New roles for astrocytes: redefining the functional architecture of the brain

Cholinergic action on cortical glial cells in vivo

This study aims at understanding complex interactions between cortical neurons, glia and blood supply developing during the transition from slow-wave sleep to wakefulness. In spite of essential advances from in vitro and culture preparations, the basic mechanisms of glial interactions with their cellular and ionic environment had remained uninvestigated in vivo. Here we approach this issue by performing simultaneous intracellular recordings of cortical neurons and glia, together with measurements of cerebral blood flow (CBF), extracellular K+ concentrations and local field potentials in both anesthetized (ketamine-xylazine) and naturally behaving cats. Under anesthesia, cortical activation was elicited with electric stimulation of cholinergic nuclei (pedunculopontine tegmental in the brainstem and/or nucleus basalis in the basal forebrain). Iontophoretic application of acetylcholine on the recorded cells was also used. In the vast majority of cases (> 80%) glial cells were hyperpolarized during electric stimulation or spontaneous activation. This result was also obtained in all cases where iontophoresis was used or when glutamatergic kainate/quisqualate receptors were blocked with 6-cyano-7-nitroquinoxaline-2,3-dione. The glial hyperpolarization was associated with steady neuronal depolarization, increased CBF, lower extracellular K+ concentration, increased membrane resistance, decreased membrane capacitance and persistent positive DC field potentials. In some cases of cortical activation (< 20%), glial cells displayed sustained depolarizing potentials, in parallel with neuronal depolarization, decreased CBF and more negative DC field potentials. The above-mentioned effects of cholinergic activation were blocked by the muscarinic antagonist scopolamine. We propose that the glial response to cholinergic activation results from the balance between the direct hyperpolarizing action of acetylcholine and the depolarizing modulation of glutamate from the neighboring neurons, in addition to the modulation of the interglial communication pathway and/or the ionic traffic across blood vessels.

Morphological Alterations and Cell Death Provoked by the Branched-Chain α-Amino Acids Accumulating in Maple Syrup Urine Disease in Astrocytes from Rat Cerebral Cortex

1. Maple syrup urine disease (MSUD) is an inherited metabolic disorder predominantly characterized by neurological dysfunction and cerebral atrophy whose patophysiology is poorly known. 2. We investigated here whether the branched-chain amino acids (BCAA) leucine (Leu), isoleucine (Ile) and valine (Val), which are the biochemical hallmark of this disorder, could alter astrocyte morphology and cytoskeleton reorganization by exposing cultured astrocytes from cerebral cortex of neonatal rats to various concentrations of the amino acids. A change of cell morphology from the usual polygonal to the appearance of fusiform or process-bearing cells was caused by the BCAA. Cell death was also observed when astrocytes were incubated in the presence of BCAA for longer periods. 3. Val-treated astrocytes presented the most dramatic morphological alterations. Immunocytochemistry with anti-actin and anti-GFAP antibodies revealed that all BCAA induced reorganization of actin and GFAP cytoskeleton. In addition, lysophosphatidic acid, an activator of RhoA GTPase pathway, was able to totally prevent the morphological alterations and cytoskeletal reorganization induced by Val, indicating that the RhoA signaling pathway was involved in these effects. 4. Furthermore, creatine attenuated the morphological alterations provoked by the BCAA, the protection being more pronounced for Val, suggesting that impairment of energy homeostasis is partially involved in BCAA cytotoxic action. The data indicate that the BCAA accumulating in MSUD are toxic to astrocyte cells, a fact that may be related to the pathogenesis of the neurological dysfunction of MSUD patients.

Neuroglial metabolism in the awake rat brain: CO2 fixation increases with brain activity

Glial cells are thought to supply energy for neurotransmission by increasing nonoxidative glycolysis; however, oxidative metabolism in glia may also contribute to increased brain activity. To study glial contribution to cerebral energy metabolism in the unanesthetized state, we measured neuronal and glial metabolic fluxes in the awake rat brain by using a double isotopic-labeling technique and a two-compartment mathematical model of neurotransmitter metabolism. Rats (n = 23) were infused simultaneously with 14C-bicarbonate and [1-13C]glucose for up to 1 hr. The 14C and 13C labeling of glutamate, glutamine, and aspartate was measured at five time points in tissue extracts using scintillation counting and 13C nuclear magnetic resonance of the chromatographically separated amino acids. The isotopic 13C enrichment of glutamate and glutamine was different, suggesting significant rates of glial metabolism compared with the glutamate-glutamine cycle. Modeling the 13C-labeling time courses alone and with 14C confirmed significant glial TCA cycle activity (V(PDH)((g)), approximately 0.5 micromol x gm(-1) x min(-1)) relative to the glutamate-glutamine cycle (V(NT)) (approximately 0.5-0.6 micromol x gm(-1) x min(-1)). The glial TCA cycle rate was approximately 30% of total TCA cycle activity. A high pyruvate carboxylase rate (V(PC), approximately 0.14-0.18 micromol x gm(-1) x min(-1)) contributed to the glial TCA cycle flux. This anaplerotic rate in the awake rat brain was severalfold higher than under deep pentobarbital anesthesia, measured previously in our laboratory using the same 13C-labeling technique. We postulate that the high rate of anaplerosis in awake brain is linked to brain activity by maintaining glial glutamine concentrations during increased neurotransmission.

Integrin alpha(v)beta8-mediated activation of transforming growth factor-beta by perivascular astrocytes: an angiogenic control switch

Brain hemorrhage is a severe complication of both neoplastic and nonneoplastic brain disease. Mice deficient in the alpha(v)beta8 integrin display defective brain vessel formation resulting in hemorrhage and perinatal death, but the mechanism of brain hemorrhage is unknown. Because the alpha(v)beta8 integrin is expressed by astrocytes and not expressed by endothelium, paracrine interactions between astrocytes and endothelial cells could contribute to the maintenance of brain vessel integrity. We have investigated the mechanisms underlying astrocytic-endothelial paracrine signaling and have found that integrin-mediated activation of transforming growth factor (TGF)-beta by astrocytes influences endothelial cell function. Thus, we identified the integrin alpha(v)beta8 in human perivascular glial cell processes surrounding developing blood vessels. Human astrocytic alpha(v)beta8 was a major cell surface receptor for latent TGF-beta, and alpha(v)beta8-dependent activation of TGF-beta was the major mechanism of TGF-beta activation in primary cultures of astrocytes or freshly dissociated fetal brain cells. This activation of TGF-beta was sufficient to inhibit endothelial migration in fibrin gels and to alter expression of genes affecting proteolytic and angiogenic pathways. Taken together, our data suggest that astrocytic alpha(v)beta8 acts as a central regulator of brain vessel homeostasis through regulation of TGF-beta activation and expression of TGF-beta-responsive genes that promote vessel differentiation and stabilization, most notably plasminogen activator inhibitor-1 and thrombospondin-1.

Possible role of 5'-adenosine triphosphate in synchronization of Ca2+ oscillations in primate luteinizing hormone-releasing hormone neurons

LHRH neurons derived from the olfactory placode region of monkey embryos exhibit spontaneous intracellular Ca2+ ([Ca2+]i) oscillations that synchronize among LHRH neurons and nonneuronal cells at a frequency similar to pulsatile LHRH release. To understand the mechanism of intercellular communication between LHRH neurons and nonneuronal cells, which leads to synchronization, we examined the possible role of ATP. 1) ATP, not ADP or AMP, stimulated both LHRH release and [Ca2+]i concentration, whereas the ATP-induced [Ca2+]i response was abolished by infusion of apyrase, which hydrolyzes ATP; 2) the ATP-induced [Ca2+]i response occurred in normal (but not low) extracellular Ca2+ and was blocked by the voltage-dependent L-type Ca2+ channel blocker, nifedipine; 3) pharmacological experiments with purinergic receptor agonists and antagonists indicated that the ATP-induced [Ca2+]i response in LHRH neurons was mediated through P2X, but not P2Y, receptors; 4) cloning and sequencing studies suggested that P2X2 and P2X4 transcripts were present in olfactory placode cultures; and 5) P2X2 receptors and P2X4 were expressed in LHRH neurons. The results suggest that ATP may play a role in intercellular communication when LHRH neurons synchronize, and raise the possibility that nonneuronal cells, such as glia, may be a crucial component of the in vivo LHRH neurosecretory system.

Astrocytes in cerebral ischemic injury: morphological and general considerations

Asrocytic responses constitute one of the earliest and most prominent changes in the CNS following ischemic injury. Astrocytes are known to carry out critical functions such as maintenance of ionic homeostasis, prevention of excitotoxicity, scavenging free radicals, provision of nutrients and growth factors, promotion of neovascularization, and support of synaptogenesis and neurogenesis that potentially may influence the outcome of ischemic injury. This article reviews ischemia-associated alterations in astrocytes and their potential significance. Interactions with neurons, microglia, and endothelial cells are also considered. This article highlights the critical role of astrocytes in the CNS response to ischemic injury.

Vascular endothelial cells promote acute plasticity in ependymoglial cells of the neuroendocrine brain

Glial and endothelial cells interact throughout the brain to define specific functional domains. Whether endothelial cells convey signals to glia in the mature brain is unknown but is amenable to examination in circumventricular organs. Here we report that purified endothelial cells of one of these organs, the median eminence of the hypothalamus, induce acute actin cytoskeleton remodeling in isolated ependymoglial cells and show that this plasticity is mediated by nitric oxide (NO), a diffusible factor. We found that both soluble guanylyl cyclase and cyclooxygenase products are involved in this endothelial-mediated control of ependymoglia cytoarchitecture. We also demonstrate by electron microscopy that activation of endogenous NO release in the median eminence induces rapid structural changes, allowing a direct access of neurosecretory axons containing gonadotropin-releasing hormone (GnRH) (the neuropeptide controlling reproductive function) to the portal vasculature. Local in vivo inhibition of NO synthesis disrupts reproductive cyclicity, a process that requires a pulsatile, coordinated delivery of GnRH into the hypothalamic-adenohypophyseal portal system. Our results identify a previously unknown function for endothelial cells in inducing neuroglial plasticity and raise the intriguing possibility that endothelial cells throughout the brain may use a similar signaling mechanism to regulate glial-neuronal interactions.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

Neurovascular mechanisms of Alzheimer's neurodegeneration

In contrast to traditional neuroncentric views of Alzheimer's disease (AD), recent findings indicate that neurovascular dysfunction contributes to cognitive decline and neurodegeneration in AD. Here, I propose the neurovascular hypothesis of AD, suggesting that faulty clearance of amyloid beta peptide (A beta) across the blood-brain barrier (BBB), aberrant angiogenesis and senescence of the cerebrovascular system could initiate neurovascular uncoupling, vessel regression, brain hypoperfusion and neurovascular inflammation. Ultimately, this would lead to BBB compromise, to chemical imbalance in the neuronal environment and to synaptic and neuronal dysfunction, injury and loss. Based on the neurovascular hypothesis, I suggest an array of new potential therapeutic approaches that could be developed for AD, to enhance A beta clearance and neurovascular repair, and to protect the neurovascular unit from divergent inducers of injury and apoptosis.

In vivo measurement of brain extracellular space diffusion by cortical surface photobleaching

Molecular diffusion in the brain extracellular space (ECS) is an important determinant of neural function. We developed a brain surface photobleaching method to measure the diffusion of fluorescently labeled macromolecules in the ECS of the cerebral cortex. The ECS in mouse brain was labeled by exposure of the intact dura to fluorescein-dextrans (M(r) 4, 70, and 500 kDa). Fluorescein-dextran diffusion, detected by fluorescence recovery after laser-induced cortical photobleaching using confocal optics, was slowed approximately threefold in the brain ECS relative to solution. Cytotoxic brain edema (produced by water intoxication) or seizure activity (produced by convulsants) slowed diffusion by >10-fold and created dead-space microdomains in which free diffusion was prevented. The hindrance to diffusion was greater for the larger fluorescein-dextrans. Interestingly, slowed ECS diffusion preceded electroencephalographic seizure activity. In contrast to the slowed diffusion produced by brain edema and seizure activity, diffusion in the ECS was faster in mice lacking aquaporin-4 (AQP4), an astroglial water channel that facilitates fluid movement between cells and the ECS. Our results establish a minimally invasive method to quantify diffusion in the brain ECS in vivo, revealing stimulus-induced changes in molecular diffusion in the ECS with unprecedented spatial and temporal resolution. The in vivo mouse data provide evidence for: (1) dead-space ECS microdomains after brain swelling; (2) slowed molecular diffusion in the ECS as an early predictor of impending seizure activity; and (3) a novel role for AQP4 as a regulator of brain ECS.

Astrocytic control of glutamatergic activity: astrocytes as stars of the show

It is a major recent finding that astrocytes can influence synaptic activity by release of glutamate, but many other glutamate-mediated activities are also controlled by astrocytes. Even the most obvious neuronal function of glutamate - its release as a transmitter - is regulated by astrocytes; these cells are needed for formation of precursors for glutamate synthesis, for reuptake of released transmitter, and for disposal of excess glutamate. Without astrocytic involvement, normal function of glutamatergic neurons is not possible, as exemplified by almost instantaneous abrogation of normal vision and learning upon inhibition of astrocyte-specific metabolic pathways. In addition, astrocytes are essential for production of the neuroprotectant glutathione, yet they can also contribute to neuronal death during ischemia by maintaining glutamine synthesis, enabling neuronal formation of neurotoxic glutamate.

Vesicle mobility studied in cultured astrocytes

Astrocytes release many neuroactive substances, which are stored in membrane bound vesicles and may play a role in synapse modulation and in the coupling between neuronal activity and the local blood flow. However, the mobility of these vesicles in astrocytes has not been studied yet. We here used a fluorescently tagged proatrial natriuretic peptide to label single vesicles and dynamic microscopy to monitor their mobility. To track and analyze labeled vesicles, we employed a computer software. We found two modes of vesicle mobility, directional and non-directional. The mobility of non-directional vesicles is likely determined mainly by free diffusion. Only directional vesicles displayed a straight-line motion. The relationship of mean square displacement with time in directional vesicles resembled a quadratic function, indicating that in addition to free diffusion other mechanisms may contribute to vesicle movements in astrocytes, the biophysical properties of which are similar to those of neurons.

Ballistic labeling and dynamic imaging of astrocytes in organotypic hippocampal slice cultures

Protoplasmic astrocytes in mammalian CNS tissues in vivo have a highly complex 3D morphology, but in dissociated cell cultures they often assume a flattened, fibroblast-like morphology bearing only a few, simple processes. By fluorescent labeling and confocal reconstruction we show that many astrocytes in organotypic hippocampal slice cultures exhibit a more native complex cytoarchitecture. Although astrocytes at the surface of slice cultures show a reactive form with several thick glial fibrillary acidic protein (GFAP)-positive processes, astrocytes situated in deeper portions of tissue slices retain a highly complex 3D morphology with many fine spine- or veil-like protrusions. Dozens of astrocytes can be labeled in single slice cultures by gene gun-mediated ballistic delivery of gold or tungsten particles carrying cDNAs (Biolistics), lipophilic dyes (DiOlistics), or fluorescent intracellular calcium indicators (Calistics). Expression of a membrane-targeted form of eGFP (Lck-GFP) is superior to soluble eGFP for resolving fine astrocytic processes. Time-lapse confocal imaging of Lck-GFP transfected astrocytes or "calistically" labeled astrocytes show structural remodeling and calcium transients, respectively. This approach provides an in vitro system for investigating the functional architecture, development and dynamic remodeling of astrocytes and their relationships to neurons and glia in live mammalian brain tissues.

Emerging complexities in identity and function of glial connexins

Recent research results indicate that glial gap-junction communication is much more complex and widespread than originally thought, and has diverse roles in brain homeostasis and the response of the brain to injury. The situation is far from clear, however. Pharmacological agents that block gap junctions can abolish neuron-glia long-range signaling and can alleviate neuronal damage whereas, intriguingly, opposite effects are observed in mice lacking connexin43, a major gap-junction subunit protein in astrocytes. How can the apparently contradictory results be explained, and how is specificity achieved within the glial gap-junction system? Another key issue in understanding glial connexin function is that oligodendrocytes and astrocytes, each of which express distinct connexin isotypes, are thought to participate in brain homeostasis by forming a panglial syncytium. Molecular analysis has revealed a surprising diversity of connexin expression and function, and this has led to new hypotheses regarding their roles in the brain, which could be tested using new approaches.

Why the frontal cortex in autism might be talking only to itself: local over-connectivity but long-distance disconnection

Although it has long been thought that frontal lobe abnormality must play an important part in generating the severe impairment in higher-order social, emotional and cognitive functions in autism, only recently have studies identified developmentally early frontal lobe defects. At the microscopic level, neuroinflammatory reactions involving glial activation, migration defects and excess cerebral neurogenesis and/or defective apoptosis might generate frontal neural pathology early in development. It is hypothesized that these abnormal processes cause malformation and thus malfunction of frontal minicolumn microcircuitry. It is suggested that connectivity within frontal lobe is excessive, disorganized and inadequately selective, whereas connectivity between frontal cortex and other systems is poorly synchronized, weakly responsive and information impoverished. Increased local but reduced long-distance cortical-cortical reciprocal activity and coupling would impair the fundamental frontal function of integrating information from widespread and diverse systems and providing complex context-rich feedback, guidance and control to lower-level systems.

Neuroglial activation and neuroinflammation in the brain of patients with autism

Autism is a neurodevelopmental disorder characterized by impaired communication and social interaction and may be accompanied by mental retardation and epilepsy. Its cause remains unknown, despite evidence that genetic, environmental, and immunological factors may play a role in its pathogenesis. To investigate whether immune-mediated mechanisms are involved in the pathogenesis of autism, we used immunocytochemistry, cytokine protein arrays, and enzyme-linked immunosorbent assays to study brain tissues and cerebrospinal fluid (CSF) from autistic patients and determined the magnitude of neuroglial and inflammatory reactions and their cytokine expression profiles. Brain tissues from cerebellum, midfrontal, and cingulate gyrus obtained at autopsy from 11 patients with autism were used for morphological studies. Fresh-frozen tissues available from seven patients and CSF from six living autistic patients were used for cytokine protein profiling. We demonstrate an active neuroinflammatory process in the cerebral cortex, white matter, and notably in cerebellum of autistic patients. Immunocytochemical studies showed marked activation of microglia and astroglia, and cytokine profiling indicated that macrophage chemoattractant protein (MCP)-1 and tumor growth factor-beta1, derived from neuroglia, were the most prevalent cytokines in brain tissues. CSF showed a unique proinflammatory profile of cytokines, including a marked increase in MCP-1. Our findings indicate that innate neuroimmune reactions play a pathogenic role in an undefined proportion of autistic patients, suggesting that future therapies might involve modifying neuroglial responses in the brain.

Cerebellar astrocytes co-express several ADP receptors. Presence of functional P2Y(13)-like receptors

Astrocytes exhibit a form of excitability based on variations of intracellular Ca²⁺ concentration in response to various stimuli, including ADP, ATP, UTP and dinucleotides. Here, we investigate the presence of the recently cloned ADP-sensitive receptors, P2Y₁₂ and P2Y₁₃ subtypes, which are negatively coupled to adenylate cyclase, in cerebellar astrocytes. We checked the effect of specific agonists, 2-methylthioadenosine diphosphate (2MeSADP) and ADP, on adenylate cyclase stimulation induced by isoproterenol. Both agonists significantly reduced the cAMP accumulation induced by isoproterenol. The inhibitory effect was concentration-dependent with IC₅₀ values of 46 ± 13 and 23 ± 14 nM for 2MeSADP and ADP, respectively. The experiments were carried out in the presence of MRS-2179, a specific antagonist of P2Y₁ receptor, to avoid any contribution of this receptor. Using fura-2 microfluorimetry we also proved that astrocytes responded to 2MeSADP stimulations with calcium responses in the absence and also in the presence of MRS-2179. Both effects, inhibition of adenylate cyclase and intracellular calcium mobilization, were not modified by 2MeSAMP, an antagonist of P2Y₁₂ receptor, suggesting that were mediated by P2Y₁₃-like receptors.

Melatonin prevents hydrogen peroxide-induced Bax expression in cultured rat astrocytes

During oxidative stress, cell apoptosis is promoted through the mitochondrial death pathway. Increased reactive oxygen species (ROS) are linked to excess cell loss and mediate the induction of apoptosis in various cell types. However, the role of ROS in the apoptotic pathway has not been clearly established. The aims of this study were to investigate the biochemical and morphological responses of rat astrocytes to hydrogen peroxide-mediated cell death and to define the role that melatonin might play in the apoptotic cascade. Hydrogen peroxide (H2O2; 0.1-1.0 mM) significantly reduced cell viability. Astrocyte death was associated with enhanced ROS production in a dose-dependent manner, as measured by 2',7'-dichloro-fluorescein fluorescence. H2O2-induced cell death was found to be mediated through an apoptotic pathway as treated cells exhibited cell shrinkage, nuclear condensation and marked DNA fragmentation. H2O2 also triggered caspase-3 activation and Bax expression. The ability of different antioxidants to prevent H2O2-induced apoptosis was examined by pre-incubating rat astrocytes with N-acetylcysteine (10 mM), glutathione (0.5 mM) or melatonin (0.1 mM and 10 nM). Results showed that N-acetylcysteine and glutathion can protect astrocytes against ROS accumulation and caspase-3 activation, whereas 0.1 mM melatonin can inhibit H2O2-induced apoptosis by regulating Bax expression and by inhibiting caspase-3 activation. Antiapoptotic effect of 10 nM melatonin associated to inhibition of Bax expression, give rise to new therapeutic approaches.

EIF2B5 mutations compromise GFAP+ astrocyte generation in vanishing white matter leukodystrophy

Vanishing white matter disease (VWM) is a heritable leukodystrophy linked to mutations in translation initiation factor 2B (eIF2B). Although the clinical course of this disease has been relatively well described, the cellular consequences of EIF2B mutations on neural cells are unknown. Here we have established cell cultures from the brain of an individual with VWM carrying mutations in subunit 5 of eIF2B (encoded by EIF2B5). Despite the extensive demyelination apparent in this VWM patient, normal-appearing oligodendrocytes were readily generated in vitro. In contrast, few GFAP-expressing (GFAP+) astrocytes were present in primary cultures, induction of astrocytes was severely compromised, and the few astrocytes generated showed abnormal morphologies and antigenic phenotypes. Lesions in vivo also lacked GFAP+ astrocytes. RNAi targeting of EIF2B5 severely compromised the induction of GFAP+ cells from normal human glial progenitors. This raises the possibility that a deficiency in astrocyte function may contribute to the loss of white matter in VWM leukodystrophy.

Human immunodeficiency virus type 1 Nef protein mediates neural cell death: a neurotoxic role for IP-10

HIV-1 Nef is expressed in astrocytes, but a contribution to neuropathogenesis and the development of HIV-associated dementia (HAD) remains uncertain. To determine the neuropathogenic actions of the HIV-1 Nef protein, the brain-derived (YU-2) and blood-derived (NL4-3) Nef proteins were expressed in neural cells using an alphavirus vector, which resulted in astrocyte death (P < 0.001). Supernatants from Nef-expressing astrocytes also caused neuronal death, suggesting the release of neurotoxic molecules by astrocytes. Analysis of pro-inflammatory gene induction in astrocytes expressing Nef revealed increased IP-10 mRNA expression (4000-fold) that was Nef sequence dependent. Recombinant IP-10 caused selective cell death in neurons (P < 0.001) but not astrocytes, and the cytotoxicity of supernatant from astrocytes expressing Nef YU-2 was blocked by an antibody directed against the chemokine receptor CXCR3 (P < 0.001). SCID/NOD mice implanted with a Nef YU-2-expressing vector displayed abnormal motor behavior (P < 0.05), neuroinflammation, and neuronal loss relative to controls. Analysis of mRNA levels in brains from patients with HAD also revealed increased expression of IP-10 (P < 0.05), which was confirmed by immunoreactivity detected principally in astrocytes. Phylogenetic and protein structure analyses of Nef sequences derived from HIV/AIDS patients with and without HAD suggested viral evolution toward a neurotropic Nef protein. These results indicate that HIV-1 Nef contributes to neuropathogenesis by directly causing astrocyte death together with indirect neuronal death through the cytotoxic actions of IP-10 on neurons. Furthermore, Nef molecular diversity was evident in brain tissue among patients with neurological disease and which may influence IP-10 production by astrocytes.

Pathophysiological roles of extracellular nucleotides in glial cells: differential expression of purinergic receptors in resting and activated microglia

Microglial cells are the major cellular elements with immune function inside the CNS and play important roles in orchestrating inflammatory brain response to hypoxia and trauma. Although a complete knowledge of the endogenous factors leading to a prompt activation of microglia is not yet available, activation of P2 purinoreceptors by extracellular ATP has been indicated as a primary factor in microglial response. A still unresolved question, however, is which subtype(s) of P2 receptors mediate(s) the response to ATP. By a combination of RT-PCR, Western blotting, and single-cell calcium imaging, we assessed the presence and the activity of P2 receptor subtypes in the mouse microglial cell line N9. All members of the P2 receptor family, including the recently reported receptor for sugar nucleotides (P2Y(14)), were found to be present in these cells at mRNA and/or protein level. The functionality of the receptors was assessed by analysis of the calcium responses evoked by specific agonists both in N9 cells and in primary microglia from rat brain. Interestingly, a different functional profile of P2 receptors was observed in resting or in LPS-activated N9 cells. Overnight exposure to LPS increased P2Y(6) and P2Y(14), decreased P2X(7), and left unchanged P2Y(1) and P2Y(2,4) receptor activity. The change in the P2 receptor profile in activated cells suggests selective roles for specific P2 receptor subtypes in microglial activation triggered by LPS. We speculate that modulation of microglial cell function via subtype-selective P2 receptor ligands may open up new strategies in the therapeutic management of inflammatory neurological diseases characterized by abnormal microglia response.

Axon branch removal at developing synapses by axosome shedding

In many parts of the developing nervous system, the number of axonal inputs to each postsynaptic cell is dramatically reduced. This synapse elimination has been extensively studied at the neuromuscular junction, but how axons are lost is unknown. Here, we combine time-lapse imaging of fluorescently labeled axons and serial electron microscopy to show that axons at neuromuscular junctions are removed by an unusual cellular mechanism. As axons disappear, they shed numerous membrane bound remnants. These "axosomes" contain a high density of synaptic organelles and are formed by engulfment of axon tips by Schwann cells. After this engulfment, the axosome's contents mix with the cytoplasm of the glial cell. Axosome shedding might underlie other forms of axon loss and may provide a pathway for interactions between axons and glia.

A potential role for β- and γ-crystallins in the vascular remodeling of the eye

We demonstrate that expression of beta- and gamma-crystallins is associated with intraocular vessels during normal vascular development of the eye and also in the Nuc1 rat, a mutant in which the hyaloid vascular system fails to regress normally. Real-Time RT PCR, Western blot and metabolic labeling studies indicate an increased expression of beta- and gamma-crystallins in Nuc1 retina. The increased expression of crystallins was localized to the astrocytes surrounding the intraocular vessels. A similar pattern of crystallin expression was also observed in the retinal vessels during normal development. Cultured human astrocytes exposed to 3-nitropropionic acid, an established model of neuronal hypoxia, increased VEGF expression, as expected, but also increased expression of crystallins. Our data suggest that crystallins may function together with VEGF during vascular remodeling. Interestingly, in human PFV (persistent fetal vasculature) disease, where the hyaloid vasculature abnormally persists after birth, we show that astrocytes express both VEGF and crystallins.

Role of glial cells in cerebral ischemia

Despite intense efforts at the bench and at the bedside, few therapeutic strategies exist to combat the consequences of cerebral ischemia. Traditionally, a "neurocentric" view has dominated research in this field. Evidence is now accumulating that glial cells, in particular astrocytes, play an active and important role in the pathophysiology of cerebral ischemia. Brain energetics, water and ion homeostasis, inflammation, trophic factor production, vascular regulation, neuroneogenesis, and vasculogenesis, among others, are all under the control of glial cells. As a consequence, glial cells have been identified as promising targets for novel therapeutic approaches in brain protection. This review aims at dissecting possible protective as well as destructive roles of astrocytes (and other glial cells) in cerebral ischemia. By emphasizing open issues in this field, we hope to stimulate further research into this relatively unexplored aspect of brain pathophysiology.

Calcium Dynamics in Cortical Astrocytes and Arterioles During Neurovascular Coupling

Neuronal activity in the brain is thought to be coupled to cerebral arterioles (functional hyperemia) through Ca 2+ signals in astrocytes. Although functional hyperemia occurs rapidly, within seconds, such rapid signaling has not been demonstrated in situ, and Ca 2+ measurements in parenchymal arterioles are still lacking. Using a laser scanning confocal microscope and fluorescence Ca 2+ indicators, we provide the first evidence that in a brain slice preparation, increased neuronal activity by electrical stimulation (ES) is rapidly signaled, within seconds, to cerebral arterioles and is associated with astrocytic Ca 2+ waves. Smooth muscle cells in parenchymal arterioles exhibited Ca 2+ and diameter oscillations (“vasomotion”) that were rapidly suppressed by ES. The neuronal-mediated Ca 2+ rise in cortical astrocytes was dependent on intracellular (inositol trisphosphate [IP 3 ]) and extracellular voltage-dependent Ca 2+ channel sources. The Na + channel blocker tetrodotoxin prevented the rise in astrocytic [Ca 2+ ] i and the suppression of Ca 2+ oscillations in parenchymal arterioles to ES, indicating that neuronal activity was necessary for both events. Activation of metabotropic glutamate receptors in astrocytes significantly decreased the frequency of Ca 2+ oscillations in parenchymal arterioles. This study supports the concept that astrocytic Ca 2+ changes signal the cerebral microvasculature and indicate the novel concept that this communication occurs through the suppression of arteriolar [Ca 2+ ] i oscillations and corresponding vasomotion. The full text of this article is available online at http://circres.ahajournals.org.

Adenoviral transfer of the heme oxygenase-1 gene protects striatal astrocytes from heme-mediated oxidative injury

Heme oxygenase-1 (HO-1) is induced in the CNS after hemorrhage, and may have an effect on injury to surrounding tissue. Hemin, the preferred substrate of HO, is a neurotoxin that is present in intracranial hematomas. In a prior study, we observed that HO inhibitors increased the vulnerability of cultured cortical astrocytes to heme-mediated oxidative injury. To investigate the effect of HO more specifically, we used an adenoviral vector encoding the human HO-1 gene to specifically increase HO-1 expression. Incubation with 100 MOI of the HO-1 adenovirus (Adv-HHO-1) for 24 h increased both HO-1 protein and HO activity; a control adenovirus lacking the HO-1 gene had no effect. Using a DNA probe that was specific for human HO-1, 80.5 +/- 7.2% of astrocytes were observed to be infected by in situ hybridization. The cell death produced by 30-60 microM hemin was significantly reduced by pretreatment with 100 MOI Adv-HHO-1, as assessed by LDH release, propidium iodide exclusion, and MTT reduction assay. The threefold increase in cell protein oxidation produced by hemin was also attenuated in cultures pretreated with Adv-HHO-1. These results support the hypothesis that HO-1 protects astrocytes from heme-mediated oxidative injury. Specifically increasing astrocytic HO-1 by gene transfer may have a beneficial effect on hemorrhagic CNS injury.

Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo

Glial cells have been identified as key signaling components in the brain; however, methods to investigate their structure and function in vivo have been lacking. Here, we describe a new, highly selective approach for labeling astrocytes in intact rodent neocortex that allows in vivo imaging using two-photon microscopy. The red fluorescent dye sulforhodamine 101 (SR101) was specifically taken up by protoplasmic astrocytes after brief exposure to the brain surface. Specificity was confirmed by immunohistochemistry. In addition, SR101 labeled enhanced green fluorescent protein (EGFP)-expressing astrocytes but not microglial cells in transgenic mice. We used SR101 labeling to quantify morphological characteristics of astrocytes and to visualize their close association with the cortical microvasculature. Furthermore, by combining this method with calcium indicator loading of cell populations, we demonstrated distinct calcium dynamics in astroglial and neuronal networks. We expect SR101 staining to become a principal tool for investigating astroglia in vivo.

Heme oxygenase-2 gene deletion attenuates oxidative stress in neurons exposed to extracellular hemin

Background

Hemin, the oxidized form of heme, accumulates in intracranial hematomas and is a potent oxidant. Growing evidence suggests that it contributes to delayed injury to surrounding tissue, and that this process is affected by the heme oxygenase enzymes. In a prior study, heme oxygenase-2 gene deletion increased the vulnerability of cultured cortical astrocytes to hemin. The present study tested the effect of HO-2 gene deletion on protein oxidation, reactive oxygen species formation, and cell viability after mixed cortical neuron/astrocyte cultures were incubated with neurotoxic concentrations of hemin.

Results

Continuous exposure of wild-type cultures to 1–10 μM hemin for 14 h produced concentration-dependent neuronal death, as detected by both LDH release and fluorescence intensity after propidium iodide staining, with an EC₅₀ of 1–2 μM; astrocytes were not injured by these low hemin concentrations. Cell death was consistently reduced by at least 60% in knockout cultures. Exposure to hemin for 4 hours, a time point that preceded cell lysis, increased protein oxidation in wild-type cultures, as detected by staining of immunoblots for protein carbonyl groups. At 10 μM hemin, carbonylation was increased 2.3-fold compared with control sister cultures subjected to medium exchanges only; this effect was reduced by about two-thirds in knockout cultures. Cellular reactive oxygen species, detected by fluorescence intensity after dihydrorhodamine 123 (DHR) staining, was markedly increased by hemin in wild-type cultures and was localized to neuronal cell bodies and processes. In contrast, DHR fluorescence intensity in knockout cultures did not differ from that of sham-washed controls. Neuronal death in wild-type cultures was almost completely prevented by the lipid-soluble iron chelator phenanthroline; deferoxamine had a weaker but significant effect.

Conclusions

These results suggest that HO-2 gene deletion protects neurons in mixed neuron-astrocyte cultures from heme-mediated oxidative injury. Selective inhibition of neuronal HO-2 may have a beneficial effect after CNS hemorrhage.

Minocycline reduces the lipopolysaccharide-induced inflammatory reaction, peroxynitrite-mediated nitration of proteins, disruption of the blood-brain barrier, and damage in the nigral dopaminergic system

We have evaluated the potential neuroprotectant activity of minocycline in an animal model of Parkinson's disease induced by intranigral injection of lipopolysaccharide. Minocycline treatment was very effective in protecting number of nigral dopaminergic neurons and loss of reactive astrocytes at 7 days postlesion. Evaluation of microglia revealed that minocycline treatment highly prevented the lipopolysaccharide-induced activation of reactive microglia as visualized by OX-42 and OX-6 immunohistochemistry. Short-term RT-PCR analysis demonstrated that minocycline partially prevented the lipopolysaccharide-induced increases of mRNA levels for interleukin-1alpha and tumor necrosis factor-alpha. In addition, lipopolysaccharide highly induced protein nitration as seen by 3-nitrotyrosine immunoreactivity in the ventral mesencephalon. Minocycline treatment strongly diminished the extent of 3-nitrotyrosine immunoreactivity. We also found a direct correlation between location of IgG immunoreactivity-a marker of blood-brain barrier disruption-and neurodegenerative processes including death of nigral dopaminergic cells and reactive astrocytes. There was also a precise spatial correlation between disruption of blood-brain barrier and 3-nitrotyrosine immunoreactivity. We discuss potential involvement of lipopolysaccharide-induced formation of peroxynitrites and cytokines in the pathological events in substantia nigra in response to inflammation. If inflammation is proved to be involved in the ethiopathology of Parkinson's disease, our data support the use of minocycline in parkinsonian patients.

P2X7 receptor inhibition improves recovery after spinal cord injury

Secondary injury exacerbates the extent of spinal cord insults, yet the mechanistic basis of this phenomenon has largely been unexplored. Here we report that broad regions of the peritraumatic zone are characterized by a sustained process of pathologic, high ATP release. Spinal cord neurons expressed P2X7 purine receptors (P2X7R), and exposure to ATP led to high-frequency spiking, irreversible increases in cytosolic calcium and cell death. To assess the potential effect of P2X7R blockade in ameliorating acute spinal cord injury (SCI), we delivered P2X7R antagonists OxATP or PPADS to rats after acute impact injury. We found that both OxATP and PPADS significantly improved functional recovery and diminished cell death in the peritraumatic zone. These observations demonstrate that SCI is associated with prolonged purinergic receptor activation, which results in excitotoxicity-based neuronal degeneration. P2X7R antagonists inhibit this process, reducing both the histological extent and functional sequelae of acute SCI.

Neurone-to-astrocyte signalling in the brain represents a distinct multifunctional unit

Astrocytes can respond to neurotransmitters released at the synapse by generating elevations in intracellular Ca(2+) concentration ([Ca(2+)](i)) and releasing glutamate that signals back to neurones. This discovery opens new perspectives for the possible participation of these glial cells in actual information processing by the brain and raises the hypothesis that astrocyte activation by neuronal signals plays a key role in distinct, functional events. Depending on the level of neuronal activity, the [Ca(2+)](i) response that is activated by neurotransmitters can either remain restricted to an astrocytic process or it can propagate as an intracellular [Ca(2+)](i) wave to other astrocytic processes in contact with different neurones, astrocytes, microglia or endothelial cells of cerebral arterioles. Glutamate release triggered by the [Ca(2+)](i) rise at the astrocytic process represents a feedback, short-distance signal that affects synaptic transmission locally. The release of glutamate as well as of other compounds far away from the site of initial activation represents a feedforward, long-distance signal that can be involved in the regulation of distinct processes. For instance, through the release of vasoactive molecules from the astrocytic processes in contact with cerebral arterioles, the neurone-astrocyte-endothelial cell signalling pathway plays a pivotal role in the neuronal control of vascular tone. In this article we will review recent results that should persuade us to reshape our current thinking on the roles of astroglial cells in the brain. We propose that neurones and astrocytes represent an integral unit that has a distinctive role in different fundamental events in brain function. Furthermore, while recent findings provide important evidences for the vesicular hypothesis of glutamate release, we discuss also the proposals for a possible physiological role of hemichannels and purinergic P2X(7) receptors in glutamate release from astrocytes. A full clarification of the functional significance of the bidirectional communication that astrocytes establish with neurones as well as with other brain cells represents one of the most intriguing challenges in neurobiological research at the moment and should fuel stimulating debates in years to come.

Synaptotagmin IV regulates glial glutamate release

Calcium-binding synaptotagmins (Syts) are membrane proteins that are conserved from nematode to human. Fifteen Syts (Syts I-XV) have been identified in mammalian species. Syt I has been well studied and is a candidate for the Ca(2+)-sensor that triggers evoked exocytosis underlying fast synaptic transmission. Whereas the functions of the other Syts are unclear, Syt IV is of particular interest because it is rapidly up-regulated after chronic depolarization or seizures, and because null mutations exhibit deficits in fine motor coordination and hippocampus-dependent memory. Screening Syts I-XIII, which are enriched in brain, we find that Syt IV is located in processes of astroglia in situ. Reduction of Syt IV in astrocytes by RNA interference decreases Ca(2+)-dependent glutamate release, a gliotransmission pathway that regulates synaptic transmission. Mutants of the C2B domain, the only putative Ca(2+)-binding domain in Syt IV, act in a dominant-negative fashion over Ca(2+)-regulated glial glutamate release, but not gliotransmission induced by changes in osmolarity. Because we find that Syt IV is expressed predominantly by astrocytes and is not in the presynaptic terminals of the hippocampus, and because Syt IV knockout mice exhibit hippocampal-based memory deficits, our data raise the intriguing possibility that Syt IV-mediated gliotransmission contributes to hippocampal-based memory.

Molecular diversity of astrocytes with implications for neurological disorders

The astrocyte represents the most abundant yet least understood cell type of the CNS. Here, we use a stringent experimental strategy to molecularly define the astrocyte lineage by integrating microarray datasets across several in vitro model systems of astrocyte differentiation, primary astrocyte cultures, and various astrocyterich CNS structures. The intersection of astrocyte data sets, coupled with the application of nonastrocytic exclusion filters, yielded many astrocyte-specific genes possessing strikingly varied patterns of regional CNS expression. Annotation of these astrocyte-specific genes provides direct molecular documentation of the diverse physiological roles of the astrocyte lineage. This global perspective in the normal brain also provides a framework for how astrocytes may participate in the pathogenesis of common neurological disorders like Alzheimer's disease, Parkinson's disease, stroke, epilepsy, and primary brain tumors.

Deletion of macrophage-inflammatory protein 1 alpha retards neurodegeneration in Sandhoff disease mice

Sandhoff disease is a prototypical lysosomal storage disorder in which a heritable deficiency of a lysosomal enzyme, beta-hexosaminidase, results in the storage of the enzyme's substrates in lysosomes. As with many of the other lysosomal storage diseases, neurodegeneration is a prominent feature. Although the cellular and molecular pathways that underlie the neurodegenerative process are not yet fully understood, macrophage/microglial-mediated inflammation has been suggested as one possible mechanism. We now show that the expanded macrophage/microglial population in the CNS of Sandhoff disease mice is compounded by the infiltration of cells from the periphery. Coincident with the cellular infiltration was an increased expression of macrophage-inflammatory protein 1alpha (MIP-1alpha), a leukocyte chemokine, in astrocytes. Deletion of MIP-1alpha expression resulted in a substantial decrease in infiltration and macrophage/microglial-associated pathology together with neuronal apoptosis in Sandhoff disease mice. These mice without MIP-1alpha showed improved neurologic status and a longer lifespan. The results indicate that the pathogenesis of Sandhoff disease involves an increase in MIP-1alpha that induces monocytes to infiltrate the CNS, expand the activated macrophage/microglial population, and trigger apoptosis of neurons, resulting in a rapid neurodegenerative course.

Glial modulation of synaptic transmission in culture

Accumulating evidence has demonstrated the existence of bidirectional communication between glial cells and neurons, indicating an important active role of glia in the physiology of the nervous system. Neurotransmitters released by presynaptic terminals during synaptic activity increase intracellular Ca(2+) concentration in adjacent glial cells. In turn, activated glia may release different transmitters that can feed back to neuronal synaptic elements, regulating the postsynaptic neuronal excitability and modulating neurotransmitter release from presynaptic terminals. As a consequence of this evidence, a new concept of the synaptic physiology, the tripartite synapse, has been proposed, in which glial cells play an active role as dynamic regulatory elements in neurotransmission. In the present article we review evidence showing the ability of astrocytes to modulate synaptic transmission directly, with the focus on studies performed on cell culture preparations, which have been proved extremely useful in the characterization of molecular and cellular processes involved in astrocyte-mediated neuromodulation.

Correlação entre Hiperglicemia e Células do SNC, com Enfoque na Atividade Glial

Introdução. Entre os mecanismos biológicos que originam o qua­dro hiperglicêmico a predominância é do diabetes melittus (DM). O DM representa um grupo de desordens metabólicas caracterizadas por hiperglicemia crônica que ocasiona severas alterações celulares e teci­duais. Objetivo. O presente trabalho analisou através de revisão da literatura o comportamento de células gliais expostas a elevadas con­centrações de glicose, similares às observadas no DM. Método. Foi realizada uma revisão literária através de artigos científicos das bases de dados Pubmed, Science Direct, Scopus e Scielo. Resultados. Foram selecionados artigos e livros entre 1988 e 2009 que discutiam hiper­glicemia, sistema nervoso central e que relacionavam hiperglicemia e células gliais. Conclusão. A hiperglicemia crônica proporcionada pelo DM pode influenciar de maneira danosa o metabolismo cerebral exercendo ações sobre a atividade glial. Podendo afetar a sobrevivência neuronal através da excitotoxicidade glutamatérgica e da produção de espécies reativas de oxigênio (ERO) e de espécies reativas de nitro­gênio (ERN) que geram como consequência o processo de neuroin­flamação. Tal processo inflamatório pode resultar em dano e morte neural caracterizando um processo neurodegerativo.