Astrocyte,the star <Emphasis Type="Italic">avatar</Emphasis>: redefined

Until recently, the neuroscience community held the belief that glial cells such as astrocytes and oligodendrocytes functioned solely as “support” cells of the brain. In this role, glial cells simply provide physical support and housekeeping functions for the more important cells of the brain, the neurons. However, this view has changed radically in recent years with the discovery of previously unrecognized and surprising functions for this underappreciated cell type. In the past decade or so, emerging evidence has provided new insights into novel glial cell activities such as control of synapse formation and function, communication, cerebrovascular tone regulation, immune regulation and adult neurogenesis. Such advances in knowledge have effectively elevated the role of the astrocyte to one that is more important than previously realized. This review summarizes the past and present knowledge of glial cell functions that has evolved over the years, and has resulted in a new appreciation of astrocytes and their value in studying the neurobiology of human brain cells and their functions. In this review, we highlight recent advances in the role of glial cells in physiology, pathophysiology and, most importantly, in adult neurogenesis and “stemness”, with special emphasis on astrocytes.     

Expression of Hyaluronectin by C-6 Glial Cells at High Density

Hyaluronectin, a brain glycoprotein that has been localized to the nodes of Ranvier in vivo and to oligodendrocytes in primary cultures of neonatal rat brain cells, was shown by using an unlabeled immunoperoxidase method to be present in C-6 glial cells grown to high density. The density-dependent expression of this glycoprotein is in accordance with the known properties of the glial stem cells, i.e., induction of differentiated properties such as 2',3'-cyclic nucleotide-3'-phosphohydrolase, glutamine synthetase, S-100 protein, and glial fibrillary acidic protein.     

Neuroglia in ageing and disease

The proper operation of the mammalian brain requires dynamic interactions between neurones and glial cells. Various types of glial cells are susceptible to morpho-functional changes in a variety of brain pathological states, including toxicity, neurodevelopmental, neurodegenerative and psychiatric disorders. Morphological modifications include a change in the glial cell size and shape; the latter is evident by changes of the appearance and number of peripheral processes. The most blatant morphological change is associated with the alteration of the sheer number of neuroglia cells in the brain. Functionally, glial cells can undergo various metabolic and biochemical changes, the majority of which reflect upon homeostasis of neurotransmitters, in particular that of glutamate, as well as on defence mechanisms provided by neuroglia. Not only glial cells exhibit changes associated with the pathology of the brain but they also change with brain aging.     

CALCIUM METABOLISM IN ISOLATED BRAIN CELLS AND SUBCELLULAR FRACTIONS

Abstract— The accumulation of calcium ions by brain mitochondria and microsomes and by fractions containing neuronal or glial cells has been studied in vitro with techniques involving ⁴⁵Ca and ultramicro-flame photometry. ATP and substrate-supported calcium accumulation by brain mitochondria was of the same magnitude as for mitochondria from other organs. Brain microsomes accumulated calcium approximately 15 times less than brain mitochondria. Variations in Na⁺/K⁺ ratios and in ATP/ADP ratios had a more marked influence on microsomal uptake than on mitochondrial uptake. The passive Ca²⁺ binding by glial cells was higher than neuronal perikarya and synaptosomes. Also the calcium accumulation ability in cell suspensions was slightly higher for glial cells as compared to neuronal perikarya. The calcium uptake by glial cells was stimulated by high external K⁺ concentration, which also was the case for nerve endings. The uptake in neuronal perikarya was unaffected by variations in K⁺ concentration. A comparison between neuronal and glial mitochondria showed that both reach a steady state level of similar magnitude, but that the rate of initial accumulation was greater for glial mitochondria. A high glial calcium accumulation was also observed for the microsomal fraction.     

Organization of a midline proliferative cluster in the embryonic brain of the grasshopper

We have studied a group of midline cells in the embryonic brain of the grasshopper by using immunocytochemical and intracellular dye injection techniques. This cluster of midline cells differentiates between the pars intercerebralis lobes of the protocerebrum during early embryogenesis, and is composed of putative midline progenitors as well as neuronal and glial cells. Annulin immunoreactive glial processes surround the borders of the midline cell cluster and also form a network of processes extending from there to the borders of proliferative clusters in the brain hemispheres. Among the cells that derive from the midline cluster are two bilaterally symmetrical pairs of identified primary commissure pioneer neurons. By navigating along the glial bound borders of the midline proliferative cluster, the axons of these pioneers establish an initial axonal bridge across the brain midline. This analysis identifies a glial-bound midline proliferative cluster in the brain and shows that neuronal and glial cells of this cluster are closely associated with neurons pioneering the primary brain commissure. Comparable features of midline cells in the ventral ganglia and similarities to other proliferative clusters in the brain hemispheres are discussed.     

Mutually exclusive expression of fibronectin and glial fibrillary acidic protein in cultured brain cells

The reported expression of the cell surface-associated, mainly mesenchymal glycoprotein fibronectin by cultured glial cells is in discrepancy with recent work on brain tissue failing to demonstrate any glial or neuronal fibronectin. We have investigated the expression of fibronectin in relation to glial fibrillary acidic protein in cultured human glial and glioma cell lines as well as in cultures derived from newborn rat brain. Using double immunofluorescence technique we found that cells containing glial fibrillary acidic protein do not express fibronectin, and vice versa. The only exception to this rule was the occasional finding of fibronectin at points of cell-to-cell adhesion also in relation to cells containing glial fibrillary acidic protein. The results were also tested by polyacrylamide gel electrophoresis of the culture media of the human cell lines, and by subcultures from the brain of newborn rat, cultures stimulated with dibutyryl cyclic AMP (db-cAMP), and by vinblastine treatment of the cells. The lack of expression of fibronectin in cells containing glial fibrillary acidic protein, a gliospecific cytoskeletal protein, is discussed with reference to glio-mesenchymal interactions and glial markers in vitro.     

Cytochrome P450 1A isoenzymes in brain cells: Expression and inducibility in cultured rat brain neuronal and glial cells

Studies initiated to determine the expression of CYP1A1/1A2 isoenzymes in the primary cultures of rat brain neuronal and glial cells revealed significant activity of CYP1A-dependent 7-ethoxyresorufin-o-dealkylase (EROD) in microsomes prepared from both rat brain neuronal and glial cells. RT-PCR and immunocytochemical studies demonstrated constitutive mRNA and protein expression of CYP1A1 and 1A2 isoenzymes in cultured neuronal and glial cells. Cultured neurons exhibited relatively higher constitutive mRNA and protein expression of CYP1A1 and 1A2 isoenzymes, associated with higher activity of EROD than the glial cells. Induction studies with 3-methylchlorantherene (MC), a known CYP1A-inducer, resulted in significant concentration dependent increase in the activity of EROD in cultured rat brain cells with glial cells exhibiting a greater magnitude of induction than the neuronal cells. This difference in the increase in enzyme activity was also observed with RT-PCR and immunocytochemical studies, indicating relatively higher increase in CYP1A1 and 1A2 mRNA as well as protein expression in the cultured glial cells when compared to the neuronal cells. The greater magnitude of induction of CYP1A1 in glial cells is of significance, as these cells are components of the blood-brain barrier and it is suggested that they have a potential role in the toxication-detoxication mechanism. Our data indicating differences in the expression and sensitivity of CYP1A1 isoenzymes in cultured rat brain cells will not only help in identifying and distinguishing xenobiotic metabolizing capability of these cells but also in understanding the vulnerability of these specific cell types towards neurotoxicants.     

Multipotent neural stem cells generate glial cells of the central complex through transit amplifying intermediate progenitors in Drosophila brain development

The neural stem cells that give rise to the neural lineages of the brain can generate their progeny directly or through transit amplifying intermediate neural progenitor cells (INPs). The INP-producing neural stem cells in Drosophila are called type II neuroblasts, and their neural progeny innervate the central complex, a prominent integrative brain center. Here we use genetic lineage tracing and clonal analysis to show that the INPs of these type II neuroblast lineages give rise to glial cells as well as neurons during postembryonic brain development. Our data indicate that two main types of INP lineages are generated, namely mixed neuronal/glial lineages and neuronal lineages. Genetic loss-of-function and gain-of-function experiments show that the gcm gene is necessary and sufficient for gliogenesis in these lineages. The INP-derived glial cells, like the INP-derived neuronal cells, make major contributions to the central complex. In postembryonic development, these INP-derived glial cells surround the entire developing central complex neuropile, and once the major compartments of the central complex are formed, they also delimit each of these compartments. During this process, the number of these glial cells in the central complex is increased markedly through local proliferation based on glial cell mitosis. Taken together, these findings uncover a novel and complex form of neurogliogenesis in Drosophila involving transit amplifying intermediate progenitors. Moreover, they indicate that type II neuroblasts are remarkably multipotent neural stem cells that can generate both the neuronal and the glial progeny that make major contributions to one and the same complex brain structure.     

Nuclear progesterone receptors are up-regulated by estrogens in neurons and radial glial progenitors in the brain of zebrafish

In rodents, there is increasing evidence that nuclear progesterone receptors are transiently expressed in many regions of the developing brain, notably outside the hypothalamus. This suggests that progesterone and/or its metabolites could be involved in functions not related to reproduction, particularly in neurodevelopment. In this context, the adult fish brain is of particular interest, as it exhibits constant growth and high neurogenic activity that is supported by radial glia progenitors. However, although synthesis of neuroprogestagens has been documented recently in the brain of zebrafish, information on the presence of progesterone receptors is very limited. In zebrafish, a single nuclear progesterone receptor (pgr) has been cloned and characterized. Here, we demonstrate that this pgr is widely distributed in all regions of the zebrafish brain. Interestingly, we show that Pgr is strongly expressed in radial glial cells and more weakly in neurons. Finally, we present evidence, based on quantitative PCR and immunohistochemistry, that nuclear progesterone receptor mRNA and proteins are upregulated by estrogens in the brain of adult zebrafish. These data document for the first time the finding that radial glial cells are preferential targets for peripheral progestagens and/or neuroprogestagens. Given the crucial roles of radial glial cells in adult neurogenesis, the potential effects of progestagens on their activity and the fate of daughter cells require thorough investigation.     

Glial patterning during postembryonic development of central neuropiles in the brain of the honeybee

 Glial cells are involved in several functions during the development of the nervous system. To understand potential glial contributions to neuropile formation, we examined the cellular pattern of glia during the development of the mushroom body, antennal lobe and central complex in the brain of the honeybee. Using an antibody against the glial-specific repo-protein of Drosophila, the location of the glial somata was detected in the larval and pupal brain of the bee. In the early larva, a continuous layer of glial cell bodies defines the boundaries of all growing neuropiles. Initially, the neuropiles develop in the absence of any intrinsic glial somata. In a secondary process, glial cells migrate into defined locations in the neuropiles. The corresponding increase in the number of neuropile-associated glial cells is most likely due to massive immigrations of glial cells from the cell body rind using neuronal fibres as guidance cues. The combined data from the three brain regions suggest that glial cells can prepattern the neuropilar boundaries. Received: 3 November 1996 / Accepted: 7 February 1997     

Calcium signal communication in the central nervous system

The communication of calcium signals between cells is known to be operative between neurons where these signals integrate intimately with electrical and chemical signal communication at synapses. Recently, it has become clear that glial cells also exchange calcium signals between each other in cultures and in brain slices. This communication pathway has received utmost attention since it is known that astrocytic calcium signals can be induced by neuronal stimulation and can be communicated back to the neurons to modulate synaptic transmission. In addition to this, cells that are generally not considered as brain cells become progressively incorporated in the picture, as astrocytic calcium signals are reported to be communicated to endothelial cells of the vessel wall and can affect smooth muscle cell tone to influence the vessel diameter and thus blood flow. We review the available evidence for calcium signal communication in the central nervous system, taking into account a basic functional unit -the brain cell tripartite- consisting of neurons, glial cells and vascular cells and with emphasis on glial-vascular calcium signaling aspects.     

Nicotine Uses Neuron-Glia Communication to Enhance Hippocampal Synaptic Transmission and Long-term Memory

Nicotine enhances synaptic transmission and facilitates long-term memory. Now it is known that bi-directional glia-neuron interactions play important roles in the physiology of the brain. However, the involvement of glial cells in the effects of nicotine has not been considered until now. In particular, the gliotransmitter D-serine, an endogenous co-agonist of NMDA receptors, enables different types of synaptic plasticity and memory in the hippocampus. Here, we report that hippocampal long-term synaptic plasticity induced by nicotine was annulled by an enzyme that degrades endogenous D-serine, or by an NMDA receptor antagonist that acts at the D-serine binding site. Accordingly, both effects of nicotine: the enhancement of synaptic transmission and facilitation of long-term memory were eliminated by impairing glial cells with fluoroacetate, and were restored with exogenous D-serine. Together, these results show that glial D-serine is essential for the long-term effects of nicotine on synaptic plasticity and memory, and they highlight the roles of glial cells as key participants in brain functions.     

Expression of Aromatase in Radial Glial Cells in the Brain of the Japanese Eel Provides Insight into the Evolution of the cyp191a Gene in Actinopterygians

The cyp19a1 gene that encodes aromatase, the only enzyme permitting conversion of C19 aromatizable androgens into estrogens, is present as a single copy in the genome of most vertebrate species, except in teleosts in which it has been duplicated. This study aimed at investigating the brain expression of a cyp19a1 gene expressed in both gonad and brain of Japanese eel, a basal teleost. By means of immunohistochemistry and in situ hybridization, we show that cyp19a1 is expressed only in radial glial cells of the brain and in pituitary cells. Treatments with salmon pituitary homogenates (female) or human chorionic gonadotrophin (male), known to turn on steroid production in immature eels, strongly stimulated cyp19a1 messenger and protein expression in radial glial cells and pituitary cells. Using double staining studies, we also showed that aromatase-expressing radial glial cells exhibit proliferative activity in both the brain and the pituitary. Altogether, these data indicate that brain and pituitary expression of Japanese eel cyp19a1 exhibits characteristics similar to those reported for the brain specific cyp19a1b gene in teleosts having duplicated cyp19a1 genes. This supports the hypothesis that, despite the fact that eels also underwent the teleost specific genome duplication, they have a single cyp19a1 expressed in both brain and gonad. Such data also suggest that the intriguing features of brain aromatase expression in teleost fishes were not gained after the whole genome duplication and may reflect properties of the cyp19a1 gene of ancestral Actinopterygians.     

Exploring glia to better understand Alzheimer’s disease

The amyloid-β (Aβ) hypothesis has been the leading explanation for the pathogenesis of Alzheimer’s disease (AD). The most common traits of AD are cognitive impairments and memory loss, which are associated with the accumulation of Aβ. Aβ aggregates activate glial cells, which in turn remove Aβ. Because microglia act as immune cells in the brain, most glia-related studies of AD have focused primarily on this cell type. However, astrocytes, another type of glial cell, also participate in the brain immune system, synaptic formation, brain homeostasis, and various other brain functions. Accordingly, many studies on the underlying mechanisms of AD have investigated not only neurons but also glial cells. Although these studies suggest that microglia and astrocytes are effective targets for AD therapeutics, other recent studies have raised questions regarding whether microglial cells and/or astrocytes serve a neuroprotective or neurotoxic function in AD. To gain a better understanding of the mechanisms of AD and identify novel targets for AD treatment, in this review, we consider the role of both microglia and astrocytes in AD.     

Progress in glial cell studies in some laboratories in China

Glial cells in the central nervous system(CNS) consist of a heterogeneous population of cell types,each characterized by distinct morphological features,physiological properties,and specific markers.In contrast to the previous view that glial cells were passive elements in the brain,accumulating evidence suggests that glial cells are active participants in various brain functions and brain disorders.This review summarizes recent progress of glial cell studies from several groups in China,ranging from studie...     

Notch2 expression negatively correlates with glial differentiation in the postnatal mouse brain

Notch family molecules are thought to be negative regulators of neuronal differentiation in early brain development. After expression in the embryonic period, Notch2 continues to be expressed postnatally in the specific regions in the rodent brain. Here, we examined Notch2 expression in the postnatal mouse brain using lacZ knockin animals at the Notch2 locus. Notch2 expression was observed in the developing cerebellum and hippocampus, characteristic regions where neurogenesis persists after birth. Double staining of sections revealed that Notch2 was expressed by Bergmann glia in the cerebellum, radial glia in the hippocampus, and some astrocytes in both regions. Notch2 expression by glial cells was clearly confirmed in dissociated cell cultures. Interestingly, neocortical glia, many of which did not express Notch2 in vivo, did express Notch2 in a dissociated culture condition. The triple staining of dissociated cell cultures revealed that stronger Notch2 expression correlated with the immature type of glial gene expressions: stronger vimentin and weaker glial fibrillary acidic protein expressions. In addition, Notch2 expression correlated with the incorporation of bromodeoxyuridine both in vivo and in vitro. Thus, these findings demonstrate that Notch2 is expressed not only by neuronal cells in the embryonic brain, but also by glial cells in the postnatal brain, and that its expression negatively correlates with glial differentiation, proposing its novel function as a negative regulator of glial differentiation in mammalian brain development.     

Oligodendrocyte–Neuron Interactions: Impact on Myelination and Brain Function

In the past, glial cells were considered to be ‘glue’ cells whose primary role was thought to be merely filling gaps in neural circuits. However, a growing number of reports have indicated the role of glial cells in higher brain function through their interaction with neurons. Myelin was originally thought to be just a sheath structure surrounding neuronal axons, but recently it has been shown that myelin exerts effects on the conduction velocity of neuronal axons even after myelin formation. Therefore, the investigation of glial cell properties and the neuron-glial interactions is important for understanding higher brain function. Moreover, since there are many neurological disorders caused by glial abnormalities, further understanding of glial cell-related diseases and the development of effective therapeutic strategies are warranted. In this review, we focused on oligodendrocyte-neuron interactions, with particular attention on (1) axonal signals underlying oligodendrocyte differentiation and myelination, (2) neuronal activity-dependent myelination and (3) the effects of myelination on higher brain function.

     

State of protein B23/nucleophosmin in brain cells

Protein B23/nucleophosmin is a polyfunctional protein existing in cells in numerous structural forms. In this work, for the immunochemical analysis of nucleophosmin we used the antibodies specific to different forms of nucleophosmin, namely, antibodies selectively revealing monomers of all the known forms of this protein and antibodies specific only to isoform B23.1. Homogenates of different rat tissues such as the brain, liver, kidney, lung and heart were used, as well as nuclei from liver and brain cells. For the first time, we show that the structural state of nucleophosmin in brain differs from its state in other tissues, including the liver that is enriched with nucleophosmin. It was revealed that on immunoblots of brain homogenates not only monomeric form of nucleophosmin but also unique SDS-resistant oligomeric forms were detected in the SDS-PAGE. Analysis of nucleophosmin in the cerebellum, cortex, amygdala, brainstem, and hippocampus showed that most enriched with nucleophosmin were hippocampus and cerebellum; on their immunoblots SDS-resistant oligomeric forms of nucleophosmin dominated. Using immunochemical analysis of the protein in primary cultures of cerebellum glial cells and neurons, significant structural differences of nucleophosmin in proliferating glial cells and non-proliferating neurons were revealed for the first time. It was found also that the nucleophosmin content in glial cells is much higher than in neurons and that the main forms of protein B23 in glial cells on immunoblots are the SDS-resistant oligomers, while a monomeric form was present in much smaller quantities. In contrast to glial cells, neurons did not contain such oligomers. In neurons, only trace amounts of a monomeric form of nucleophosmin were found, which were undetectable by the antibodies specific to isoform B23.1.     

Immunohistochemistry of Atrial Natriuretic Peptide in Brain Infarction

Atrial natriuretic peptide (ANP) was originally isolated from cardiac atria, and has potent natriuretic, diuretic, and vasorelaxant properties. It has been localized in neurons and astrocytes in the cerebral cortex and the white matter. We hypothesize that glial ANP may contribute to the regulation of cerebral blood flow in brain infarction. In order to elucidate this possible role, the immunohistochemistry of ANP was studied in cases of brain infarction and in other cases of brain trauma for comparison. A statistically significant increase in the number of ANP-immunoreactive glial cells (mainly astrocytes) was observed in the white matter surrounding the brain infarction compared with the intact area. No statistically significant increase in ANP-immunoreactive glial cell number was observed in the cerebral white matter from brain haemorrhage, contusion and control cases. Our results indicate that glial ANP may increase in number in brain infarction, and that it may be involved in the regulation of the cerebral blood flow in the infarcted area.     

Dynamics of surface neurotransmitter receptors and transporters in glial cells: Single molecule insights

The surface dynamics of neurotransmitter receptors and transporters, as well as ion channels, has been well-documented in neurons, revealing complex molecular behaviour and key physiological functions. However, our understanding of the membrane trafficking and dynamics of the signalling molecules located at the plasma membrane of glial cells is still in its infancy. Yet, recent breakthroughs in the field of glial cells have been obtained using combination of superresolution microscopy, single molecule imaging, and electrophysiological recordings. Here, we review our current knowledge on the surface dynamics of neurotransmitter receptors, transporters and ion channels, in glial cells. It has emerged that the brain cell network activity, synaptic activity, and calcium signalling, regulate the surface distribution and dynamics of these molecules. Remarkably, the dynamics of a given neurotransmitter receptor/transporter at the plasma membrane of a glial cell or neuron is unique, revealing the existence of cell-type specific regulatory pathways. Thus, investigating the dynamics of signalling proteins at the surface of glial cells will likely shed new light on our understanding of glial cell physiology and pathology.