Studies of glial lineage and proliferation in vitro using an early marker for committed oligodendrocytes

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type. One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.

Re-evaluation of nestin as a marker of oligodendrocyte lineage cells

Maturation of oligodendrocyte progenitors (O2A) is characterized by morphological changes and the sequential expression of specific antigens leading to the formation of myelin membrane. Monoclonal antibodies A2B5, A007, anti-vimentin, and anti-galactocerebroside, recognize oligodendroglia at different stages of development. The neuroepithelial precursor marker nestin is also expressed by the oligodendroglial lineage; we have used enriched populations of progenitors isolated from neonatal rat brain cultures to further examine the cellular distribution of this intermediate filament protein. The phenotypic distribution of nestin positive cells among the oligodendrocyte lineage showed that 65% reacted with A2B5, whereas only 5% were A007(+), and 4% galactocerebroside(+). The remaining 25% of the cells were not labeled and had small cellular bodies devoid of processes, characteristic of the pre-O2A progenitor. Further analysis of the nestin(+) population showed that the majority of the cells were also vimentin(+). Antibody-dependent complement mediated cytolysis of A2B5(+) (O2A cells) and galactocerebroside(+) (mature oligodendrocytes) cells left a population of nestin(+) cells that were induced to proliferate in the presence of growth factors and to differentiate into A2B5(+) and galactocerebroside(+) cells. Proliferating cells maintained in the presence of platelet-derived growth factor or basic fibroblast growth factor retained nestin expression along with A2B5. By contrast, in serum-free medium nestin expression decreased while postmitotic cells acquired A007 and galactocerebroside. Our results suggest that nestin expression is a marker of pre-O2A cells that is maintained in proliferating glial progenitors, but is quickly down-regulated in postmitotic oligodendrocytes (A007(+)/galacto-cerebroside(+)) along with A2B5 and vimentin. However, other glial cells including type 2 astrocytes and some amoeboid microglia also share nestin expression.

Effects of TNFalpha on immature and mature oligodendrocytes and their progenitors in vitro

Tumor necrosis factor alpha (TNFalpha) appears to take part in the pathogenesis of multiple sclerosis and to contribute to the degeneration of oligodendrocytes as well as neurons. TNFalpha is produced by microglia and astrocytes, which also produce hormones and cytokines that influence its biological activity. Thus, in mixed cultures the effects of exogenous TNFalpha might be modified by products of astrocytes and microglia. The effects of TNFalpha in oligodendrocyte-enriched cultures are reported below. We prepared the cultures by shaking oligodendrocytes off primary mixed glial-cell cultures from brains of 2-day-old rats at 7 days in vitro and plating them (0 days post-shake, DPS). Platelet-derived growth factor and fibroblast growth factor were included in the media at 1-5 DPS in order to encourage proliferation. At 2 DPS media were added with no TNFalpha (controls) or 1000, 2000 or 5000 U/ml of TNFalpha, and at 5 DPS media were replaced with fresh serum-free media. Cultures were fixed with 4% paraformaldehyde at 5, 7, 9 and 12 DPS and immunostained. Oligodendrocyte progenitors were not reduced in numbers immediately after the incubation with TNFalpha (i. e. at 5 DPS). However, after an additional 4 days in culture fewer progenitors remained in the cultures that had been treated with TNFalpha than in the untreated cultures. In the absence of the growth factors there were fewer progenitors, but their numbers also were reduced by TNFalpha. Maturation to the myelin basic protein (MBP)-positive stage was inhibited by about 36% at 9 DPS by 1000-2000 U/ml of TNFalpha, while numbers of O4+/MBP- precursors were unaffected. It is interesting that the steady-state number of O4-positive precursors was unchanged by TNFalpha at 9 DPS, when there were reductions in the numbers of A2B5-positive progenitors and MBP-positive mature oligodendrocytes. That observation suggests that the rates of proliferation, death and maturation are controlled by multiple factors, with a particularly vulnerable time at the maturation to the MBP-positive stage. At 5000 U/ml TNFalpha the specific effect on maturation was overtaken cytotoxicity. These data and a summary of the literature suggest that inhibition of MBP expression is sensitive to lower TNFalpha concentrations and incubation times than is cell survival. Specific effects on numbers of MBP-positive cells, morphology and MBP expression occur at 1000-2000 U/ml for 48-72 h or at up to 10000 U/ml for</=24 h, and the deficits remain after removal of the TNFalpha.

Maturation of oligodendrocytes is more sensitive to TNF alpha than is survival of precursors and immature oligodendrocytes

TNF alpha is a cytokine recently found at high levels in multiple sclerosis plaques. The present study addressed the questions whether TNF alpha might affect oligodendrocytes at various stages of maturation and whether the effects of transient incubation with TNF alpha would last during subsequent differentiation. Primary glial-cell cultures were treated with 1000 U/ml of TNF alpha for 48 h, beginning on days 1, 2, 6, 8 and 10 days in vitro, and allowed to grow for up to 3 weeks (total) in vitro. A significant deficit of O4+/MBP+ cells in the TNF alpha-treated cultures became obvious during the second week. Moreover, the morphology of the O4-positive cells became more complex with time in the control cultures, whereas fewer cells in TNF alpha-treated cultures developed into cells with sheets of membrane or > four processes. In TNF alpha-treated cultures, the numbers of O4-positive cells increased by about four-fold during weeks 2 and 3, but the numbers of MBP-positive cells did not and were significantly lower than the numbers of MBP-positive cells in control cultures. The effects of TNF alpha were apparent 1 to 14 days after treatment, suggesting long-term influences, and could be initiated at diverse stages of maturation. Future testing of hypothetical mechanisms by which TNF alpha may inhibit oligodendrocyte differentiation should impact on our understanding of the apparent limitations on remyelination in the mature CNS.

Glial-restricted precursors are derived from multipotent neuroepithelial stem cells

Neuroepithelial cells in the developing ventricular zone differentiate into neurons, astrocytes, and oligodendrocytes. It is not known, however, whether this differentiation occurs in a single step or is a pathway utilizing intermediate more restricted precursor cells. To characterize the generation of glial cells from multipotent stem cells we have cultured neuroepithelial (NEP) cells from E10.5 rat embryos. Cultured NEP cells do not express any glial differentiation markers when grown on fibronectin/laminin under nondifferentiation conditions. NEP cells, however, differentiate into A2B5 immunoreactive cells which can subsequently give rise to oligodendrocytes and astrocytes. Clonal analysis of NEP cells demonstrates that the A2B5 immunoreactive cells arise in clones that contain neurons and astrocytes, indicating that A2B5(+) cells arise from multipotent NEP precursor cells. A2B5(+) cells, maintained as undifferentiated cells over multiple passages, can subsequently give rise to both oligodendrocytes and astrocytes. A2B5(+) cells, however, do not generate neurons. Thus A2B5(+) cells represent a restricted progenitor cell population that differentiates from a multipotent NEP cell. Based on our results we propose that differentiation of the multipotential NEP cells to terminally differentiated glial cells occurs via intermediate restricted precursors.

The Lineages of Neuroglial Cells

The study of neuroglial cell lineages in the CNS identifies the time in development, when astrocytes and oligodendrocytes diverge from a common precursor cell. Recent studies using retroviral tracing show that the lineages for astrocytes and oligodendrocytes begin to diverge as early as embryonic day 13 (E13) in the cerebellum and as early as E15 in the forebrain. A very small percentage of glial precursor cells present in late pre- and postnatal development are pluripotential, but the vast majority of astrocytes and oligodendrocytes in the brain are derived from “committed” precursors. The precursors for these postmitotic astrocytes and oligodendrocytes are immature astrocytes and oligodendrocytes (progenitors) that express molecular properties unique to each of these cell types.

It is critical to distinguish glial lineage (thought of in terms of a glial cell's ancestry in normal development) from glial plasticity (the potential of a glial cell to alter its fate when its normal environment is changed). In tissue culture, a bipotential cell known as the O-2A cell generates oligodendrocytes under one set of culture conditions and retains plasticity to become a subtype of astrocyte under another set. Whether all cells phenotyped as O-2A cells in culture are bipotential or whether only a subset displays this capacity is still unclear.

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.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Carbonic anhydrase II in microglia in forebrains of neonatal rats

In the brains of adult rodents carbonic anhydrase II (CA) immunoreactivity has been observed in the choroid plexus and in oligodendrocytes, astrocytes, and myelin. Localization and functions of CA in the neonatal brain, however, have been controversial. One issue is whether the CAII-immunopositive round and ameboid cells in the corpus callosum and cingulum in the rat CNS during the first postnatal week are oligodendrocytes or microglia. Colocalization of CAII with the microglial antigen, ED1, and the microglia-specific isolectin, BSI-B4, suggested that most (approx. 60%) of the CAII-positive round and ameboid cells in rat brain during the first postnatal week were, indeed, macrophages and microglia. During that initial week, some CAII-positive protoplasmic astrocytes (approx. 40%) were observed as well. At the end of the first postnatal week smooth-surfaced CAII-positive cells began to appear in the corpus callosum. Those cells also bound MAbO4, a marker for the oligodendrocyte cell line. We conclude that during the first postnatal week most of the CAII-positive cells are macrophages and microglia, and that some are protoplasmic astrocytes. During the second postnatal week CAII-positive cells in the oligodendrocyte lineage become apparent, and by the end of that week there are few CAII-positive microglia. Confocal microscopy suggests that in brains of three-day-old rats the ameboid microglia are associated with nerve fibers, where they may perform phagocytosis of axons, directional guidance of axons, or disinhibition of axonal growth.

Do oligodendrocytes divide?

Remyelination occurs in the adult central nervous system following a wide variety of experimental and naturally occurring demyelinating conditions, including multiple sclerosis. Remyelination is preceded by the appearance of new oligodendrocytes. These new cells may be generated from glial precursor cells, or from pre-existing differentiated oligodendrocytes that re-enter the cell cycle, which may first dedifferentiate, or both processes may occur. The evidence for the source of new oligodendrocytes following toxic or immune-mediated lesions is reviewed. Good evidence exists that fully differentiated oligodendrocytes can incorporate [3H]thymidine but this may be a rare event. Most of the evidence points towards glial precursor cells as the source of new oligodendrocytes in the adult, but definitive experiments have not yet been done. Research strategies, using our current knowledge and techniques, are outlined for solving this problem.

Developing oligodendroglia express mRNA for insulin-like growth factor-I, a regulator of oligodendrocyte development

Insulin-like growth factors IGF-I and IGF-II are potent inducers of oligodendrocyte development. Because IGF-I is produced, in some cases, by the same cells that respond to it (autocrine/paracrine action), we examined the possibility that IGF-I is expressed by developing oligodendroglial cells. We employed a sensitive method, reverse transcriptase-polymerase chain reaction (RT-PCR), to detect IGF-I mRNA in purified populations of oligodendroglial cells isolated from rat brain during the period of oligodendrocyte development. Cells were purified by fluorescence activated cell sorting (FACS), using antibodies to the cell surface antigenic markers O4 and galactocerebroside (GC). RNA was isolated from the sorted cells, reverse-transcribed, and PCR-amplified, using a strategy that recognizes IGF-I mRNA but not DNA. The amplified band was identified as IGF-I by size, hybridization to an IGF-I-specific antisense probe, and restriction analysis. IGF-I mRNA was detected in O4-positive/GC-negative oligodendrocyte precursors and, more weakly, in GC-positive oligodendrocytes. IGF-I mRNA could be detected reproducibly in RNA extracted from 100-cell samples of O4-positive cells, making it unlikely that the mRNA was derived from contaminants in the FACS-sorted cell populations. We conclude that IGF-I is expressed by developing oligodendroglia. Autocrine expression of IGF-I by developing oligodendroglial cells suggests that oligodendrocyte development is, in part, autoregulatory.

Phenotypic and cell cycle properties of human oligodendrocytes in vitro

The remyelination, albeit limited, which occurs at the lesion sites in the central nervous in multiple sclerosis has been attributed to both myelin production by previously myelinating cells and to precursor cells which mature into myelin-producing cells. Oligodendrocyte (OL) number may be increased at the periphery of the lesions. In this study, we assessed the state of maturation and cell cycle-dependent properties of OLs derived from surgically resected adult human cerebral cortex specimens. In 6-day-old OL cultures, a small proportion of cells (14.1 +/- 3.5%: range 4-24%) expressed an immature phenotype, defined as A007+:myelin basic protein (MBP)-negative. Using lack of statin expression as an index of cells exiting the G0 phase of the cell cycle, we observed that 4.6 +/- 1.6% of A007+ cells, but only rare MBP+ cells (0.4 +/- 1.8%) were non-reactive with the anti-statin antibody, S44. The proportion of non-statin-reactive cells was not affected by treatment with basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF) or insulin-like growth factor (IGF). The oligodendrocytes did not incorporate BrdU during a 48-h pulse and did not immunoreact with Ki-67 antibody. In 4-week-old cultures, we found that all A007+ cells were also MBP+ and that 99.5 +/- 0.7% were statin-positive. Exposing 4-week-old OLs to conditions of serum deprivation or to 1,000 units/ml of recombinant human TNF-beta for 4 days induced nuclear fragmentation in a high proportion (> 70%) of cells, as measured by a TUNEL technique; in these cultures, a similarly high proportion of cells were non-immunoreactive with anti-statin antibody. Our results suggest that a small number of phenotypic 'pre-oligodendrocytes' can be derived from the adult human CNS and that a proportion of these cells have exited the G0 phase of the cell cycle. Attempt at cell cycling, however, could reflect abortive mitosis and activation of programmed cell death.

Postmitotic oligodendrocytes generated during postnatal cerebral development are derived from proliferation of immature oligodendrocytes

The phenotype of proliferating glia is examined during postnatal rodent development by combining immunocytochemistry (ICC) with 3H-thymidine autoradiography (ARG) to identify cells in the S phase of the cell cycle. Antibodies (ABs) which are specific for cells in the oligodendrocyte (OL) lineage were utilized, with emphasis placed upon the proliferation of OLs as it remains unclear whether this cell type divides in situ. The results show that proliferating cells stain with ABs which are specific for OLs and myelin glycolipids. The proliferating OLs (oligodendroblasts), although they do not appear to have formed myelin sheaths, have quite elaborate and distinctive morphologies. These oligodendroblasts give rise to very long, thin processes which in turn have additional branches. Their cytoarchitecture corresponds closely to cells described as oligodendroblasts with electron microscopy and whose processes often appear to be in the initial phase of myelination (Skoff et al: J. Comp. Neurol. 169:291-312, 1976a). These proliferating OLs are still quite immature because the expression of myelin specific proteins is only occasionally observed in 3H-thymidine labeled cells. The phenotype of the oligodendroblasts is quite different from that of proliferating astrocytes (astroblasts). As shown in previous studies (Skoff; Dev. Biol. 139:149-163, 1990), the astroblasts, which are identified by the presence of glial fibrillary acidic protein (GFAP), usually have thick, stubby processes, and both their nucleus and cytoplasm are larger and of lighter density than those found in oligodendroblasts. In early myelinating regions of the cerebrum, glycolipid positive cells account for the majority of the 3H-thymidine labeled cells. This data, when combined with the quantification of proliferating astrocytes (ASs) from previous immunocytochemical and electron microscopic studies, indicate that oligodendroblasts and astroblasts constitute the vast majority of the proliferating glia in the brain and in optic nerve at times when ASs and OLs are being generated. In normal postnatal cerebral development, the immature ASs and OLs which proliferate are the direct, immediate precursors for most postmitotic ASs and OLs.

Rat oligodendrocytes express muscarinic receptors coupled to phosphoinositide hydrolysis and adenylyl cyclase

Muscarinic receptors expressed by rat oligodendrocyte primary cultures were examined by measuring changes in second messengers following exposure to carbachol, an acetylcholine analog, and by polymerase chain reaction. Inositol phosphate levels were measured in [3H]myo-inositol-labelled young oligodendrocyte cultures following stimulation with carbachol. Atropine, a specific muscarinic antagonist, prevented the carbachol-induced accumulation of inositol phosphates. The formation of inositol trisphosphate was concentration- and time-dependent, with the peak at 100 microM carbachol and 10 min. Carbachol increased intracellular calcium levels, which were dependent both on the mobilization of intracellular stores and influx of extracellular calcium. In initial experiments with more selective antagonists, the mobilization of intracellular calcium was preferentially inhibited by pirenzepine, a selective M1 antagonist, but not methoctramine, a selective M2 antagonist, suggesting M1 muscarinic receptor involvement. A role for protein kinase C in the regulation of carbachol-stimulated inositol phosphate formation and intracellular calcium mobilization was demonstrated, as acute pretreatment with phorbol-12,13-myristate acetate abolished the formation of both second messengers. Pretreatment with 100 microM carbachol abolished the 40% increase in the cyclic AMP accumulation stimulated by isoproterenol, a specific beta-adrenergic agonist. In turn, the inhibition was alleviated by pretreatment with atropine, suggesting muscarinic receptor involvement. Polymerase chain reaction carried out with specific m1 and m2 muscarinic receptor oligonucleotide primers, confirmed that these cells express, at least, the two muscarinic receptor subtypes. Without excluding the expression of other subtypes, these results suggest that developing oligodendrocytes express m1 (M1) and m2 (M2) muscarinic receptors capable of mediating phosphoinositide hydrolysis, mobilization of intracellular calcium and the attenuation of beta-adrenergic stimulation of cyclic AMP formation.

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.

Differentiation of glial precursor cells from developing rat brain in vitro

We have previously shown that enriched preparations of oligodendrocytes from mature bovine brain or 30d rat brain contain 4-10% ganglioside-GD3+ glial precursor cells, which differentiate into astrocytes in culture. These findings are in contrast to those described by others in cultures of neonatal rat brain, which contain precursors that differentiate into both astrocytes and oligodendrocytes. We have extended this study to determine whether the properties of glial precursor cells vary during development. Cells isolated by the same technique from 5-, 10- and 20-day-old rats, were placed in culture and double-immunostained at 1,2,3,6 and 10 days in vitro (DIV) for GD3/glial fibrillary acidic protein (GFAP), galactosylceramide (GC)/GFAP, GD3/GC, GD3/antigen O4, GC/O4 and GFAP/O4. After 1 DIV the isolates from 5 day rats contained 30% GD3+ cells, 1% oligodendrocytes (GC+) and 3.5% astrocytes (GFAP+). The corresponding percentages from 10 day rats were: 35% GD3+, 3% GC+ and 3% GFAP+; and from 20 day rats: 28% GD3+, 35% GC+ and 1% GFAP+. Thus the number of oligodendrocytes in the initial isolate increased dramatically between 10 and 20 days. At all 3 ages immature cells were a major component of the total isolate. GFAP+ cells increased rapidly in all cultures. In all cultures the numbers of GFAP+/GD3+ cells reached a maximum and then declined coincident with the increase of GFAP+/GD3- cells, but there were many more of these double-stained cells in cultures from 20 day rats. Moreover, all cultures at 1 DIV contained some GFAP+/GD3- cells. Thus astrocytes appeared to derive both from pre-existing GFAP+/GD3- cells and from GD3+ cells, the latter pathway being more significant in the older rats. GC+ cells increased in cultures prepared from 5 day and 10 day rats, but remained relatively constant in cultures from 20 day rats. The number of GD3+/O4+ cells decreased coincident with an increase of GC+/O4+ cells (all GC+ cells were O4+), but the number of GC+/GD3+ cells was insignificant in any culture from any age animal. These findings in developing animals support the scheme shown by others in neonates that oligodendrocytes derive from GD3+ cells via O4+/GC-intermediate cells. These data show that more GD3+ cells differentiate into oligodendrocytes in 5- and 10-day-old animals than in 20-day-old animals, and a larger percentage differentiate into astrocytes in 20-day-old animals than in the younger animals. The most reasonable explanation of these results is that two committed populations of GD3+ precursor cells exist in the brain, and that the ratio of these populations changes during development. The implication of this conclusion is that bipotential progenitor cells do not persist beyond the neonatal period, but become committed to separate lineages during development.

Proliferation and differentiation of O4+ oligodendrocytes in postnatal rat cerebellum: analysis in unfixed tissue slices using anti-glycolipid antibodies

We report the study of the in vivo morphology, differentiation, and proliferation of oligodendrocytes (OLs) and their progenitors identified by the antiglycolipid antibodies O4, R-mAb, and O1 in postnatal rat cerebellum, using a novel immunocytochemical staining protocol which allows the analysis of the expression of OL-specific glycolipids in live, unfixed brain slices. An analysis of the individual cells identified in double label immunocytochemistry indicated that the order of antigen expression in OLs during in vivo development is, first, antigens recognized by O4, second, antigens recognized R-mAb, and third, antigens recognized by O1. This order of antigen expression is correlated with increasing morphological complexity and is a pattern mimicked in many culture systems. In vivo O4 identified 3 distinct stages of the OL lineage: (1) morphologically simple proligodendrocyte antigen+ (POA+) R-mAb- blast cells localized at the leading edge of myelinogenesis; (2) morphologically more complex R-mAb+O1- cells; and (3) actively myelinating O1+ [i.e., galactocerebroside+ (GalC)] OLs residing within the white matter. Only the POA+R-mAb- cells incorporated BrdU in animals that were prelabeled 3 hr before immunocytochemistry. We have demonstrated in vivo the subdivision of pre-GalC+ OL progenitors into shorter, biologically noteworthy, stages of maturation. A spatial comparison of the cell populations identified by O4, R-mAb, and O1 demonstrated a progressive wave of OL maturation from the base of the cerebellum toward the folia. The data are consistent with the hypothesis that multiprocessed O4+GalC- progenitors are the most mature stage of the OL lineage with significant proliferative capacity and the first postmigratory stage in normal development.

Identification of mouse type-2-like astrocytes: Demonstration of glutamate and GABA transmitter activated responses

We have identified mouse type-2-like astrocytes and examined some of their electrophysiological properties. Cultures were prepared from P4 mouse neopallia. We demonstrate that mouse type-2-like astrocytes can be identified using the following criteria: presence of glial fibrillary acidic protein (GFAP), presence of chondroitin sulfate polysaccharide, and presence of gamma-aminobutyric acid (GABA). A2B5-binding is not a sufficient criterion to identify O2A lineage cells in mouse neopallial glial cultures since the monoclonal antibody A2B5 binds not only to O2A lineage cells but also to a subpopulation of large, flat type-1-like astrocytes. Mouse type-2-like astrocytes have resting membrane potentials of -76.2 +/- 2.1 mV-i.e., similar to that of mouse type-1-like astrocytes. The input resistance of 44.2 +/- 0.5 M omega is an order of magnitude greater than that of type-1-like astrocytes suggesting the type-2-like astrocytes are not extensively electrically coupled either to each other or to type-1-like astrocytes. Glutamate application caused an 8.8 +/- 1.7 mV depolarization of type-2-like astrocytes. Application of glutamate to barium treated astrocytes caused a fast depolarization with a peak amplitude of 21.4 +/- 1.8 mV; the cells repolarized from this peak by about 10 mV and upon removal of glutamate returned to its pre-glutamate value. Application of GABA caused a transient depolarization of 14.0 +/- 1.7 mV. The presence of barium resulted in a steady-state GABA-induced depolarization of 10.3 +/- 2.0 mV. Neither SITS nor beta-alanine interfered with the amplitude of the glutamate and GABA responses.

Galactocerebroside and sulfatide independently mediate Ca2+ responses in oligodendrocytes

Galactocerebroside (GalC) and sulfated galactocerebroside (sulfatide) are sphingolipids highly enriched in myelin. The binding of antibodies reactive with either sulfatide or GalC to cultured oligodendrocytes causes a Ca2+ influx, followed by microtubule depolymerization; however, antisulfatide is less effective than anti-GalC in altering cytoskeleton. Typical Ca2+ responses are delayed for both antibodies but are transient for sulfatide-reactive antibodies in contrast to the sustained responses previously reported for anti-GalC (Dyer and Benjamins, J Cell Biol 111: 625-633, 1990). Approximately one-half as many oligodendrocytes respond to sulfatide-reactive antibodies (about 39%) as to anti-GalC (about 75%). Subpopulations of oligodendrocytes were identified that responded to neither antibody, only one antibody, or both antibodies, indicating that sulfatide and GalC independently mediate Ca2+ responses. These results suggest that sulfatide and GalC have different physiologic roles in regulating elaboration of myelin membrane by oligodendrocytes in vivo and support the possibility that viral or immune attack via GalC or sulfatide on oligodendrocytes may mimic normal signals in a manner that disrupts the sequence of events that coordinates myelination or maintenance of myelin in vivo.

Division of astroblasts and oligodendroblasts in postnatal rodent brain: evidence for separate astrocyte and oligodendrocyte lineages

What precursor cells are the source of the macroglia generated during postnatal development? In order to answer this question, we studied the expression of glial specific antigens in proliferating neuroglia in postnatal rodent brain and optic nerve. Immunocytochemistry using antibodies to oligodendrocyte (OL) specific markers (sulfatide and galactocerebroside) and an astrocyte (AS) specific marker (glial fibrillary acidic protein) was combined with thymidine autoradiography. During the first week of postnatal development when most ASs are being generated, one third to one half of the proliferating cells in the optic system are positive for glial fibrillary acidic protein after a 1 h injection of thymidine (Skoff, Dev. Biol., 139:149-168, 1990). During the second postnatal week when OLs are being generated, 30 to 100% of the proliferating cells in presumptive white matter tracts are sulfatide positive and at least 10% are galactocerebroside positive. This finding demonstrates that ASs and OLs divide during postnatal development. These results confirm previous electron microscopic autoradiographic studies showing that the vast majority of proliferating cells in postnatal rat optic nerve have the morphologic characteristics of differentiating ASs or OLs (Skoff, J. Comp. Neurol., 169:291-312, 1976). Since proliferating ASs (astroblasts) and OLs (oligodendroblasts) constitute the majority of the dividing cells at the time that ASs and OLs are being generated, these glioblasts must be the major source for the macroglia generated postnatally. The findings strongly suggest that separate lineages exist for ASs and OLs during postnatal development. There is no compelling in vivo evidence for a bipotential progenitor cell that generates the majority of OLs and certain ASs in postnatal rodent brain. There may, of course, be distinct lineages for the subtypes of ASs and possibly even for subtypes of OLs. We review the concepts of commitment and plasticity and apply these terms to glial differentiation. In situ, the presence of oligodendroblasts and astroblasts demonstrates the COMMITMENT of proliferating cells to a specific glial lineage during normal development. Culture conditions may provide an environment that permits proliferating glial cells to vacillate in their selection of a specific lineage. This situation demonstrates developmental PLASTICITY and the ability of glia to adapt to an altered environment. Whether committed glial cells in situ can be induced to switch their lineage when normal CNS conditions are altered is an intriguing question that remains to be answered.