Glial cells more than support cells?

Glial cells are the most abundant cells in the human brain and have long been considered as passive supporting cells for neurons. In contrast to the extensive studies on various neuronal functions in the nervous system, we still have limited knowledge about glial cells. Recently a number of pioneering studies have provided convincing evidence that glia play active roles in development and function of the central nervous system. This review discusses recent advances in our understanding of the molecular mechanisms underlying glial cell differentiation. We then highlight some of the novel findings about glial function, i.e. the role of glia in synaptogenesis and the intricate relationship between astrocytes and adult neural stem cells. Finally, we summarize the emerging studies that implicate abnormalities in the formation or maintenance of glia leading to severe brain diseases, such as Alexander disease, glioblastoma and multiple sclerosis, and potential therapeutic strategies to tackle these diseases.     

Passive Glial Cells, Fact or Artifact?

Astrocytes that are recorded in acute tissue slices of rat hippocampus using whole-cell patch-clamp, commonly exhibit voltage-activated Na⁺ and K⁺ currents. Some reports have described astrocytes that appear to lack voltage-activated currents and proposed that these cells constitute a subpopulation of electrophysiologically passive astrocytes. We show here that these cells can spontaneously change during a recording unmasking expression of previously suppressed voltage-activated currents, suggesting that such cells do not represent a subpopulation of passive astrocytes. Superfusion of a low Ca²⁺/EGTA solution was able to reversibly suppress voltage-activated K⁺ currents in cultured astrocytes. Currents were restored upon addition of normal bath Ca²⁺. These effects of Ca²⁺ on both outward and inward K⁺ currents were dose- and time-dependent, with increasing concentrations of Ca²⁺ (from 0 to 800 μm) leading to a gradual unmasking of voltage-dependent outward and inward K⁺ currents. The transition from an apparently passive cell to one exhibiting prominent voltage-activated currents was not associated with any changes in membrane capacitance or access resistance. By contrast, in cells in which low access resistance or poor seal accounted for the absence of voltage-activated currents, improvement of cell access was always accompanied by changes in series resistance and membrane capacitance. We propose that spillage of pipette solution containing low Ca²⁺/EGTA during cell approach in slice recordings and/or poor cell access, lead to a transient masking of voltage-activated currents even in astrocytes that express prominent voltage-activated currents. These cells, however, do not constitute a subpopulation of electrophysiologically passive astrocytes. Received: 22 April 1998/Revised: 8 September 1998     

“Glial inhibition” of memory in Alzheimer's disease

<正>Two recent studies on Nature Medicine and Nature Communications reported the unusual roles of astrocytes in the brain,that is,release gamma-aminobutyric acid(GABA)and impair hippocampal memory in Alzheimer’s disease(AD).AD represents one common form of neurodegenerative diseases,and is characterized by progressive dementia.The pathological feature of an AD brain includes the appearance     

Glial localization of interleukin-1α in invertebrate ganglia

1. Mytilus pedal ganglion contains a small population of glial cells that are immunopositive for interleukin-1 alpha. Positively stained fibers can also be seen in the neuropil of these sections. 2. The marine worm Nereis diversicolor also exhibits positive neural immunostaining for interleukin-1 alpha. 3. Both organisms contain hemocytes that contain immunoactivity for interleukin-1 alpha. The study suggests interleukin-1 alpha to be an ancient cytokine given its presence in organisms that evolved significantly earlier than mammals.     

Activated Scavenger Receptor A Promotes Glial Internalization of Aβ

Beta-amyloid (Aβ) aggregates have a pivotal role in pathological processing of Alzheimer’s disease (AD). The clearance of Aβ monomer or aggregates is a causal strategy for AD treatment. Microglia and astrocytes are the main macrophages that exert critical neuroprotective roles in the brain. They may effectively clear the toxic accumulation of Aβ at the initial stage of AD, however, their functions are attenuated because of glial overactivation. In this study, we first showed that heptapeptide XD4 activates the class A scavenger receptor (SR-A) on the glia by increasing the binding of Aβ to SR-A, thereby promoting glial phagocytosis of Aβ oligomer in microglia and astrocytes and triggering intracellular mitogen-activated protein kinase (MAPK) signaling cascades. Moreover, XD4 enhances the internalization of Aβ monomers to microglia and astrocytes through macropinocytosis or SR-A-mediated phagocytosis. Furthermore, XD4 significantly inhibits Aβ oligomer-induced cytotoxicity to glial cells and decreases the production of proinflammatory cytokines, such as TNF-α and IL-1β, in vitro and in vivo. Our findings may provide a novel strategy for AD treatment by activating SR-A.     

General Anesthetics Inhibit LPS-Induced IL-1β Expression in Glial Cells

Background

Glial cells, including microglia and astrocytes, are considered the primary source of proinflammatory cytokines in the brain. Immune insults stimulate glial cells to secrete proinflammatory cytokines that modulate the acute systemic response, which includes fever, behavioral changes, and hypothalamic-pituitary-adrenal (HPA) axis activation. We investigated the effect of general anesthetics on proinflammatory cytokine expression in the primary cultured glial cells, the microglial cell line BV-2, the astrocytic cell line A-1 and mouse brain.

Methodology/Principal Findings

Primary cultured glial cells were exposed to lipopolysaccharide (LPS) in combination with general anesthetics including isoflurane, pentobarbital, midazolam, ketamine, and propofol. Following this treatment, we examined glial cell expression of the proinflammatory cytokines interleukin (IL)-1β, IL-6, and tumor necrosis factor-alpha (TNF-α). LPS-induced expression of IL-1β mRNA and protein were significantly reduced by all the anesthetics tested, whereas IL-6 and TNF-α mRNA expression was unaffected. The anesthetics suppressed LPS-induced extracellular signal-regulated kinase 1/2 (ERK 1/2) phosphorylation, but did not affect nuclear factor-kappaB and activator protein-1 activation. The same effect was observed with BV-2, but not with A-1 cells. In the mouse experiments, LPS was injected intraperitoneally, and isoflurane suppressed IL-1β in the brain and adrenocorticotropic hormone in plasma, but not IL-1β in plasma.

Conclusions/Significance

Taken together, our results indicate that general anesthetics inhibit LPS-induced IL-1β upregulation in glial cells, particularly microglia, and affects HPA axis participation in the stress response.     

Neuroprotective Agents: Is Effective on Toxicity in Glial Cells?

1. Glial cells are the most abundant cell population in the central nervous system. The aim of this study was to examine the effects of melatonin, 7-nitroindazole, and riluzole on glutamate toxicity in primary glial cell culture. 2. Glutamate toxicity was induced by addition of 100 μM glutamate to the cultures, and then 100 μM melatonin, 500 μM 7-nitroindazole, and 10 (M riluzole were administered in different groups. Lactate Dehydrogenase activity and nitrite levels were determined at the 1st, 6th, and 24th h. 3. Melatonin, 7-nitroindazole, and riluzole decrease Lactate Dehydrogenase activity at the 1st, 6th, and 24th h (at all hours, p<0.05). Nitrite levels were decreased by melatonin and riluzole at the 1st, 6th, and 24th h. 4. In this study, we observed that melatonin, 7-nitroindazole, and riluzole are effective as protective agents on glutamate toxicity in mixed glial cells.     

Glial adenosine kinase – A neuropathological marker of the epileptic brain

Experimental research over the past decade has supported the critical role of astrocytes activated by different types of injury and the pathophysiological processes that underlie the development of epilepsy. In both experimental and human epileptic tissues astrocytes undergo complex changes in their physiological properties, which can alter glio-neuronal communication, contributing to seizure precipitation and recurrence. In this context, understanding which of the molecular mechanisms are crucially involved in the regulation of glio-neuronal interactions under pathological conditions associated with seizure development is important to get more insight into the role of astrocytes in epilepsy.     

Targeting Glial Cells to Elucidate the Pathogenesis of Huntington’s Disease

Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by expended CAG repeats in the Huntingtin (Htt) gene. The resultant mutant Htt (mHtt) forms aggregates in neurons and causes neuronal dysfunctions. The major characteristic of HD is the selective loss of neurons in the striatum and cortex, which leads to movement disorders, dementia, and eventual death. Expression of mHtt was also found in non-neuronal cells in the brain, suggesting non-cell-autonomous neurotoxicity in HD. As was documented in many different neurodegenerative disorders, elevated inflammatory responses are also reported in HD. To date, effective treatments for this devastating disease remain to be developed. This review focuses on the importance of glial cells and inflammation in HD pathogenesis. Potential anti-inflammatory interventions for HD are also discussed.     

Glial Fibrillary Acidic Protein: GFAP-Thirty-One Years (1969–2000)

It is now well established that the glial fibrillary acidic protein (GFAP) is the principal 8-9 nm intermediate filament in mature astrocytes of the central nervous system (CNS). Over a decade ago, the value of GFAP as a prototype antigen in nervous tissue identification and as a standard marker for fundamental and applied research at an interdisciplinary level was recognized (Raine, 135). As a member of the cytoskeletal protein family, GFAP is thought to be important in modulating astrocyte motility and shape by providing structural stability to astrocytic processes. In the CNS of higher vertebrates, following injury, either as a result of trauma, disease, genetic disorders, or chemical insult, astrocytes become reactive and respond in a typical manner, termed astrogliosis. Astrogliosis is characterized by rapid synthesis of GFAP and is demonstrated by increase in protein content or by immunostaining with GFAP antibody. In addition to the major application of GFAP antisera for routine use in astrocyte identification in the CNS, the molecular cloning of the mouse gene in 1985 has opened a new and rich realm for GFAP studies. These include antisense, null mice, and numerous promoter studies. Studies showing that mice lacking GFAP are hypersensitive to cervical spinal cord injury caused by sudden acceleration of the head have provided more direct evidence for a structural role of GFAP. While the structural function of GFAP has become more acceptable, the use of GFAP antibodies and promoters continue to be valuable in studying CNS injury, disease, and development.     

Expression of αB-Crystallin in the Peripapillary Glial Cells of the Developing Chick Retina

Peripapillary glial cells (PPGCs) are a peculiar macroglia in avian species, located in the central retina adjacent to the optic nerve head. PPGCs have a similar shape and orientation to Müller cells, which traverse the entire layer of the retina; however, there are differences in protein expression between the two cell types. In the present study, we first demonstrated that PPGCs expressed αB-crystallin, which is not expressed in Müller cells, during retinal development. αB-crystallin was first faintly expressed in PPGCs of the E5 retina, adjacent to the optic nerve head. Further, αB-crystallin was exclusively expressed in PPGCs up to E14. The shape of these cells was bipolar with vitread and ventricular processes. The vitread processes of αB-crystallin+ PPGCs became finer at E18. Double labeling analysis clearly demonstrated that only vimentin+ or GFAP+ astrocytes were located in the optic nerve head and were demarcated from the retina by αB-crystallin+ PPGCs. Furthermore, we determined that αB-crystallin+ PPGCs, with a number of processes, completely wrapped the optic nerve head and were densely located in the junction of the optic nerve head and the retina in a whole mount preparation and in vertical-sectioned retinae. The results of present study, together with reports that retinal astrocytes migrate from the optic nerve head, suggest that PPGCs prevent astrocytes from migrating into the retina in avian species.     

Glial tumor necrosis factor alpha (TNFα) generates metaplastic inhibition of spinal learning

Injury-induced overexpression of tumor necrosis factor alpha (TNFα) in the spinal cord can induce chronic neuroinflammation and excitotoxicity that ultimately undermines functional recovery. Here we investigate how TNFα might also act to upset spinal function by modulating spinal plasticity. Using a model of instrumental learning in the injured spinal cord, we have previously shown that peripheral intermittent stimulation can produce a plastic change in spinal plasticity (metaplasticity), resulting in the prolonged inhibition of spinal learning. We hypothesized that spinal metaplasticity may be mediated by TNFα. We found that intermittent stimulation increased protein levels in the spinal cord. Using intrathecal pharmacological manipulations, we showed TNFα to be both necessary and sufficient for the long-term inhibition of a spinal instrumental learning task. These effects were found to be dependent on glial production of TNFα and involved downstream alterations in calcium-permeable AMPA receptors. These findings suggest a crucial role for glial TNFα in undermining spinal learning, and demonstrate the therapeutic potential of inhibiting TNFα activity to rescue and restore adaptive spinal plasticity to the injured spinal cord. TNFα modulation represents a novel therapeutic target for improving rehabilitation after spinal cord injury.     

Absence of Glial α-Dystrobrevin Causes Abnormalities of the Blood-Brain Barrier and Progressive Brain Edema

The blood-brain barrier (BBB) plays a key role in maintaining brain functionality. Although mammalian BBB is formed by endothelial cells, its function requires interactions between endotheliocytes and glia. To understand the molecular mechanisms involved in these interactions is currently a major challenge. We show here that α-dystrobrevin (α-DB), a protein contributing to dystrophin-associated protein scaffolds in astrocytic endfeet, is essential for the formation and functioning of BBB. The absence of α-DB in null brains resulted in abnormal brain capillary permeability, progressively escalating brain edema, and damage of the neurovascular unit. Analyses in situ and in two-dimensional and three-dimensional in vitro models of BBB containing α-DB-null astrocytes demonstrated these abnormalities to be associated with loss of aquaporin-4 water and Kir4.1 potassium channels from glial endfeet, formation of intracellular vacuoles in α-DB-null astrocytes, and defects of the astrocyte-endothelial interactions. These caused deregulation of tight junction proteins in the endothelia. Importantly, α-DB but not dystrophins showed continuous expression throughout development in BBB models. Thus, α-DB emerges as a central organizer of dystrophin-associated protein in glial endfeet and a rare example of a glial protein with a role in maintaining BBB function. Its abnormalities might therefore lead to BBB dysfunction.     

Limbic Structures Show Altered Glial–Neuronal Metabolism in the Chronic Phase of Kainate Induced Epilepsy

A better understanding is needed of how glutamate metabolism is affected in mesial temporal lobe epilepsy (MTLE). Here we investigated glial–neuronal metabolism in the chronic phase of the kainate (KA) model of MTLE. Thirteen weeks following systemic KA, rats were injected i.p. with [1-¹³C]glucose. Brain extracts from hippocampal formation, entorhinal cortex, and neocortex, were analyzed by ¹³C and ¹H magnetic resonance spectroscopy to quantify ¹³C labeling and concentrations of metabolites, respectively. The amount and ¹³C labeling of glutamate were reduced in the hippocampal formation and entorhinal cortex of epileptic rats. Together with the decreased concentration of NAA, these results indicate neuronal loss. Additionally, mitochondrial dysfunction was detected in surviving glutamatergic neurons in the hippocampal formation. In entorhinal cortex glutamine labeling and concentration were unchanged despite the reduced glutamate content and label, possibly due to decreased oxidative metabolism and conserved flux of glutamate through glutamine synthetase in astrocytes. This mechanism was not operative in the hippocampal formation, where glutamine labeling was decreased. In neocortex labeling and concentration of GABA were increased in epileptic rats, possibly representing a compensatory mechanism. The changes in the hippocampus might be of pathophysiological importance and merit further studies aiming at resolving metabolic causes and consequences of MTLE. Special issue article in honor of Dr. Frode Fonnum.     

Use of Genetically Modified Glial Cells Overexpressing Laminin α1-Chain Peptides in Neurite Outgrowth Studies

SUMMARY 1. C6 glioma cells were transfected with two constructs carrying C-terminal laminin 1-chain sequences of 117 and 114 bp length, respectively. These sequences are specifically known to code for peptides which have neurite-promoting activity.2. The stable expression and secretion of the two peptides was detected by Northern and Western blot analysis.3. Primary neuronal cultures derived from embryonic mouse forebrain were cocultured with these transfected cells and exhibited a substantial increase in neurite outgrowth and in survival time. Conditioned media from the transfected cells generated similar effects.4. Organotypic cultures from embryonic mouse brain were used as a second system as being closer to the in vivo situation. Again, coculture of brain slices with transfected cells or treatment with laminin peptide-containing media increased neuronal outgrowth.     

Pigment Epithelium-Derived Factor Released by Müller Glial Cells Exerts Neuroprotective Effects on Retinal Ganglion Cells

Survival of retinal ganglion cells (RGC) is compromised in several vision-threatening disorders such as ischemic and hypertensive retinopathies and glaucoma. Pigment epithelium-derived factor (PEDF) is a naturally occurring pleiotropic secreted factor in the retina. PEDF produced by retinal glial (Müller) cells is suspected to be an essential component of neuron-glial interactions especially for RGC, as it can protect this neuronal type from ischemia-induced cell death. Here we show that PEDF treatment can directly affect RGC survival in vitro. Using Müller cell-RGC-co-cultures we observed that activity of Müller-cell derived soluble mediators can attenuate hypoxia-induced damage and RGC loss. Finally, neutralizing the activity of PEDF in glia-conditioned media partially abolished the neuroprotective effect of glia, leading to an increased neuronal death in hypoxic condition. Altogether our results suggest that PEDF is crucially involved in the neuroprotective process of reactive Müller cells towards RGC.     

The RET–Glial Cell-derived Neurotrophic Factor (GDNF) Pathway Stimulates Migration and Chemoattraction of Epithelial Cells

Embryonic development requires cell migration in response to positional cues. Yet, how groups of cells recognize and translate positional information into morphogenetic movement remains poorly understood. In the developing kidney, the ureteric bud epithelium grows from the nephric duct towards a group of posterior intermediate mesodermal cells, the metanephric mesenchyme, and induces the formation of the adult kidney. The secreted protein GDNF and its receptor RET are required for ureteric bud outgrowth and subsequent branching. However, it is unclear whether the GDNF–RET pathway regulates cell migration, proliferation, survival, or chemotaxis. In this report, we have used the MDCK renal epithelial cell line to show that activation of the RET pathway results in increased cell motility, dissociation of cell adhesion, and the migration towards a localized source of GDNF. Cellular responses to RET activation include the formation of lamellipodia, filopodia, and reorganization of the actin cytoskeleton. These data demonstrate that GDNF is a chemoattractant for RET-expressing epithelial cells and thus account for the developmental defects observed in RET and GDNF mutant mice. Furthermore, the RET-transfected MDCK cells described in this report are a promising model for delineating RET signaling pathways in the renal epithelial cell lineage.     

Human ESC-Derived Chimeric Mouse Models of Huntington’s Disease Reveal Cell-Intrinsic Defects in Glial Progenitor Cell Differentiation

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Altered Neuronal–Glial Signaling in Glutamatergic Transmission as a Unifying Mechanism in Chronic Pain and Mental Fatigue

Recent experimental results have demonstrated a glial activation during long-term pain that produces and releases cytokines, free oxygen radicals, nitric oxide, and other neuroactive substances in the spinal cord dorsal horns. Such activation might generate a vicious circle by increasing the neuronal excitability level due to a decreased astroglial glutamate uptake and thereby reinforce pain signals that travel up to the thalamus and further up into the parietal cortex for identification and interpretation. In this paper, we adapt new knowledge on neuronal-glial signaling in the CNS to develop tentative explanations at the cellular level for the maintenance of pain signals in the brain, for formation of "pain memory," and even for the increased pain sensitivity that persons with chronic pain often experience in body regions other than those originally affected. We also suggest a hypothetical mechanism at the cellular level underlying the mental fatigue from which persons with chronic pain may suffer. This hypothesis relies on the impaired astroglial glutamate uptake capacity due to the production of neuroactive substances, altered conditions in the chronic pain state, and the anxiety and stress reactions that may occur secondary to the pain. Neuronal activity over time in the dysfunctional state of the astroglial network leads to an increase in extracellular glutamate levels in the vicinity of glutamate synapses. In turn, this increase leads over time to less precision in glutamate transmission. The increased extracellular glutamate levels lead to increased excitability and increased energy requirements. When cellular energy decreases the glutamate transmission decreases, and according to our hypothesis, this is one cause of mental fatigue. New strategies for treatment of chronic pain and the associated mental fatigue are formulated and should be explored.