of the AD brain. Furthermore, co-localization of the GFAP-CCP Ab and an antibody specific to active caspase-3 was demonstrated within damaged astrocytes of the AD brain. These data suggest that the activation of caspases and cleavage of cellular proteins such as GFAP may contribute to astrocyte injury and damage in the AD brain.

Human astrocytes are resistant to Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis

Human astrocytes express Fas yet are resistant to Fas-induced apoptosis. Here, we report that calcium/calmodulin-dependent protein kinase II (CaMKII) is constitutively activated in human astrocytes and protects the cells from apoptotic stimulation by Fas agonist. Once stimulated, Fas recruits Fas-associated death domain and caspase-8 for the assembly of the death-inducing signaling complex (DISC); however, caspase-8 cleavage is inhibited in the DISC. Inhibition of CaMKII kinase activity inhibits the expression of phosphoprotein enriched astrocytes-15 kDa/phosphoprotein enriched in diabetes (PEA-15/PED) and cellular Fas-associated death domain-like interleukin-1beta-converting enzyme-inhibitory protein (c-FLIP), thus releasing their inhibition of caspase-8 cleavage. Inhibition of PEA-15/PED or c-FLIP by small interfering RNA sensitizes human astrocytes to Fas-induced apoptosis. In contrast, inhibition of CaMKII, PEA-15, or c-FLIP does not affect the sensitivity of human astrocytes to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL death receptors (DR4, DR5) are weakly expressed at mRNA, protein, and cell surface levels and thus fail to mediate the assembly of the DISC in human astrocytes. Overexpression of DR5 restores TRAIL signaling pathways and sensitizes the human astrocytes to TRAIL-induced apoptosis if CaMKII kinase activity or expression of PEA-15 and c-FLIP is inhibited; the results suggest that CaMKII-mediated pathways prevent TRAIL-induced apoptosis in human astrocytes under conditions in which TRAIL death receptors are upregulated. This study has therefore identified the molecular mechanisms that protect normal human astrocytes from apoptosis induced by Fas ligand and TRAIL.

Streptozotocin-induced diabetes causes astrocyte death after ischemia and reperfusion injury

Diabetes exacerbates neuronal cell death induced by cerebral ischemia. One contributing factor is enhanced acidosis during ischemia. Astrocytes are vulnerable to hypoxia under acidic conditions in vitro and may be targets of ischemia under diabetic conditions. The objective of this study was to determine whether diabetes would cause damage to astrocytes after an ischemic brain injury in vivo. Diabetic and nondiabetic rats were subjected to 5 min of forebrain ischemia and followed by 30 min, 6 h, or 1 or 3 days of recovery. The results showed that ischemia caused activation of astrocytes in nondiabetic rats. In contrast, diabetes caused astrocyte activation in early stage of reperfusion and astrocyte death in late stage of reperfusion. Remarkable astrocyte death was preceded by increased DNA oxidation. Further studies revealed that increased astrocyte damage coincided with enhanced production of free radicals. These data suggest that hyperglycemic ischemia worsens outcome in astrocytes, as it does in neurons.

7beta-hydroxycholesterol reduces the extent of reactive gliosis caused by iron deposition in the hippocampus but does not attenuate the iron-induced seizures in rats

7beta-Hydroxycholesterol has been previously demonstrated to inhibit astrocytosis in injured cortex or spinal cord of rats. In this study, we explored the inhibitory effects of the liposome containing 7beta-hydroxycholesterol on the reactive astrocytosis caused by the injection of iron into the hippocampus of rats and furthermore evaluated the involvement of reactive astrocytosis in iron-induced epilepsy. Injection of ferric chloride solution unilaterally into the hippocampus of rats induced spontaneous spiking activity ipsilaterally then developed into bilateral hippocampi and generalized convulsive seizures within the first week post-operation, and spontaneous epileptiform activity and generalized seizures lasted as long as 2 weeks post-operation, whereas none of the rats injected with sodium chloride solution unilaterally into the hippocampus developed generalized seizures. With immunohistochemistry and Western blot analyses, apparent reactive astrocytosis in bilateral hippocampi was detected using antibody against glial fibrillary acidic protein 14 days after the injection of ferric chloride solution, but no significant differences were found in the amount of synaptophysin protein, a presynaptic vesicle protein, as compared with the rats injected with sodium chloride solution. Infusion of liposome suspension containing 7beta-hydroxycholesterol into the same site immediately after the injection of ferric chloride solution reduced the extent of the reactive astrocytosis by 50%-55% of the amount of glial fibrillary acidic protein in the hippocampi of both hemispheres, and non-significantly elevated the amount of synaptophysin protein in both sides of hippocampus. However, these effects did not significantly modify the seizure latency and the incidence of generalized seizures in the rats. These findings demonstrate the effects of 7beta-hydroxycholesterol on the inhibition of reactive astrocytosis caused by iron deposition in the hippocampus of rats, and suggest that the reactive astrocytosis may not play a causal role in the development of iron-induced seizures.

Motheaten (me/me) mice deficient in SHP-1 are less susceptible to focal cerebral ischemia

We have demonstrated previously that the protein tyrosine phosphatase SHP-1 seems to play a role in glial development and is upregulated in non-dividing astrocytes after injury. The present study examines the effect of loss of SHP-1 on the CNS response to permanent focal ischemia. SHP-1 deficient (me/me) mice and wild-type littermates received a permanent middle cerebral artery occlusion (MCAO). At 1, 3, and 7 days after MCAO, infarct volume, neuronal survival and cell death, gliosis, and inflammatory cytokine levels were quantified. SHP-1 deficient me/me mice display smaller infarct volumes at 7 days post-MCAO, increased neuronal survival within the ischemic penumbra, and decreased numbers of cleaved caspase 3+ cells within the ischemic core compared with wild-type mice. In addition, me/me mice exhibit increases in GFAP+ reactive astrocytes, F4-80+ microglia, and a concomitant increase in the level of interleukin 12 (IL-12) over baseline compared with wild-type. Taken together, these results demonstrate that loss of SHP-1 results in greater healing of the infarct due to less apoptosis and more neuronal survival in the ischemic core and suggests that pharmacologic inactivation of SHP-1 may have potential therapeutic value in limiting CNS degeneration after ischemic stroke.

Reactive astrogliosis induces astrocytic differentiation of adult neural stem/progenitor cells in vitro

Neural stem cells reside in defined areas of the adult mammalian brain, including the dentate gyrus of the hippocampus. Rat neural stem/progenitor cells (NSPCs) isolated from this region retain their multipotency in vitro and in vivo after grafting into the adult brain. Recent studies have shown that endogenous or grafted NSPCs are activated after an injury and migrate toward lesioned areas. In these areas, reactive astrocytes are present and secrete numerous molecules and growth factors; however, it is not currently known whether reactive astrocytes can influence the lineage selection of NSPCs. We investigated whether reactive astrocytes could affect the differentiation, proliferation, and survival of adult NSPCs by modelling astrogliosis in vitro, using mechanical lesion of primary astrocytes. Initially, it was found that conditioned medium from lesioned astrocytes induced astrocytic differentiation of NSPCs without affecting neuronal or oligodendrocytic differentiation. In addition, NSPCs in coculture with lesioned astrocytes also displayed increased astrocytic differentiation and some of these NSPC-derived astrocytes participated in glial scar formation in vitro. When proliferation and survival of NSPCs were analyzed, no differential effects were observed between lesioned and nonlesioned astrocytes. To investigate the molecular mechanisms of the astrocyte-inducing activity, the expression of two potent inducers of astroglial differentiation, ciliary neurotrophic factor and leukemia inhibitory factor, was analyzed by Western blot and shown to be up-regulated in conditioned medium from lesioned astrocytes. These results demonstrate that lesioned astrocytes can induce astroglial differentiation of NSPCs and provide a mechanism for astroglial differentiation of these cells following brain injury.