Bidirectional signaling between neurons and glial cells has been demonstrated in brain slices and is believed to mediate glial modulation of synaptic transmission in the CNS. Our laboratory has characterized similar neuron–glia signaling in the mammalian retina. We find that light-evoked neuronal activity elicits Ca2+ increases in Müller cells, which are specialized retinal glial cells. Neuron to glia signaling is likely mediated by the release of ATP from neurons and is potentiated by adenosine. Glia to neuron signaling has also been observed and is mediated by several mechanisms. Stimulation of glial cells can result in either facilitation or depression of synaptic transmission. Release of D-serine from Müller cells might also potentiate NMDA receptor transmission. Müller cells directly inhibit ganglion cells by releasing ATP, which, following hydrolysis to adenosine, activates neuronal A1 receptors. The existence of bidirectional signaling mechanisms indicates that glial cells participate in information processing in the retina.

The astrocytic enzyme adenosine kinase (ADK) is a key negative regulator of the brain’s endogenous anticonvulsant adenosine. Astrogliosis with concomitant upregulation of ADK is part of the epileptogenic cascade and contributes to seizure generation. To molecularly dissect the respective roles of astrogliosis and ADK-expression for seizure generation, we used a transgenic approach to uncouple ADK-expression from astrogliosis: in Adk-tg mice the endogenous Adk-gene was deleted and replaced by a ubiquitously expressed Adk-transgene with novel ectopic expression in pyramidal neurons, resulting in spontaneous seizures. Here, we followed a unique approach to selectively injure the CA3 of these Adk-tg mice. Using this strategy, we had the opportunity to study astrogliosis and epileptogenesis in the absence of the endogenous astrocytic Adk-gene. After triggering epileptogenesis we demonstrate astrogliosis without upregulation of ADK, but lack of seizures, whereas matching wild-type animals developed astrogliosis with upregulation of ADK and spontaneous recurrent seizures. By uncoupling ADK-expression from astrogliosis, we demonstrate that global expression levels of ADK rather than astrogliosis per se contribute to seizure generation.

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.

Broad interest in the rapidly advancing field of microglial involvement in forming neural circuits is evident from the fresh findings published in leading journals. This special issue of Neuron Glia Biology contains a special collection of research articles and reviews concerning the new appreciation of microglial function in the normal physiology of the brain that extends beyond their traditionally understood role in pathology.

cDNA microarrays were utilized to identify abnormally expressed genes in a malignant peripheral nerve sheath tumor (MPNST)-derived cell line, T265, by comparing the mRNA abundance profiles with that of normal human Schwann cells (nhSCs). The findings characterize the molecular phenotype of this important cell-line model of MPNSTs, and elucidate the contribution of Schwann cells in MPNSTs. In total, 4608 cDNA sequences were screened and hybridizations replicated on custom cDNA microarrays. In order to verify the microarray data, a large selection of differentially expressed mRNA transcripts were subjected to semi-quantitative reverse transcription PCR (LightCycler). Western blotting was performed to investigate a selection of genes and signal transduction pathways, as a further validation of the microarray data. The data generated from multiple microarray screens, semi-quantitative RT–PCR and Western blotting are in broad agreement. This study represents a comprehensive gene-expression analysis of an MPNST-derived cell line and the first comprehensive global mRNA profile of nhSCs in culture. This study has identified ∼900 genes that are expressed abnormally in the T265 cell line and detected many genes not previously reported to be expressed in nhSCs. The results provide crucial information on the T265 cells that is essential for investigation using this cell line in experimental studies in neurofibromatosis type I (NF1), and important information on normal human Schwann cells that is applicable to a wide range of studies on Schwann cells in cell culture.

Various animal models are available for studying human multiple sclerosis (MS). Most of them model the initial phase of MS, including the immune-triggered attack of the myelin membrane and/or oligodendrocytes and, occasionally, demonstrate the remission and relapsing phases. However, few mimic the late chronic demyelinating phase. Overexpression of the proteolipid protein gene (Plp) causes a unique demyelinating disorder in mice in which normal-appearing myelin forms early in life and chronic demyelination occurs later. We found that remyelination is severely affected in this late demyelinating phase, but is not caused by deprivation of oligodendrocyte progenitors expressing PDGF receptor alpha (PDGFRα) and Olig2, which are present at an even higher number in the demyelinated white matter of the mutants than in wild-type controls. Furthermore, mature oligodendrocytes containing PLP were observed, but failed to remyelinate. The ability of oligodendrocytes from older transgenic animals to produce a myelin membrane-like structure was not impaired when cultured in vitro, which indicates that the lack of remyelination is not simply caused by changes in the intrinsic properties of the oligodendrocytes. Glial activation also occurred much earlier than active demyelination in mutant mice. Thus, in addition to intrinsic mechanisms, extrinsic mechanisms might also have an important role in defects of remyelination. These features are also observed in patients at a late stage of MS, leading to chronic demyelinating lesions. Thus, this mouse model partly mimics the late stage of MS and can be used to study the cause of inhibition of remyelination.

Transplantation of olfactory ensheathing cells (OECs) into the injured spinal cord has been shown to exert neuroprotective effects and promote functional recovery. In the present study, we investigated the potential modulatory effects of OECs on the inflammatory reaction developed after photochemical injury to the spinal cord. OEC cultures were obtained from olfactory bulbs of adult Sprague-Dawley rats. Photochemical spinal cord injury was induced in adult rats at T8. Thirty minutes after the insult, either a suspension of OECs (180 000 cells in 12 µl DMEM) or DMEM alone was injected into the lesioned spinal cord. At 3, 7 and 14 days post-operation (dpo), five animals from each group were processed for histology. Double-fluorescent labeling of transverse sections of the cord were made by combination of immunohistochemistry for inflammatory markers, interleukin 1β (IL-1β) and inducible nitric oxide synthase (iNOS), and for selective markers of astrocytes (glial fibrillar acidic protein; GFAP) and microglia/macrophages (tomato lectin; LEC). Differences in the intensity and time course of glial response, and IL-1β and iNOS expression were found between the two groups of rats. The reactivity grade against IL-1β, iNOS, GFAP and LEC in OEC-transplanted rats was higher at 7 dpo and lower at 14 dpo compared with DMEM-injected rats. These results indicate that the mechanisms underlying neuroprotection by OECs might be caused by earlier, higher and shorter duration of microglia/macrophage and astrocyte responses after injury.

The complexity of the adult brain is the result of an integrated series of developmental events that depends on appropriate timing of differentiation. The importance of transcriptional regulatory networks and epigenetic mechanisms of regulation of gene expression is becoming increasingly evident. Among these mechanisms, previous work has revealed the importance of histone deacetylation in oligodendrocyte differentiation. In this manuscript we define the region of interaction between transcription factor Yin-Yang 1 (YY1) and histone deacetylase 1, and characterize the functional consequences of YY1 overexpression on the differentiation of oligodendrocyte progenitors.

The regional heterogeneity of neuronal phenotypes is a well-known phenomenon. Whether or not glia derived from different brain regions are phenotypically and functionally distinct is less clear. Here, we show that microglia, the resident immune cells of the brain, display region-specific responses for activating agents including glutamate (GLU), lipopolysaccharide (LPS) and adenosine 5′-triphosphate (ATP). Primary microglial cultures were prepared from brainstem (Brs), cortex (Ctx), hippocampus (Hip), striatum (Str) and thalamus (Thl) of 1-day-old rats and were shown to upregulate the release of nitric oxide (NO) and brain-derived neurotrophic factor (BDNF) in a region- and activator-specific manner. With respect to ATP specifically, ATP-induced changes in microglial tumor necrosis factor-α (TNF-α) release, GLU uptake and purinergic receptor expression were also regionally different. When co-cultured with hypoxia (Hyp)-injured neurons, ATP-stimulated microglia from different regions induced different levels of neurotoxicity. These region-specific responses could be altered by pre-conditioning the microglia in a different neurochemical milieu, with taurine (TAU) being one of the key molecules involved. Together, our results demonstrate that microglia display a regional heterogeneity when activated, and this heterogeneity likely arises from differences in the environment surrounding the microglia. These findings present an additional mechanism that may help to explain the regional selectiveness of various brain pathologies.

Current concepts of invertebrate phylogeny are reviewed. Annelida and Arthropoda, previously regarded as closely related, are now placed in separate clades. Myelin, a sheath of multiple layers of membranes around nerve axons, is found in members of the Annelida, Arthropoda and Chordata. The structure, composition and function of the sheaths in Annelida and Arthropoda are examined and evidence for the separate evolutionary origins of myelin in the three clades is presented. That myelin has arisen independently at least three times, namely in Annelids, Arthropodas and Chordates, provides a remarkable example of convergent evolution.

Extensive literature documented that astrocytes release neurotransmitters, cytokines and other signaling molecules to modulate migration, maturation and myelin synthesis of oligodendrocytes through mechanisms primarily converging on cytosolic [Ca2+] transients. Considering the long-term effects, it is expected that astrocyte-conditioned medium is a major regulator of gene expression in oligodendrocytes even in the absence of cytosol-to-cytosol communication via astrocyte–oligodendrocyte gap junction channels. Indeed, by comparing the transcriptomes of immortalized precursor oligodendrocyte (Oli-neu) cells when cultured alone and co-cultured with non-touching astrocytes we found profound changes in the gene expression level, control and networking. Remarkably, the astrocyte proximity was more effective in remodeling the myelination (MYE) gene fabric and its control by cytokine receptor (CYR)-modulated intercellular Ca2+-signaling (ICS) transcriptomic network than the dibutyryl-cAMP (db-cAMP) treatment-induced transformation into myelin-associated glycoprotein-positive oligodendrocyte-like cells. Moreover, astrocyte proximity up-regulated 37 MYE genes and switched on another 14 MYE, 23 ICS and 4 CYR genes, enhancing the roles of the leukemia inhibitory factor receptor and connexins Cx29 and Cx47. The novel prominent gene analysis identified the enhancer of zeste homolog 2 as the most relevant MYE gene in the astrocyte proximity, notch gene homolog 1 in control and B-cell leukemia/lymphoma 2 in differentiated Oli-neu cells.

In the cerebellum, lamellar Bergmann glial (BG) appendages wrap tightly around almost every Purkinje cell dendritic spine. The function of this glial ensheathment of spines is not entirely understood. The development of ensheathment begins near the onset of synaptogenesis, when motility of both BG processes and dendritic spines are high. By the end of the synaptogenic period, ensheathment is complete and motility of the BG processes decreases, correlating with the decreased motility of dendritic spines. We therefore have hypothesized that ensheathment is intimately involved in capping synaptogenesis, possibly by stabilizing synapses. To test this hypothesis, we misexpressed GluR2 in an adenoviral vector in BG towards the end of the synaptogenic period, rendering the BG α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) Ca2+-impermeable and causing glial sheath retraction. We then measured the resulting spine motility, spine density and synapse number. Although we found that decreasing ensheathment at this time does not alter spine motility, we did find a significant increase in both synaptic pucta and dendritic spine density. These results indicate that consistent spine coverage by BG in the cerebellum is not necessary for stabilization of spine dynamics, but is very important in the regulation of synapse number.

Cerebral white matter injury in premature infants, known as periventricular leukomalacia (PVL), is common after hypoxia–ischemia (HI). While ionotropic glutamate receptors (iGluRs) can mediate immature white matter injury, we have previously shown that excitotoxic injury to premyelinating oligodendrocytes (preOLs) in vitro can be attenuated by group I metabotropic glutamate receptor (mGluR) agonists. Thus, we evaluated mGluR expression in developing white matter in rat and human brain, and tested the protective efficacy of a central nervous system (CNS)-penetrating mGluR agonist on injury to developing oligodendrocytes (OLs) in vivo. Group I mGluRs (mGluR1 and mGluR5) were strongly expressed on OLs in neonatal rodent cerebral white matter throughout normal development, with highest expression early in development on preOLs. Specifically at P6, mGluR1 and mGLuR5 were most highly expressed on GalC-positive OLs compared to neurons, axons, astrocytes and microglia. Systemic administration of (1S,3R) 1-aminocyclopentane-trans-1,3,-dicarboxylic acid (ACPD) significantly attenuated the loss of myelin basic protein in the white matter following HI in P6 rats. Assessment of postmortem human tissue showed both mGluR1 and mGluR5 localized on immature OLs in white matter throughout development, with mGluR5 highest in the preterm period. These data indicate group I mGluRs are highly expressed on OLs during the peak period of vulnerability to HI and modulation of mGluRs is protective in a rodent model of PVL. Group I mGluRs may represent important therapeutic targets for protection from HI-mediated white matter injury.

Recent studies have implicated glial cells in novel physiological roles in the CNS, such as modulation of synaptic transmission, so it is possible that glial cells might have a functional role in the hyperexcitability that is characteristic of epilepsy. Indeed, alterations in distinct astrocyte membrane channels, receptors and transporters have all been associated with the epileptic state. This paper focuses on the potential roles of the glial water channel aquaporin-4 (AQP4) in modulating brain excitability and in epilepsy. We review studies of seizure phenotypes, K+ homeostasis and extracellular space physiology of mice that lack AQP4 (AQP4−/− mice) and discuss the human studies demonstrating alterations of AQP4 in specimens of human epilepsy tissue. We conclude with new studies of AQP4 regulation by seizures and discuss its potential role in the development of epilepsy (epileptogenesis). Although many questions remain unanswered, the available data indicate that AQP4 and its molecular partners might represent important new therapeutic targets.

Members of the major histocompatibility complex (MHC) class I family of proteins are well known for their central role in the adaptive immune system, where they present self and non-self peptides for immune surveillance. Although the brain has been long considered immune privileged, in part because of an apparent lack of neuronal MHC class I, it has since been shown that MHC class I proteins are expressed by normal, uninfected neurons. Moreover, expression of MHC class I is unusually dynamic in the developing and adult brain, and MHC class I levels in neurons can be regulated by endogenous and exogenous electrical activity. Unexpectedly, several recent studies find that MHC class I is required for distinct activity-dependent events during brain development, adult plasticity, and in response to injury. Together, these studies indicate a novel role for MHC class I proteins in translating electrical activity into changes in synaptic strength and neuronal connectivity in vivo.

Synapse formation in the CNS is a complex process that involves the dynamic interplay of numerous signals exchanged between pre- and postsynaptic neurons as well as perisynaptic glia. Members of the neurotrophin family, which are widely expressed in the developing and mature CNS and are well-known for their roles in promoting neuronal survival and differentiation, have emerged as key synaptic modulators. However, the mechanisms by which neurotrophins modulate synapse formation and function are poorly understood. Here, we summarize our work on the role of neurotrophins in synaptogenesis in the CNS, in particular the role of these signaling molecules and their receptors, the Trks, in the development of excitatory and inhibitory hippocampal synapses. We discuss our results that demonstrate that postsynaptic TrkB signaling plays an important role in modulating the formation and maintenance of NMDA and GABAA receptor clusters at central synapses, and suggest that neurotrophin signaling coordinately modulates these receptors as part of mechanism that promotes the balance between excitation and inhibition in developing circuits. We also discuss our results that demonstrate that astrocytes promote the formation of GABAergic synapses in vitro by differentially regulating the development of inhibitory presynaptic terminals and postsynaptic GABAA receptor clusters, and suggest that glial modulation of inhibitory synaptogenesis is mediated by neurotrophin-dependent and -independent signaling. Together, these findings extend our understanding of how neuron–glia communication modulates synapse formation, maintenance and function, and set the stage for defining the cellular and molecular mechanisms by which neurotrophins and other cell–cell signals direct synaptogenesis in the developing brain.

The biochemical effects triggered by the action of glutamate, the main excitatory amino acid, on a specialized type of glia cells, Bergmann glial cells of the cerebellum, are a model system with which to study glia–neuronal interactions. Neuron to Bergmann glia signaling is involved in early stages of development, mainly in cell migration and synaptogenesis. Later, in adulthood, these cells have an important role in the maintenance and proper function of the synapses that they surround. Major molecular targets of this cellular interplay are glial glutamate receptors and transporters, both of which sense synaptic activity. Glutamate receptors trigger a complex network of signaling cascades that involve Ca2+ influx and lead to a differential gene-expression pattern. In contrast, Bergmann glia glutamate transporters participate in the removal of the neurotransmitter from the synaptic cleft and act also as signal transducers that regulate, in the short term, their own activity. These exciting findings strengthen the concept of active participation of glial cells in synaptic transmission and the involvement of neuron–glia circuits in the processing of brain information.

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeats within the coding sequence of the ataxin-1 protein. In the present study, we used a conditional transgenic mouse model of SCA1 to investigate very early molecular and morphological changes related to the behavioral phenotype. In mice with neural deficits detected by rotarod performance, and simultaneous spatial impairments in exploratory activity and uncoordinated gait, we observed both significant altered expression and patchy distribution of excitatory amino acids transporter 1. The molecular changes observed in astroglial compartments correlate with changes in synapse morphology; synapses have a dramatic reduction of the synaptic area external to the postsynaptic density. By contrast, Purkinje cells demonstrate preserved structure. In addition, severe reactive astrocytosis matches changes in the glial glutamate transporter and synapse morphology. We propose these morpho-molecular changes are the cause of altered synaptic transmission, which, in turn, determines the onset of the neurological symptoms by altering the synaptic transmission in the cerebellar cortex of transgenic animals. This model might be suitable for testing drugs that target activated glial cells in order to reduce CNS inflammation.

Live-cell imaging of glioblastoma U373 and U87 cells transfected with actin cytoskeleton markers has been used to study the re-arrangements that are associated with migration in two- and three-dimensional matrices and in brain tissue. In collagen gels and in brain slices, both cell types developed neuronal-like processes with ruffling membranes and filopodia. Blebbing cells were also observed, but these were mainly immobile. The retraction of trailing cell processes in a tissue environment was associated with the transient development and contraction of bundles of axial stress fibers. The inhibition of Rho-kinase caused glioblastoma cells in brain slices to become immobile and develop neurite-like processes at random, which indicates the requirement of Rho signaling and contractility for migration. Actin stress fibers were also observed in glioblastoma cells injected into the brains of living mice. Thus, invading glioblastoma cells use neurite-like extensions to penetrate between neuronal fibers and contractile actin bundles for traction of the cell body.