Extracellular matrix of cultured glial cells: selective expression of chondroitin 4-sulfate by type-2 astrocytes and their progenitors

Activity Shapes Neural Circuit Form and Function: A Historical Perspective

The brilliant and often prescient hypotheses of Ramon y Cajal have proven foundational for modern neuroscience, but his statement that "In adult centers the nerve paths are something fixed, ended, immutable … " is an exception that did not stand the test of empirical study. Mechanisms of cellular and circuit-level plasticity continue to shape and reshape many regions of the adult nervous system long after the neurodevelopmental period. Initially focused on neurons alone, the field has followed a meteoric trajectory in understanding of activity-regulated neurodevelopment and ongoing neuroplasticity with an arc toward appreciating neuron-glial interactions and the role that each neural cell type plays in shaping adaptable neural circuity. In this review, as part of a celebration of the 50th anniversary of Society for Neuroscience, we provide a historical perspective, following this arc of inquiry from neuronal to neuron-glial mechanisms by which activity and experience modulate circuit structure and function. The scope of this consideration is broad, and it will not be possible to cover the wealth of knowledge about all aspects of activity-dependent circuit development and plasticity in depth.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

Glycogen synthase kinase 3 beta (GSK3β) at the tip of neuronal development and regeneration

Gaining a basic understanding of the inhibitory molecules and the intracellular signaling involved in axon development and repulsion after neural lesions is of clear biomedical interest. In recent years, numerous studies have described new molecules and intracellular mechanisms that impair axonal outgrowth after injury. In this scenario, the role of glycogen synthase kinase 3 beta (GSK3β) in the axonal responses that occur after central nervous system (CNS) lesions began to be elucidated. GSK3β function in the nervous tissue is associated with neural development, neuron polarization, and, more recently, neurodegeneration. In fact, GSK3β has been considered as a putative therapeutic target for promoting functional recovery in injured or degenerative CNS. In this review, we summarize current understanding of the role of GSK3β during neuronal development and regeneration. In particular, we discuss GSK3β activity levels and their possible impact on cytoskeleton dynamics during both processes.

Roles of ES cell-derived gliogenic neural stem/progenitor cells in functional recovery after spinal cord injury

Transplantation of neural stem/progenitor cells (NS/PCs) following the sub-acute phase of spinal cord injury (SCI) has been shown to promote functional recovery in rodent models. However, the types of cells most effective for treating SCI have not been clarified. Taking advantage of our recently established neurosphere-based culture system of ES cell-derived NS/PCs, in which primary neurospheres (PNS) and passaged secondary neurospheres (SNS) exhibit neurogenic and gliogenic potentials, respectively, here we examined the distinct effects of transplanting neurogenic and gliogenic NS/PCs on the functional recovery of a mouse model of SCI. ES cell-derived PNS and SNS transplanted 9 days after contusive injury at the Th10 level exhibited neurogenic and gliogenic differentiation tendencies, respectively, similar to those seen in vitro. Interestingly, transplantation of the gliogenic SNS, but not the neurogenic PNS, promoted axonal growth, remyelination, and angiogenesis, and resulted in significant locomotor functional recovery after SCI. These findings suggest that gliogenic NS/PCs are effective for promoting the recovery from SCI, and provide essential insight into the mechanisms through which cellular transplantation leads to functional improvement after SCI.

Transcriptional regulation of scar gene expression in primary astrocytes

The failure of the adult injured spinal cord to support axonal regeneration is in part attributed to the glial scar. Reactive astrocytes constitute a major cellular component of the glial scar and are heterogeneous with respect to the extracellular matrix proteins that they secrete. Astrocytes may produce antiregenerative molecules such as chondroitin sulphate proteoglycans (CSPGs) or proregenerative molecules such as laminin and fibronectin. While many different CSPGs are expressed after spinal cord injury (SCI) they all rely on the same enzymes, xylosyltransferase-I and -II (XT-I, XT-II) and chondroitin 4-sulfotransferase (C4ST) to add the repulsive chondroitin sulfate side chains to their core proteins. We show that XT-I, XT-II, and C4ST are part of a CSPG biosynthetic gene (CBG) battery. Using primary astrocyte cultures and quantitative PCR we demonstrate that TGFbeta2, PDGF, and IL-6 induce the expression of CBGs, laminin and fibronectin by several-fold. We further show that over-expression of the transcription factor SOX9 also strongly induces the expression of CBGs but does not increase the expression of laminin or fibronectin. Correspondingly, SOX9 knock-down in primary astrocytes causes a decrease in CBG and an increase in laminin and fibronectin mRNA levels. Finally, we show that the in vivo expression profiles of TGFbeta2, PDGF, IL-6, and SOX9 are consistent with their potential roles in differentially regulating CBGs, laminin and fibronectin gene expression in the injured spinal cord. This work suggests that SOX9 levels may be pivotal in determining the balance of pro- and anti-regenerative extracellular matrix molecules produced by astrocytes.

Roles of Eph receptors and ephrins in the normal and damaged adult CNS

Injury to the central nervous system (CNS) usually results in very limited regeneration of lesioned axons, which are inhibited by the environment of the injury site. Factors that have been implicated in inhibition of axonal regeneration include myelin proteins, astrocytic gliosis and cell surface molecules that are involved in axon guidance during development. This review examines the contribution of one such family of developmental guidance molecules, the Eph receptor tyrosine kinases and their ligands, the ephrins in normal adult CNS and following injury or disease. Eph/ephrin signaling regulates axon guidance through contact repulsion during development of the CNS, inducing collapse of neuronal growth cones. Eph receptors and ephrins continue to be expressed in the adult CNS, although usually at lower levels, but are upregulated following neural injury on different cell types, including reactive astrocytes, neurons and oligodendrocytes. This upregulated expression may directly inhibit regrowth of regenerating axons; however, in addition, Eph expression also regulates astrocytic gliosis and formation of the glial scar. Therefore, Eph/ephrin signaling may inhibit regeneration by more than one mechanism and modulation of Eph receptor expression or signaling could prove pivotal in determining the outcome of injury in the adult CNS.

Astrocytes derived from glial-restricted precursors promote spinal cord repair

Background

Transplantation of embryonic stem or neural progenitor cells is an attractive strategy for repair of the injured central nervous system. Transplantation of these cells alone to acute spinal cord injuries has not, however, resulted in robust axon regeneration beyond the sites of injury. This may be due to progenitors differentiating to cell types that support axon growth poorly and/or their inability to modify the inhibitory environment of adult central nervous system (CNS) injuries. We reasoned therefore that pre-differentiation of embryonic neural precursors to astrocytes, which are thought to support axon growth in the injured immature CNS, would be more beneficial for CNS repair.

Results

Transplantation of astrocytes derived from embryonic glial-restricted precursors (GRPs) promoted robust axon growth and restoration of locomotor function after acute transection injuries of the adult rat spinal cord. Transplantation of GRP-derived astrocytes (GDAs) into dorsal column injuries promoted growth of over 60% of ascending dorsal column axons into the centers of the lesions, with 66% of these axons extending beyond the injury sites. Grid-walk analysis of GDA-transplanted rats with rubrospinal tract injuries revealed significant improvements in locomotor function. GDA transplantation also induced a striking realignment of injured tissue, suppressed initial scarring and rescued axotomized CNS neurons with cut axons from atrophy. In sharp contrast, undifferentiated GRPs failed to suppress scar formation or support axon growth and locomotor recovery.

Conclusion

Pre-differentiation of glial precursors into GDAs before transplantation into spinal cord injuries leads to significantly improved outcomes over precursor cell transplantation, providing both a novel strategy and a highly effective new cell type for repairing CNS injuries.

Inhibition of axon growth by oligodendrocyte precursor cells

The glial scar that forms at the site of injury is thought to be a biochemical and physical barrier to successful regeneration, although the molecules responsible for this barrier function are not well understood. Glia scars contain large numbers of oligodendrocyte precursor cells (OPCs) and these cells can produce several different growth-inhibitory chondroitin sulfate proteoglycans (CSPGs), including NG2, neurocan, and phosphacan. Here, we used membrane-based assays to show that the surface of OPCs is both nonpermissive and inhibitory for neurite outgrowth. Inhibition of growth by OPC is reversed by treatment with antibodies against the NG2 CSPG and the expression of NG2 is sufficient to change a growth-permissive cell surface to a nonpermissive surface. These result suggest that the OPCs that accumulate rapidly at sites of CNS injury can contribute to the creation of an environment that inhibits nerve regeneration and that NG2 is a necessary feature of that environment.

Developmental and regional distribution of aspartoacylase in rat brain tissue

The function of N-acetyl-aspartate (NAA), a predominant molecule in the brain, has not yet been determined. However, NAA is commonly used as a putative marker of viable neurones. To investigate the possible function of NAA, we determined the anatomical, developmental and cellular distribution of aspartoacylase, which catalyses the hydrolysis of NAA. Levels of aspartoacylase activity were measured during postnatal development in several brain regions. The differential distribution of aspartoacylase activity in purified populations of cells derived from the rat CNS was also investigated. The developmental and anatomical distribution of aspartoacylase correlated with the maturation of white matter tracts in the rat brain. Activity increased markedly after 7 days and coincided with the time course for the onset of myelination in the rat brain. Gray matter showed little activity or developmental trend. There was a 60-fold excess in optic nerve (a white matter tract) when compared with cortex at 21 days of development. In the adult brain there was a 18-fold difference in corpus callosum compared with cortex (stripped of corpus callosum). Cellular studies demonstrated that purified cortical neurons and cerebellar granular neurones have no activity. Primary O-2A progenitor cells had moderate activity, with three-fold higher activity in immature oligodendrocyte and 13-fold increase in mature oligodendrocytes (myelinating cells of the CNS). The highest activity was seen in type-2 astrocytes (20-fold difference compared with O-2A progenitors) derived from the same source. Aspartoacylase activity increased with time in freshly isolated astrocytes, with significantly higher activity after 15 days in culture. We conclude that aspartoacylase activity in the developing postnatal brain corresponds with maturation of myelination, and that the cellular distribution is limited to glial cells.

Expression and regulation of the LIM-class homeobox gene rlim-1 in neuronal progenitors of the rat cerebellum

To investigate LIM gene function in the rat cerebellar system, we analyzed expression and regulation of the rat homologue of frog Xlim-1 (rlim-1) in vivo and in cultured cells. In developing cerebellum, peak levels of rlim-1 mRNA at postnatal day 8 (p8) are coincident with the peak period of granule cell proliferation. Analysis of rlim-1 protein with a specific antibody showed that expression was also maximal at p8. In situ hybridization showed that at p8 rlim-1 mRNA was expressed in Purkinje and granule cells. Both the proliferative and the premigratory granule cells in the external germinal zone displayed high levels of rlim-1 mRNA expression. Immunocytochemical staining demonstrated that at p8 rlim-1 protein was also present in proliferative and premigratory granule cells. In adult cerebellum (p30), rlim-1 mRNA and protein expression in granule cells was strongly attenuated. The down-regulation of rlim-1 mRNA occurred in granule cells just after the time of final division, coinciding with the onset of their migration. rlim-1 protein was detected in migratory granule neurons. The developmental decrease in rlim-1 mRNA and protein found in vivo was reproduced in pure cerebellar granule cell cultures. In these cultures, granule neurons were postmitotic 1 day after plating but still displayed high levels of rlim-1 protein expression up to 3 days in vitro. Our findings indicate that 1) rlim-1 is likely to act in concert with other genes to specify granule cell fate, 2) rlim-1 expression in granule neurons is regulated autonomously, and 3) rlim-1 protein may also play an important role in granule neuron differentiation and survival. Published 2001 Wiley-Liss, Inc.

Perineuronal nets ensheath fast spiking, parvalbumin-immunoreactive neurons in the medial septum/diagonal band complex

Perineuronal nets, composed of extracellular matrix material, have previously been associated with parvalbumin-immunoreactive neurons in the medial septum/diagonal band (MS/DB) complex of the rat. The aim of this study was to correlate the presence of perineuronal nets with electrophysiological properties and parvalbumin immunoreactivity in MS/DB neurons. Intracellular recordings were made from cells in a brain slice preparation maintained in vitro, and neurons were characterized into four populations: (i) slow-firing neurons, (ii) burst-firing neurons, (iii) fast spiking neurons with narrow action potentials and a small degree of spike frequency adaptation, and (iv) regular spiking neurons with broader action potentials and a high degree of spike frequency adaptation. Following electrophysiological characterization, neurons were filled with biocytin, processed for parvalbumin immunoreactivity and stained for perineuronal nets using Wisteria floribunda lectin. The three substances were viewed with triple fluorescence. Fast spiking, nonadapting neurons, shown previously to contain parvalbumin immunoreactivity, were nearly all ensheathed by perineuronal nets. There was a population of small parvalbumin-immunoreactive neurons which did not possess perineuronal nets, and which were not encountered with the intracellular electrodes. The other three neuron types in the MS/DB did not contain parvalbumin immunoreactivity or perineuronal nets. In keeping with this neurochemical profile for electrophysiologically identified neurons, burst-firing neurons had action potential parameters more similar to those of regular spiking than of fast spiking neurons. We conclude that fast spiking neurons, presumed to be GABAergic septohippocampal projection neurons, are surrounded by supportive structures to enable the high level of neuronal discharge required for producing disinhibition of hippocampal pyramidal neurons.

Activated macrophages and the blood-brain barrier: inflammation after CNS injury leads to increases in putative inhibitory molecules

The cellular responses to spinal cord or brain injury include the production of molecules that modulate wound healing. This study examined the upregulation of chondroitin sulfate proteoglycans, a family of molecules present in the wound healing matrix that may inhibit axon regeneration in the central nervous system (CNS) after trauma. We have demonstrated increases in these putative inhibitory molecules in brain and spinal cord injury models, and we observed a close correlation between the tissue distribution of their upregulation and the presence of inflammation and a compromised blood-brain barrier. We determined that the presence of degenerating and dying axons injured by direct trauma does not provide a sufficient signal to induce the increases in proteoglycans observed after injury. Activated macrophages, their products, or other serum components that cross a compromised blood-brain barrier may provide a stimulus for changes in extracellular matrix molecules after CNS injury. While gliosis is associated with increased levels of proteoglycans, not all reactive astrocytes are associated with augmented amounts of these extracellular matrix molecules, which suggests a heterogeneity among glial cells that exhibit a reactive phenotype. Chondroitin sulfate also demarcates developing cavities of secondary necrosis, implicating these types of boundary molecules in the protective response of the CNS to trauma.

Expression of glial antigens in mouse astrocytes: species differences and regulation in vitro

Expression of developmentally regulated antigens was used to characterize glial cells in cultures from embryonic mouse cerebral cortex. Over 90% of the cells had a flat morphology, and about 50% of these flat cells also expressed the ganglioside GD3. Up to 40% of all the GD3 expressing cells also expressed A2B5 antigen. Flat cells expressing either glial fibrillary acidic protein (GFAP), or GD3 or both were present at all times in vitro. These three populations of flat cells could not be further distinguished on the basis of NG2 or fibronectin expression, or with respect to their responses to the mitogens FGF-2, PDGF, or EGF. The glial cultures also contain a small number (approximately 5%) of process bearing cells with the morphological and immunocytochemical characteristics of oligodendrocyte precursors. The expression of GD3 by flat cells changed with time in culture as the fraction of flat cells expressing only GD3 declined and the fraction of cells expressing GFAP (with or without GD3) increased. The data are consistent with those flat cells expressing only GD3 being astrocyte precursors. Furthermore, between 1 and 3 weeks in vitro GD3/GFAP cells lose GD3 while retaining GFAP. Cells expressing only GFAP could be induced to express GD3 and A2B5 by treatment with FGF-2. The widespread and regulated expression of GD3 and A2B5 by murine glia is different from the restricted pattern of expression previously reported for these antigens in rat brain cell cultures. These results demonstrate that expression of GD3 and A2B5 by murine astrocytes depends on both culture age and extracellular signals and that these gangliosides are not markers for cell lineage in the mouse.

The use of transplanted glial cells to reconstruct glial environments in the CNS

Transplantation studies have demonstrated that glia-depleted areas of the CNS can be reconstituted by the introduction of cultured cells. Thus, the influx of Schwann cells into glia-free areas of demyelination in the spinal cord can be prevented by the combined introduction of astrocytes and cells of the O-2A lineage. Although Schwann cell invasion of areas of demyelination is associated with destruction of astrocytes, the transplantation of rat tissue culture astrocytes ("type-1") alone cannot suppress this invasion, indicating a role for cells of the O-2A lineage in reconstruction of glial environments. By transplanting different glial cell preparations and manipulating lesions so as to prevent meningeal cell and Schwann cell proliferation it is possible to demonstrate that the behaviour of tissue culture astrocytes ("type-1") and astrocytes derived from O-2A progenitor cells ("type-2") is different. In the presence of meningeal cells, tissue culture astrocytes clump together to form cords of cells. In contrast, type-2 astrocytes spread throughout glia-free areas in a manner unaffected by the presence of meningeal cells or Schwann cells. Thus, progenitor-derived astrocytes show a greater ability to fill glia-free areas than tissue culture astrocytes. Similarly, when introduced into infarcted white matter in the spinal cord, progenitor-derived astrocytes fill the malacic area more effectively than tissue culture astrocytes, although axons do not regenerate into the reconstituted area.

A keratan sulfate proteoglycan marks the boundaries in the cortical barrel fields of the adult rat

The present study uses the monoclonal antibody TED 15 to examine the distribution of a keratan sulfate proteoglycan, ABAKAN, in the barrel field of the rat somatosensory cortex. At birth there is very little ABAKAN present in the somatosensory cortex. The levels of this proteoglycan increase during development until the highest levels are reached in the adult rat. Immunohistochemistry reveals that the TED 15 immunoreaction product marks the boundaries between cortical barrels. At postnatal day 7 (P7) the proteoglycan is localized specifically to the boundary regions. As the brain matures, the levels of ABAKAN increase in the barrel hollows and in the surrounding cortex; however the boundary regions maintain a higher level of expression even in the adult animal. The high levels of ABAKAN observed in the adult barrel fields indicate that unlike other boundary molecules this proteoglycan may be involved in maintaining the structure of the adult somatosensory cortex.

AMPA receptors shape Ca2+ responses in cortical oligodendrocyte progenitors and CG-4 cells

Intracellular calcium signals triggered by glutamate receptor activation were studied in primary cortical oligodendrocyte lineage cells and in the oligodendrocyte cell line CG-4. Glutamate, kainate, and AMPA (30-300 microM) increased [Ca2+]i in both types of cells at the stage of oligodendrocyte progenitors (O-2A; GD3+) or pro-oligodendroblasts (O4+). The peak amplitude of Ca2+ responses to glutamate receptor agonists was significantly larger in cortical cells. In CG-4 and in cortical cells, the majority (more than 90%) of bipolar GD3+ or multipolar O4+ cells responded to kainate. In all the cells analyzed, kainate was more efficacious than AMPA and glutamate. The percentage of bipolar or multipolar cells responding to glutamate was significantly lower in the CG-4 cell line than in primary cultures. Cellular responses typical of metabotropic glutamate receptor activation were observed in 20% of the cortical O-2A progenitors, but in none of the CG-4 cells. The AMPA-selective antagonist GYKI 52466 blocked kainate-induced Ca2+ responses in cortical O-2A cells. The selective AMPA receptor modulator cyclothiazide (30 microM) greatly potentiated the effects of AMPA (30-100 microM) on [Ca2+]i in cortical and CG-4 cells. Our findings indicate that Ca2+ responses in cells of the oligodendrocyte lineage are primarily shaped by functional AMPA receptors.

Chondroitin sulfate proteoglycan staining in astrocyte-Schwann cell co-cultures

Transplantation of Schwann cells (SCs) in the central nervous system (CNS) for remyelination in pathological situations has been considered a promising approach. However, numerous studies have indicated that astrocytes have a restrictive effect on SC migration within the CNS. We have previously established an in vitro model which demonstrates the restrictive effect of astrocytes on SCs (Ghirnikar and Eng, Glia 4:367-377, 1994). Using this culture model, in the present study, we have characterized the molecular basis underlying astrocyte-SC interaction and demonstrated chondroitin sulfate proteoglycan (CSP) staining in the co-cultures. Following 1-2 weeks of incubation, CSP staining was specifically associated with SCs co-cultured with astrocytes. Staining with antibodies specific for the different chondroitin sulfate isomers revealed the presence of both, chondroitin-4- and 6-sulfates in SCs. In contrast, SCs when cultured alone, or in the presence of astrocytes conditioned medium did not show CSP staining. These data suggest that CSP staining is associated with SCs following co-culture with astrocytes and mediated by cell to cell contact. We hypothesize that the CSP, alone or in combination with other molecules expressed by astrocytes and/or SCs, may be involved in the restrictive effects of astrocytes on SCs. Identification of molecules involved in the unfavorable interaction between astrocytes and SCs will have an important bearing on efforts to remyelinate demyelinated axons by SC transplantation within the damaged CNS.

Restricted appearance of tenascin and chondroitin sulphate proteoglycans after transection and sprouting of adult rat postcommissural fornix

Transected fibres of the adult rat postcommissural fornix sprout over short distances but fail to traverse the lesion site and terminate in close vicinity to the wound. As a step in defining the molecular environment responsible for regeneration failure at the lesion site, we have used immunocytochemistry to analyse the spatio-temporal expression pattern of two putative growth-inhibitory extracellular matrix components, tenascin and chondroitin sulphate proteoglycans and their topographical relationship to the sprouting axons. Both tenascin and chondroitin sulphate proteoglycan labelling appeared after fornix transection and were confined to the immediate vicinity of the lesion site. While tenascin-labelling was associated with astrocytes and microglia/macrophages, which accumulate preferentially at the tract borders, chondroitin sulphate proteoglycan labelling appeared as a homogeneous meshwork around the wound. Tenascin-like immunoreactivity disappeared between 17 days and 4 weeks, but chondroitin sulphate proteoglycan staining persisted at least up to 14 months after transection. Regrowing fornix fibres invaded and elongated within the chondroitin sulphate proteoglycan-immunopositive region up to the lesion site, where they terminated. This zone of axonal growth inhibition was neither characterized by an increase of chondroitin sulphate proteoglycan immunoreactivity nor by the presence of tenascin-immunopositive structures. The spatio-temporal distribution patterns of tenascin and chondroitin sulphate proteoglycan and the permeability of the chondroitin sulphate proteoglycan-immunopositive region for sprouting axons do not support the hypothesis that chondroitin sulphate proteoglycan alone and/or tenascin inhibit the advance of sprouting fornix fibres.

Glutamate regulates intracellular calcium and gene expression in oligodendrocyte progenitors through the activation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Oligodendrocytes and their progenitors (O-2A) express functional kainate- and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring glutamate receptors. The physiological consequences of activation of these receptors were studied in purified rat cortical O-2A progenitors and in the primary oligodendrocyte cell line CG-4. Changes in the mRNA levels of a set of immediate early genes were studied and were correlated to intracellular Ca2+ concentration, as measured by fura-2 Ca2+ imaging. Both in CG-4 and in cortical O-2A progenitors, basal mRNA levels of NGFI-A were much higher than c-fos, c-jun, or jun-b. Glutamate, kainate, and AMPA greatly increased NGFI-A mRNA and protein by activation of membrane receptors in a Ca(2+)-dependent fashion. Agonists at non-N-methyl-D-aspartate receptors promoted transmembrane Ca2+ influx through voltage-dependent channels as well as kainate and/or AMPA channels. The influx of Ca2+ ions occurring through glutamate-gated channels was sufficient by itself to increase the expression of NGFI-A mRNA. AMPA receptors were found to be directly involved in intracellular Ca2+ and NGFI-A mRNA regulation, because the effects of kainate were greatly enhanced by cyclothiazide, an allosteric modulator that selectively suppresses desensitization of AMPA but not kainate receptors. Our results indicate that glutamate acting at AMPA receptors regulates immediate early gene expression in cells of the oligodendrocyte lineage by increasing intracellular calcium. Consequently, modulation of these receptor channels may have immediate effects at the genomic level and regulate oligodendrocyte development at critical stages.

Fibrous and protoplasmic astrocytes express GABAA receptors that differ in benzodiazepine pharmacology

Astrocytes cultured from spinal cord contain two morphologically distinguishable types of astrocytes: fibrous and protoplasmic cells. Both astrocyte subtypes, in culture, are able to express GABAA receptors, and their activation results in inward currents at the resting potential. Using patch-clamp electrophysiology we characterized their basic receptor pharmacology and compared it to spinal cord neurons that were also present in small numbers in these cultures. As in neuronal GABAA receptors, the local anesthetic pentobarbital effectively potentiated GABA-induced currents in both astrocyte subtypes. Similarly, the benzodiazepine diazepam, on average doubled GABA-induced currents in both astrocytes subtypes. In contrast to these effects that were similar in both astrocytes types and similar to spinal cord neurons, the response to the convulsant methyl-4-ethyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM), which is an inverse benzodiazepine agonist differs between astrocyte subtypes. DMCM reduced GABA-induced currents by about 50% in fibrous astrocytes as we also observed with spinal cord neurons. In contrast, DMCM increased GABA currents in protoplasmic astrocytes by up to 150%, an effect never observed in neurons. DMCM potentiations of GABA currents have recently been attributed to differences in receptor subunit composition. Our results thus indicate that subtypes of astrocytes express GABAA receptors that differ pharmacologically and likely differ also in subunit composition.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Perineuronal nets provide a polyanionic, glia-associated form of microenvironment around certain neurons in many parts of the rat brain

The nature and function of previously described perineuronal nets are still obscure. In the present study their polyanionic components were demonstrated in the rat brain using colloidal iron hydroxide (CIH) staining. In subcortical regions, such as the red nucleus, cerebellar, and vestibular nuclei, most neurons were ensheathed by CIH-binding material. In the cerebral cortex perineuronal nets were seen around numerous nonpyramidal neurons. Biotinylated hyaluronectin revealed that hyaluronan occurs in perineuronal nets. Two plant lectins [Wisteria floribunda agglutinin (WFA) and Vicia villosa agglutinin (VVA)] with affinity for N-acetylgalactosamine visualized perineuronal nets similar to those rich in anionic components. Glutamic acid decarboxylase (GAD)-immunoreactive synaptic boutons were shown to occupy numerous meshes of perineuronal VVA-positive nets. Electron microscopically, VVA binding sites were scattered throughout perisynaptic profiles, but accumulated at membranes and in the extracellular space except not in synaptic clefts. To investigate the spatial relationship between glial cell processes and perineuronal nets, two astrocytic markers (S100-protein and glutamine synthetase) were visualized at the light and electron microscopic level. Two methods to detect microglia by the use of Griffonia simplicifolia agglutinin (GSA I-B4) and the monoclonal antibody, OX-42, were also applied. Labelled structures forming perineuronal nets were observed with both astrocytic, but not with microglial, markers. It is concluded that perineuronal nets are composed of a specialized type of glia-associated extracellular matrix rich in polyanionic groups and N-acetylgalactosamine. The net-like appearance is due to perisynaptic arrangement of the astrocytic processes and these extracellular components. Similar to the ensheathment of nodes of Ranvier, perineuronal nets may provide a special ion buffering capacity required around various, perhaps highly active, types of neurons.

Cycloheximide downsynthesizes inhibitory molecules for neurite outgrowth in neural transplantation

Rat fetal substantia nigra treated with cycloheximide, a protein synthesis inhibitor, were implanted to adult rat striatum. After 4 weeks, tyrosine hydroxylase (TH)-like immunoreactive (-LI) fibers of host striatum penetrated grafts, and TH-LI neurites in the grafts elongated and mingled with the host striatal neurites. Astrocytes proliferated in the grafts without glial scar between the graft-host border. A few chondroitin sulfate- or tenascin-LI glial cells were found in the grafts, while in transplants without cycloheximide, glial scar expressed strong immunoreactivity for these molecules. Downsynthesis of these inhibitory molecules may alter the glial character and permit neurites traversing the border.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Development and differentiation of glial precursor cells in the rat cerebellum

The development and differentiation of bipotential glial precursor cells has been studied extensively in tissue culture, but little is known about the distribution and fate of these cells within intact animals. To analyze the development of glial progenitor cells in the developing rat cerebellum, we utilized immunofluorescent, immunocytochemical, and autoradiographic techniques. Glial progenitor cells were identified with antibodies against the NG2 chondroitin-sulfate proteoglycan, a cell-surface antigen of 02A progenitor cells in vitro, and the distribution of this marker antigen was compared to that of marker antigens that identify immature astrocytes, mature astrocytes, oligodendrocyte precursors, and mature oligodendrocytes. Cells expressing the NG2 antigen appeared in the cerebellum during the last 3-4 days of embryonic life. Over the first 10 days of postnatal life, the NG2-labeled cells incorporated 3H-thymidine into their nuclei and their total number increased. At all ages examined, the NG2-labeled cells did not contain either vimentin-like or glial fibrillary acidic protein (GFAP)-like immunoreactivity, suggesting that they do not develop along an astrocytic pathway. NG2-labeled cells of embryonic animals expressed GD3 ganglioside antigens, a property of oligodendrocyte precursors, whereas NG2-positive cells of postnatal animals did not express GD3 immunoreactivity. Nevertheless, the NG2-labeled cells of the nascent white matter expressed oligodendrocyte-specific marker antigens. Cells lying outside of the white matter continued to express the NG2 antigen. In adult animals, the NG2-labeled cells incorporated 3H-thymidine. Glial cells isolated from adult animals and grown in tissue culture express the NG2 antigen and display the phenotypic plasticity characteristic of 02A progenitor cells. These findings demonstrate that a population of glial progenitor cells is extensive within both young and adult animals.

Postnatal development and localization of an N-acetylgalactosamine containing glycoconjugate associated with nonpyramidal neurons in cat visual cortex

We have analyzed the appearance of N-acetylgalactosamine containing glycoconjugates by staining sections of fixed cat visual cortex with the lectins from Vicia villosa (VVA) and Glycine max (SBA) conjugated to fluorescent labels, horseradish peroxidase, or biotin. The appearance of lectin staining during postnatal development followed an inside-out gradient starting in the deep cortical layers at the fourth postnatal week, successively including more of the superficial layers during the first three postnatal months until labelled cells were present throughout layers II-VI at the seventh postnatal month. Staining was associated with the soma and proximal dendrites of bipolar and multipolar nonpyramidal neurons. Outside layer IV large neurons with basket cell and neurogliaform morphology predominated, whereas small multipolar cells were stained in layer IV. High power observation revealed a lattice-like staining on neuronal surfaces. In the cortical white matter staining was found at the nodes of Ranvier. The ultrastructural localization of lectin binding sites was assessed by a pre-embedding histochemical procedure with biotinylated lectin visualized with HRP-conjugated avidin and diaminobenzidine as the chromogen. The reaction product was found exclusively in close association with synaptic terminals on somata and proximal dendrites of nonpyramidal neurons. There was no preference with respect to the morphology of the synaptic structure, i.e., symmetric synapses with ovoid or asymmetric synapses with round vesicles. In the vicinity of synaptic contacts the staining was associated with membranes of astrocytic processes ensheathing the synapses. In the cortical white matter astrocytic processes running along myelinated axons were strongly labelled at the nodes of Ranvier. Comparison of Western blots from 4-week-old and adult cat cortex membranes revealed one lectin-positive protein band with an apparent molecular weight of about 24 kD in conjunction with the histochemical expression of perisynaptic staining in the adult tissue. It is concluded that the employed lectins selectively visualize distal astroglial processes ensheathing synapses on a subpopulation of cortical interneurons and nodes of Ranvier. On the basis of the developmental appearance and localization we consider and discuss the possibility that the N-acetylgalactosamine containing glycoconjugate is involved in the stabilization of synaptic contacts on GABA-ergic interneurons.

Transforming growth factor-betas inhibit mitogen-stimulated proliferation of astrocytes

We have studied the influence of three members of the transforming growth factor-beta (TGF-beta) family of multifunctional growth factors on the proliferation of cultured astrocytes isolated from newborn mouse cerebral cortex. Although TGF-beta s 1, 2, and 3 cause only a small reduction in the low level of astrocyte proliferation occurring in chemically defined medium, they each inhibit the effects of five astrocyte mitogens (bFGF, EGF, PDGF, IL-1 alpha, and IL-2). Inhibition is observed when astrocytes are exposed to mitogen and TGF-beta at the same time and when they are exposed to TGF-beta prior to, and separately from, mitogen. This latter effect appears to be due to the binding of TGF-beta s to astrocyte-secreted extracellular matrix. These findings raise the possibility that TGF-beta may co-operate with other growth factors to control astrocyte proliferation in vivo.

Effects of transforming growth factor-beta 1 on the extracellular matrix and cytoskeleton of cultured astrocytes

The present study was performed on primary cultures and subcultures of cerebellar astrocytes in order to investigate the effects of transforming growth factor-beta 1 (TGF beta 1) on proliferation, extracellular matrix (ECM) components, and cytoskeletal structures in relation to morphological changes. The expression and cellular distribution of the ECM components laminin and fibronectin and the cytoskeletal proteins glial fibrillary acidic protein (GFAP) and actin were investigated by immunoblotting, immunocytochemistry, and phalloidin staining. The proliferation of primary cultures was strongly inhibited by TGF beta 1. Treated cells became enlarged and spread onto the substratum. TGF beta 1 promoted the appearance of actin stress fibers and increased the cell actin content. It elicited a slight increase in GFAP expression and induced dispersion of thin filaments of GFAP. TGF beta 1 also stimulated the production of laminin and fibronectin and their incorporation into the ECM of primary cultures grown in medium with or without serum. Astrocytes grown in serum-containing medium for 1 day after subculturing responded strongly to TGF beta 1. Changes promoted by TGF beta 1 in cell shape, cytoskeleton, and ECM production of cultured astrocytes may have relevance for understanding the mechanisms of action of TGF beta 1 during brain development.

Binding of secreted human neuroblastoma proteoglycans to the Alzheimer's amyloid A4 peptide

Proteoglycans (PGs) may play a fundamental role in all forms of amyloidosis. In Alzheimer's disease, proteoglycans are found deposited in senile plaques and in neurofibrillary tangles. However, the cellular source of these deposited PGs and their role in amyloidosis in Alzheimer's disease is unknown. Proteoglycans were purified from conditioned medium of human neuroblastoma cells (SKNSH-SY 5Y). Two species of proteoglycans were identified by enzyme susceptibility including a heparan sulfate proteoglycan and a dermatan sulfate proteoglycan. A monoclonal antibody to the protein core of a vascular basement membrane heparan sulfate proteoglycan found in senile plaques in Alzheimer's disease cross-reacted with the proteoglycans secreted by human neuroblastoma cells. Binding between 35SO4-labelled neuroblastoma proteoglycans and the Alzheimer amyloid (A4) peptide was demonstrated by affinity chromatography. Specificity studies demonstrated that binding of human neuroblastoma proteoglycans to the amyloid peptide was specific for a heparan sulfate glycosaminoglycan, with some binding to a dermatan sulfate proteoglycan. Binding to A4 was also demonstrated by a chemically deglycosylated protein core preparation. No significant binding of neuroblastoma proteoglycans was found to two other basic peptides derived from the extracellular domain of the beta-amyloid precursor, demonstrating the specificity of proteoglycan binding to the A4 peptide. Human neuroblastoma proteoglycans may bind to the-Alzheimer amyloid A4 peptide in a region with a heparin binding consensus sequence [VHHQKL] which also contains the cleavage site of the beta-amyloid precursor protein. Neuronal proteoglycans may either regulate the secretion of the amyloid protein precursor or modify the binding of the amyloid protein precursor to other cellular adhesion molecules. Alterations in this binding may be related to the pathogenesis of amyloid deposition in Alzheimer's disease.

Expression of fibronectin and laminin by different types of mouse glial cells cultured in a serum-free medium

The expression of fibronectin and laminin by cultured glial cells was studied. The glial culture from neonatal mouse cerebra maintained in a chemically defined, serum-free medium consisted of type-1 astrocytes, oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, oligodendrocytes and type-2 astrocytes. Double-labelling immunofluorescent experiments performed using the mixed glial culture indicated that fibronectin and laminin are expressed in different patterns among the glial subtypes. The staining intensities with anti-fibronectin or anti-laminin antibodies decreased in the order: type-1 astrocytes, O-2A progenitor cells and type-2 astrocytes. Both molecules were deposited in a fibrillar matrix underneath type-1 astrocytes, whereas only intracytoplasmic localization of these molecules was observed with O-2A progenitor cells and type-2 astrocytes. Western blot analysis showed that glial fibronectin has a slightly higher molecular weight than mouse plasma fibronectin (230 kDa) and that glial laminin is a variant with a 220 kDa B chain present and the 400 kDa A chain missing. Using enzyme-linked immunosorbent assays (ELISA), these molecules were detected in the glial extracellular matrix at the concentration of 4 ng/10(6) cells. A large amount of fibronectin (82 ng/10(6) cells) was secreted into the culture medium, while secretion of laminin was not detected.

Chondroitin sulfate proteoglycan surrounds a subset of human and rat CNS neurons

Chondroitin sulfate proteoglycan (CS-PG) bearing glycosaminoglycan (GAG) chains containing unsulfate (COS) and 6-sulfate (C6S) disaccharides was immunolocalized in rat and human CNS by using monoclonal antibodies (MAb) specific for the two disaccharides. The immunostaining with both MAb was restricted to the periphery of a neuronal subset in rat and human CNS. Double immunofluorescence showed codistribution of the antigens around the same neuronal population. The staining with anti-COS MAb was stronger than with anti-C6S MAb, suggesting that the proteoglycan (PG) contains mainly COS disaccharides. In different rat cortical areas, 40-60/mm2 positive interneurons were found, the visual cortex showing the highest value. In human cortex, positivity was also observed around the soma of some pyramidal cells. In the rat, positive neurons were also localized in deep cerebellar nuclei, reticular nucleus of the thalamus, and other structures of the midbrain and hindbrain. CA3 region of hippocampus and the external layer of pyriform cortex were characterized by positivity of the neuropil. Immunoelectronmicroscopy showed the antigens in the extracellular space around the neuronal soma, the synaptic elements and the cell processes of the neuropil. The neuronal surface of the soma and of the proximal dendrites were positive, but the pre- and postsynaptic membranes and clefts were negative.

Developmental profiles of arylsulfatases A and B in rat cerebral cortex and spinal cord

Arylsulfatases A (EC 3.1.6.1) and B (EC 3.1.6.12) are lysosomal enzymes that can remove sulfate groups from sulfatides and sulfo-glycosaminoglycans, respectively. The activities of these enzymes in cerebral cortex and in spinal cord of developing rat pups were measured. The tissues were homogenized and the arylsulfatases A and B in the soluble fraction were separated from each other by anion exchange chromatography on DE-52 cellulose. Subsequently, the enzyme activities were assayed with p-nitrocatechol sulfate as substrate at 37 degrees C and pH 5.6. We observed a developmental profile of arylsulfatase A, similar to that previously reported for cerebroside sulfatase (EC 3.1.6.8; (Van der Pal et al. (1990) Biochim. Biophys. Acta 1043, 91-96]. The activity of arylsulfatase A increased gradually during development, whereas arylsulfatase B rose more steeply, peaked around day 15 and declined thereafter. As a consequence the ratio between B and A forms of arylsulfatase dropped from about 4 in 1-week-old pups to 2.2 (cortex) and 0.7 (cord) in 7-week-old rat pups.

Glial cell lineage in the cerebral cortex: a review and synthesis

The present review is focused on the cell lineage relationships underlying gliogenesis in the cerebral cortex. Studies conducted both in vivo and in vitro suggest that the process of cortical gliogenesis involves a hierarchy of progressively restricted progenitor cell pools. In the cerebral cortex, as well as other areas of the central nervous system, glial cells differentiate from one another through a series of steps that can be defined at molecular, structural, and functional levels. Although the precise timing, sequence, and diversity of the steps involved in cortical gliogenesis are still not fully defined, the emerging picture suggests that both cell lineage and cell-cell interactions play a synergetic role in the determination and maintenance of the proper blend of glial cells in the cerebrum.

Chondroitin 4-sulfate proteoglycan forms an extracellular network in human and rat central nervous system

Chondroitin 4-sulfate proteoglycan (C4S-PG) was localized both in rat and human central nervous system (CNS) by monoclonal and polyclonal antisera recognizing the 4-sulfate disaccharide (C4S). In the rat the whole CNS was studied in serial coronal sections. A positive extracellular meshwork was observed both in white and grey matters. In the white matter (WM) C4S-PG formed a network around myelinated axons, sparing myelin sheaths and axoplasms. The neuropil of the grey matter (GM) showed a positive meshwork constituted by delicate intermingling filaments. The cytoplasms of neuronal, glial and endothelial cells were negative. Stronger straining than in the neuropil was observed around the soma and the proximal part of the cell processes of some neurons located in the cortex, in the deep cerebellar nuclei and in some other CNS nuclei. A similar pattern was also observed in human CNS, the only difference being a smaller amount of cortical neurons surrounded by a rim of C4S-PG. This study shows that a PG bearing C4S disaccharide is located extracellularly in the rodent and human CNS and that C4S disaccharides can be present in different types of CNS proteoglycans (PGs).