Multiple and novel specificities of monoclonal antibodies O1, O4, and R-mAb used in the analysis of oligodendrocyte development

Fyn tyrosine kinase regulates oligodendroglial cell development but is not required for morphological differentiation of oligodendrocytes

The non-receptor protein tyrosine kinase Fyn, which is a member of the Src family of kinases, has been shown to be essential for normal myelination and has been suggested to play a role in oligodendrocyte development. However, oligodendrocyte development has not been studied directly in cells lacking Fyn. Additionally, because Fyn is expressed in neurons as well as oligodendrocytes, it is possible that normal myelination requires Fyn expression in neurons but not in oligodendrocytes. To address these issues, we analyzed the development of oligodendrocytes in neuron-free glial cell cultures from fyn(-/-) mice that express no Fyn protein. We observed that oligodendrocytes develop to the stage where they elaborate an extensive network of membranous processes and express the antigenic components of mature oligodendrocytes in the complete absence of Fyn. However, as compared with fyn(+/+) controls, fewer oligodendroglia developed in fyn(-/-) cell cultures, and a smaller proportion of them matured to the stage characterized by a high degree of morphological complexity. In addition, we found that insulin-like growth factor-I, a potent stimulator of oligodendrocyte development, failed to stimulate morphological maturation of fyn(-/-) oligodendroglia. The pyrazolopyrimidine PP2, believed to be a selective inhibitor of Fyn, did not prevent the development of morphologically complex oligodendrocytes. Unexpectedly, however, it was toxic to both fyn(+/+) and fyn(-/-) glial cells, indicating that this class of inhibitors can have significant effects that are independent of Fyn.

Sonic hedgehog contributes to oligodendrocyte specification in the mammalian forebrain

This study addresses the role of Sonic hedgehog (Shh) in promoting the generation of oligodendrocytes in the mouse telencephalon. We show that in the forebrain, expression of the early oligodendrocyte markers Olig2, plp/dm20 and PDGFR(alpha) corresponds to regions of Shh expression. To directly test if Shh can induce the development of oligodendrocytes within the telencephalon, we use retroviral vectors to ectopically express Shh within the mouse embryonic telencephalon. We find that infections with Shh-expressing retrovirus at embryonic day 9.5, result in ectopic Olig2 and PDGFR(alpha) expression by mid-embryogenesis. By postnatal day 21, cells expressing ectopic Shh overwhelmingly adopt an oligodendrocyte identity. To determine if the loss of telencephalic Shh correspondingly results in the loss of oligodendrocyte production, we studied Nkx2.1 mutant mice in which telencephalic expression of Shh is selectively lost. In accordance with Shh playing a role in oligodendrogenesis, within the medial ganglionic eminence of Nkx2.1 mutants, the early expression of PDGFR(alpha) is absent and the level of Olig2 expression is diminished in this region. In addition, in these same mutants, expression of both Shh and plp/dm20 is lost in the hypothalamus. Notably, in the prospective amygdala region where Shh expression persists in the Nkx2.1 mutant, the presence of plp/dm20 is unperturbed. Further supporting the idea that Shh is required for the in vivo establishment of early oligodendrocyte populations, expression of PDGFR(alpha) can be partially rescued by virally mediated expression of Shh in the Nkx2.1 mutant telencephalon. Interestingly, despite the apparent requirement for Shh for oligodendrocyte specification in vivo, all regions of either wild-type or Nkx2.1 mutant telencephalon are competent to produce oligodendrocytes in vitro. Furthermore, analysis of CNS tissue from Shh null animals definitively shows that, in vitro, Shh is not required for the generation of oligodendrocytes. We propose that oligodendrocyte specification is negatively regulated in vivo and that Shh generates oligodendrocytes by overcoming this inhibition. Furthermore, it appears that a Shh-independent pathway for generating oligodendrocytes exists.

Synergistic neurotoxicity of opioids and human immunodeficiency virus-1 Tat protein in striatal neurons in vitro

Human immunodeficiency virus (HIV) infection selectively targets the striatum, a region rich in opioid receptor-expressing neural cells, resulting in gliosis and neuronal losses. Opioids can be neuroprotective or can promote neurodegeneration. To determine whether opioids modify the response of neurons to human immunodeficiency virus type 1 (HIV-1) Tat protein-induced neurotoxicity, neural cell cultures from mouse striatum were initially characterized for mu and/or kappa opioid receptor immunoreactivity. These cultures were continuously treated with morphine, the opioid antagonist naloxone, and/or HIV-1 Tat (1-72) protein, a non-neurotoxic HIV-1 Tat deletion mutant (TatDelta31-61) protein, or immunoneutralized HIV-1 Tat (1-72) protein. Neuronal and astrocyte viability was examined by ethidium monoazide exclusion, and by apoptotic changes in nuclear heterochromatin using Hoechst 33342. Morphine (10nM, 100nM or 1microM) significantly increased Tat-induced (100 or 200nM) neuronal losses by about two-fold at 24h following exposure. The synergistic effects of morphine and Tat were prevented by naloxone (3microM), indicating the involvement of opioid receptors. Furthermore, morphine was not toxic when combined with mutant Tat or immunoneutralized Tat. Neuronal losses were accompanied by chromatin condensation and pyknosis. Astrocyte viability was unaffected. These findings demonstrate that acute opioid exposure can exacerbate the neurodegenerative effect of HIV-1 Tat protein in striatal neurons, and infer a means by which opioids may hasten the progression of HIV-associated dementia.

Selective expression of Nkx-2.2 transcription factor in chicken oligodendrocyte progenitors and implications for the embryonic origin of oligodendrocytes

Recent studies have demonstrated that oligodendrocytes originate from the ventral region of the developing spinal cord. However, the precise neuroepithelial origin of oligodendrocytes remains controversial, and the transcriptional control of oligodendrocyte lineage specification is largely unknown. Here we present evidence that oligodendrocytes in the embryonic chicken spinal cord can be generated from neuroepithelial cells that express the Nkx-2.2 homeodomain transcription factor. Nkx-2.2 expression is initially confined to a narrow stripe of neuroepithelium flanking the floor plate. Later, Nkx-2.2+ cells migrate ventrally and dorsolaterally into the surrounding gray and white matter regions where they undergo rapid proliferation. Double labeling experiments revealed that Nkx-2.2+ cells coexpress markers specific for oligodendrocyte progenitors, e.g., PDGFRalpha+, O4, and R-mAb antigens. In the brain, the Nkx-2.2 cells are also highly migratory and can generate oligodendrocytes. The persistent expression of the Nkx-2.2 homeodomain transcription factor in the oligodendrocyte lineage suggests its important role in the control of oligodendrocyte development.

The yin and yang of tenascin-R in CNS development and pathology

An important biological consequence of the initial interactions between the cell surface and its extracellular environment is the diversity of cellular responses ranging from overt repulsion or avoidance reaction to stable adhesion or final positioning. It is now evident that positive and negative guiding mechanisms are equally relevant to normal pattern formation during development and decisive for the outcome of a regenerative process. In this context, the present review summarizes the knowledge about the extracellular matrix glycoprotein tenascin-R, a member of the tenascin gene family. In contrast to all other known family members, tenascin-R is exclusively expressed in the central nervous system of vertebrates by oligodendrocytes and neuronal subsets at later developmental stages and in adulthood. We focus on the glycoprotein's structure, tissue distribution and functional implications in the molecular control of axon targeting, neural cell adhesion, migration and differentiation during nervous system morphogenesis and pathology.

Imprinting as a rapid technique for assessing the morphology of the central nervous system by immunofluorescence

This paper describes a technique that has been developed to assess the in vivo morphology of central nervous system (CNS) tissue by immunofluorescence. This technique permits the study of tissue that is mainly just a monolayer of cells. Unlike routine cryosections that are much thicker (10-15 microm), imprinting does not section the cells, but can result in the detachment of whole cells onto a glass surface for subsequent staining. The imprinting technique is simple and rapid and does not require prior fixation or embedding of the tissue. It has been used to evaluate antigens expressed at the cell surface, in myelin and in the cytoskeleton in the studies of normal and myelin mutant mice. Using the imprinting/immunofluorescence technique one can now assay the genotype of mouse strains that differ in their expression of cell surface antigens within 2 h.

Novel processive and nonprocessive glycosyltransferases from Staphylococcus aureus and Arabidopsis thaliana synthesize glycoglycerolipids, glycophospholipids, glycosphingolipids and glycosylsterols

A processive diacylglycerol glucosyltransferase has recently been identified from Bacillus subtilis [Jorasch, P., Wolter, F.P., Zähringer, U., and Heinz, E. (1998) Mol. Microbiol. 29, 419-430]. Now we report the cloning and characterization of two other genes coding for diacylglycerol glycosyltransferases from Staphylococcus aureus and Arabidopsis thaliana; only the S. aureus enzyme shows processivity similar to the B. subtilis enzyme. Both glycosyltransferases characterized in this work show unexpected acceptor specificities. We describe the isolation of the ugt106B1 gene (GenBank accession number Y14370) from the genomic DNA of S. aureus and the ugt81A1 cDNA (GenBank accession number AL031004) from A. thaliana by PCR. After cloning and expression of S. aureus Ugt106B1 in Escherichia coli, SDS/PAGE of total cell extracts showed strong expression of a protein having the predicted size of 44 kDa. Thin-layer chromatographic analysis of the lipids extracted from the transformed E. coli cells revealed several new glycolipids and phosphoglycolipids not present in the controls. These lipids were purified from lipid extracts of E. coli cells expressing the S. aureus gene and identified by NMR and mass spectrometry as 1, 2-diacyl-3-[O-beta-D-glucopyranosyl]-sn-glycerol, 1, 2-diacyl-3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyrano-+ ++syl] -sn-glycerol, 1, 2-diacyl-3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl-( 1-->6)-O-beta-D-glucopyranosyl]-sn-glycerol, sn-3'-[O-beta-D-glucopyranosyl]-phosphatidylglycerol and sn-3'-[O-(6"'-O-acyl)-beta-D-glucopyranosyl-(1"'-->6")-O-beta-D-gluco pyranosyl]-sn-2'-acyl-phospha-tidylglycerol. A 1, 2-diacyl-3-[O-beta-D-galactopyranosyl]-sn-glycerol was isolated from extracts of E. coli cells expressing the ugt81A1 cDNA from A. thaliana. The enzymatic activities expected to catalyze the synthesis of these compounds were confirmed by in vitro assays with radioactive substrates. Experiments with several of the above described glycolipids as 14C-labeled sugar acceptors and unlabeled UDP-glucose as glucose donor, suggest that the ugt106B1 gene codes for a processive UDP-glucose:1, 2-diacylglycerol-3-beta-D-glucosyltransferase, whereas ugt81A1 codes for a nonprocessive diacylglycerol galactosyltransferase. As shown in additional assays with different lipophilic acceptors, both enzymes use diacylglycerol and ceramide, but Ugt106B1 also accepts glucosyl ceramide as well as cholesterol and cholesterol glucoside as sugar acceptors.

Effects of galactolipid elimination on oligodendrocyte development and myelination

The galactolipids galactocerebroside and sulfatide, which require the enzyme UDP-galactose:ceramide galactosyltransferase (CGT) for their synthesis, are among the most prevalent molecules in the myelin sheath. Numerous studies, mainly using antibody perturbation methods in vitro, have suggested that these molecules are crucial mediators of oligodendrocyte differentiation and myelin formation. Although we have previously demonstrated that myelin formation occurs in CGT null mutant mice, which are incapable of synthesizing the myelin galactolipids, here we show that there are developmental alterations in the CNS of these animals. There is a significant decrease in the number of myelinated axon segments in the mutant spinal cord despite normal levels of myelin gene-specific mRNAs and proteins. Also, there is an increased cellularity in the mature mutant spinal cord and the distinctive morphology of the additional cells suggests that they are actively myelinating oligodendrocytes. Using in situ hybridization techniques, we show that there is a 50% increase in the number of oligodendrocytes in the mutant spinal cord. The data suggest that galactolipids play an important developmental role in regulating the maturation program and final number of oligodendrocytes.

Redistribution of cholesterol in oligodendrocyte membrane sheets after activation of distinct signal transduction pathways

Cultured oligodendrocytes produce extensive membrane sheets that contain an internal lacy network of vein-like structures composed of microtubules, actin filaments, and 2'3'-cyclic nucleotide 3'-phosphohydrolase (CNPase). These cytoplasmic vein-like structures surround domains of myelin basic protein (MBP). Using the antibiotic filipin, that binds to cholesterol, the relationship between plasma membrane cholesterol and cytoskeleton in membrane sheets was examined. Our results show that cholesterol was relatively uniformly distributed within the plasma membranes of prefixed control oligodendrocyte membrane sheets. When live cultures were extracted with Triton X-100, however, a subpopulation of cholesterol molecules remained colocalized with cytoskeleton in the membrane sheets. Activation of two well-characterized signaling pathways that differentially affect microtubule and actin filament stability in membrane sheets resulted in an apparent massive lateral movement of cholesterol molecules away from membrane regions overlying internal MBP domains to membrane tracts directly overlying cytoplasmic cytoskeletal veins. Depolymerization of microtubules by colchicine resulted in redistribution of cholesterol directly over actin filaments, whereas depolymerization of actin filaments by cytochalasin B resulted in redistribution of cholesterol directly over CNPase/microtubular veins. These data suggest that cholesterol forms an association with cytoskeletal components or proteins associated with cytoskeleton. These data also suggest that cholesterol, via interactions with cytoskeleton, plays a role in signaling pathways in oligodendrocyte membrane sheets.

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.

GFAP-positive and myelin marker-positive glia in normal and pathologic environments

The data herein demonstrate that in addition to the well-characterized myelin marker-positive, glial fibrillary acidic protein (GFAP)-negative, membrane sheet-bearing oligodendrocytes, another type of myelin marker-positive, process-bearing glia exists in normal and pathologic conditions. This second type of myelin marker-positive glia expresses GFAP, and therefore these cells have been referred to as mixed phenotype glia. Although mixed phenotype glia have been documented previously, their identity and function have remained a mystery. The goal of this immunocytochemical study was to further characterize these cells. Using the MBPlacZ transgenic mouse in which beta-galactosidase is under the control of the myelin basic protein (MBP) gene promoter, GFAP-positive/beta-galactosidase-positive and myelin/oligodendrocyte-specific protein (MOSP)-positive/beta-galactosidase-positive cells were detected in subcortical white matter and in perivascular locations within cerebral white and gray matter. In cultures prepared from highly enriched myelin marker-positive immature glia, mixed phenotype glia were detected that were GFAP-positive and either MOSP-, MBP-, O1-, and O4-positive. The expression of multiple myelin markers by mixed phenotype glia may suggest that these cells are of oligodendrocyte origin. Increased numbers of MOSP-positive/GFAP-positive mixed phenotype glia were detected in sections from adult hypomyelinated brain from shiverer, quaking, and PKU mice compared to myelinated control adult mouse brain. Similarly, cultures from control brain exposed to elevated pH for 2-3 weeks showed dramatically increased numbers of mixed phenotype glia (80%) compared to control (<10%). Increased numbers of mixed phenotype glia also were detected in shiverer cultures (40%). Since increases in the number of mixed phenotype glia occur in shiverer, quaking, and PKU mouse brain, these data suggest that mixed phenotype glia contribute to gliosis in pathologic white matter.

Chondroitin sulfates expressed on oligodendrocyte-derived tenascin-R are involved in neural cell recognition. Functional implications during CNS development and regeneration

Tenascin-R (TN-R), an extracellular matrix constituent of the central nervous system (CNS), has been implicated in a variety of cell-matrix interactions underlying axon growth inhibition/guidance, myelination and neural cell migration during development and regeneration. Although most of the functional analyses have concentrated exclusively on the role of the core protein, the contribution of TN-R glycoconjugates present on many potential sites for N- and O-glycosylation is presently unknown. Here we provide first evidence that TN-R derived from whole rat brain or cultured oligodendrocytes expresses chondroitin sulfate (CS) glycosaminoglycans (GAGs), i.e., C-4S and C-6S, that are recognized by CS-56, a CS/dermatan sulfate-specific monoclonal antibody. Based on different in vitro approaches utilizing substrate-bound glycoprotein, we found that TN-R-linked CS GAGs (1) promote oligodendrocyte migration from white matter microexplants and increase the motility of oligodendrocyte lineage cells; (2) similar to soluble CS GAGs, induce the formation of glial scar-like structures by cultured cerebral astrocytes; and (3) contribute to the antiadhesive properties of TN-R for neuronal cell adhesion in an F3/F11-independent manner, but not to neurite outgrowth inhibition, by mechanism(s) sensitive to chondroitinase or CS-56 treatments. Furthermore, after transection of the postcommissural fornix in adult rat, CS-bearing TN-R was found to be stably upregulated at the lesion site. Our findings suggest the functional impact of TN-R-linked CS on neural cell adhesion and migration during brain morphogenesis and the contribution of TN-R to astroglial scar formation (CS-dependent) and axon growth inhibition (CS-independent), i.e., suppression of axon regeneration after CNS injury.

MAL, a proteolipid in glycosphingolipid enriched domains: functional implications in myelin and beyond

The myelin and lymphocyte protein MAL (VIP17/MVP17) is a proteolipid of 17 kD with a hydrophobicity pattern that indicates a four transmembrane domain structure. The MAL cDNA has been cloned from human T-cells, rat oligodendrocytes and the Madin-Darby canine kidney (MDCK) cell line. In the nervous system both myelinating cells, oligodendrocytes and Schwann cells, express MAL protein. MAL expression parallels myelin formation, and MAL is predominantly localized in compact myelin. Prior to myelin formation MAL is also found in immature Schwann cells. Outside the nervous system MAL expression is found in T-cells and in distinct epithelial cells, e.g. in kidney, stomach and thyroid gland, where MAL is localised in the apical plasma membrane. Specific glycosphingolipids, e.g. galactosylceramide and sulfatide, are enriched in such apical kidney and stomach membranes as well as in myelin. MAL copurifies with these glycosphingolipids in detergent insoluble domains, indicating a close association and possible functional interactions of MAL with glycosphingolipids in these tissues. Moreover, recent reports point to additional functions of MAL-glycosphingolipid complexes in signalling, cell differentiation and apical sorting. The role of MAL in the formation, stabilisation and maintenance of glycosphingolipid-enriched membrane microdomains and its contribution to specific membrane properties in myelin and epithelial cells are discussed.

Spatiotemporal development of oligodendrocytes in the embryonic brain

In the central nervous system (CNS), oligodendrocytes have long been considered to be the last cell type to be generated during development. In rodents, the progenitor cells that give rise to oligodendrocytes have been reported to originate in the subventricular zone. Here, we review recent data demonstrating the existence of oligodendrocyte precursor cells in the ventricular layer of the neural tube that emerge prior to the progenitor stage. Oligodendrocyte precursors arise in restricted foci that are distributed along the rostrocaudal axis of the neural tube, for the most part ventrally. The generation of oligodendrocyte precursor cells occurs either simultaneously with, or follows closely upon the emergence of the first neurons. Experiments with quail-chick chimeras provide evidence that oligodendrocyte progenitors derived from ventricular precursors migrate either tangentially or radially to colonize extensive or segmentally restricted territories of the brain. The choice depends on their site of origin. Finally, we discuss the possibility that oligodendrocytes could be a mosaic population that originates from at least two types of precursor cells.

Rat oligodendroglia express c-met and focal adhesion kinase, protein tyrosine kinases implicated in regulating epithelial cell motility

Oligodendrocytes, the myelinating cells of the central nervous system, arise from a profilerating pool of motile progenitor cells. The proliferation and survival of these cells is dependent on signal transduction via several protein tyrosine kinases (PTKs) including receptors for fibroblast growth factor -2, the platelet-derived growth factor receptors and the neurotrophin receptor, trkC. We hypothesized that additional PTKs could also influence oligodendroglial development. Utilizing RTPCR, we amplified from post-natal day 6 rat oligodendroglia 17 distinct kinase domain sequences, 14 of which were not previously known to be expressed by oligodendroglia. Amongst the sequences identified were the c-met and Fak genes, whose protein products regulate the motility of other epithelial cell types. Utilizing immunohistochemistry, we confirmed that both c-met and Fak are expressed by cultured oligodendroglia, suggesting that these proteins could also be implicated in regulating the motility of these cells.

Adhesion molecule expression and phenotype of glial cells in the olfactory tract

These data illustrate that OBECs have a highly plastic nature in keeping with their need to respond rapidly to changing environmental cues. This relates to their required function in supporting axonal extension throughout life. Future studies using antibodies to PSA-NCAM and L-NGFr together with FACS sorting to purify the two types of OBEC should give us a clearer understanding of the lineage relationship of the two phenotypes. With purified populations of the astrocyte-like and Schwann cell-like OBEC we should be able to determine if these cells have different functions in vivo, using several approaches namely: i) identifying the growth factors that regulate their growth and differentiation, ii) measuring the ability of the purified cells to remyelinate the experimentally-created CNS lesions and iii) carry out more detailed cellular and molecular comparisons of the two phenotypes.

Identification, isolation, and promoter-defined separation of mitotic oligodendrocyte progenitor cells from the adult human subcortical white matter

Previous studies have suggested the persistence of oligodendrocyte progenitor cells in the adult mammalian subcortical white matter. To identify oligodendrocyte progenitors in the adult human subcortical white matter, we transfected dissociates of capsular white matter with plasmid DNA bearing the gene for green fluorescence protein (hGFP), placed under the control of the human early promoter (P2) for the oligodendrocytic protein cyclic nucleotide phosphodiesterase (P/hCNP2). Within 4 d after transfection with P/hCNP2:hGFP, a discrete population of small, bipolar cells were noted to express GFP. These cells were A2B5-positive (A2B5(+)), incorporated bromodeoxyuridine in vitro, and constituted <0.5% of all cells. Using fluorescence-activated cell sorting (FACS), the P/hCNP2-driven GFP(+) cells were then isolated and enriched to near-purity. In the weeks after FACS, most P/hCNP2:hGFP-sorted cells matured as morphologically and antigenically characteristic oligodendrocytes. Thus, the human subcortical white matter harbors mitotically competent progenitor cells, which give rise primarily to oligodendrocytes in vitro. By using fluorescent transgenes of GFP expressed under the control of an early oligodendrocytic promoter, these oligodendrocyte progenitor cells may be extracted and purified from adult human white matter in sufficient numbers for implantation and cell-based therapy.

Early specification of oligodendrocytes in the chick embryonic brain

Oligodendrocytes are the myelin-forming cells in the central nervous system of vertebrates. In the rodent embryo, these cells have been shown to emerge from restricted territories of the neuroepithelium. However, a comprehensive view of the development of oligodendroglial populations from their ventricular sources remains to be established. As a first step toward this aim, we have examined in vivo the spatiotemporal emergence of oligodendrocytes in the chick embryonic brain. We have detailed the patterns of expression of three early markers of the oligodendroglial lineage: the plp/dm-20 and PDGFRalpha transcripts and the O4-reactive antigen. During embryonic development, these molecules showed a similar segmental pattern of expression. However, plp/dm-20(+) cells were already observed, in the ventricular layer, at E2.5, i.e., 2 days before the appearance of O4(+) and PDGFRalpha(+) cells, suggesting that oligodendrocyte precursors arise nearly simultaneously with neurons. In the chick embryonic brain, the onset of expression of plp/dm-20 appears therefore to be the earliest event indicative of oligodendroglial specification and we propose, based on the expression of plp/dm-20 transcript, a ventricular map of the foci at which oligodendrocytes originate. In addition, we document the precocious segregation, from E5, of plp/dm-20(+) and PDGFRalpha(+) oligodendroglial cells in the subventricular and mantle layers of the brain.

The neurotoxicant, cuprizone, retards the differentiation of oligodendrocytes in vitro

The effects of oxalyldihydrazone (cuprizone) on weanling rodents provided an early protocol for toxic demyelination in vivo, in which degeneration of oligodendrocytes preceded disruption of the myelin sheath, and in which remyelination could take place. We administered cuprizone to oligodendrocyte-enriched glial-cell cultures and to mixed glial-cell cultures from neonatal rat brains. The cultures were treated with cuprizone for 1 h and allowed to continue differentiating on subsequent days. Treated cultures and respective control cultures were fixed with 4% paraformaldehyde (w/v) and immunostained with double immunofluorescence. MAbO4 was used to mark precursors and mature oligodendrocytes, and anti-myelin basic protein (MBP) to mark mature oligodendrocytes (O4+/MBP+), as distinguished from precursors, which were O4+/MBP-. Cell counts suggested that cuprizone inhibited the maturation of oligodendrocytes without diminishing the numbers of precursors, and appeared to affect the mitochondria in those cells.

Matrix metalloproteinase-9/gelatinase B is required for process outgrowth by oligodendrocytes

Oligodendrocytes (OLs) extend processes to contact axons as a prerequisite step in myelin formation. As the OL processes migrate toward their axonal targets, they modify adhesion to their substrate, an event that may be regulated by matrix metalloproteinases (MMPs). In the mouse optic nerve, MMP-9/gelatinase B increases during myelin formation. Although tissue inhibitor of metalloproteinase (TIMP)-3 also increases in parallel, the developing optic nerve has focally active MMPs demonstrable by in situ zymography. The distribution of proteolytic activity is similar to that of myelin basic protein, a marker of myelin formation. OLs in culture secrete MMP-9 and express active cell-associated metalloproteinases at the growing tips of their processes. TIMP-1 and a function-perturbing anti-MMP-9 antibody attenuate outgrowth of processes by OLs, indicating a requirement for MMP-9 in process outgrowth. Process reformation is retarded significantly in OLs cultured from MMP-9 null mice, as compared with controls, providing genetic evidence that MMP-9 is necessary for process outgrowth. These data show that MMP-9 facilitates process outgrowth by OLs in vivo and in culture.

Negative regulation of oligodendrocyte differentiation by galactosphingolipids

Galactocerebroside and sulfatide, major galactosphingolipid components of oligodendrocyte plasma membranes and myelin, are first expressed at a critical point, when progenitors cease to proliferate and commence terminal differentiation. We showed previously that an antibody to galactocerebroside/sulfatide arrested terminal differentiation, suggesting a role for these galactolipids in oligodendrocyte differentiation. We have now investigated the differentiation of oligodendrocytes (1) in response to other anti-galactolipid antibodies, showing that anti-sulfatide O4 but not anti-galactocerebroside O1 blocks terminal differentiation, perhaps by mimicking an endogenous ligand, and (2) in a transgenic mouse unable to synthesize these lipids because of mutation of the gene for ceramide galactosyltransferase, a key enzyme for galactosphingolipid synthesis. We find that galactosyltransferase mRNA expression begins at the late progenitor [pro-oligodendroblast (Pro-OL)] stage of the lineage and that the late progenitor marker pro-oligodendroblast antigen is not synthesized in the absence of galactosyltransferase. The principal outcome of the elimination of these galactolipids is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo. Because the general pattern of differentiation and the level of progenitor proliferation and survival appear to be unaltered in the mutant cultures, we conclude that the increased number of oligodendrocytes is caused by an increased rate and probability of differentiation. In agreement with these two experimental approaches, we present a model in which galactosphingolipids (in particular galactocerebroside and/or sulfatide) act as sensors and/or transmitters of environmental information, interacting with endogenous ligands to function as negative regulators of oligodendrocyte differentiation, monitoring the timely progress of Pro-OLs into terminally differentiating, myelin-producing oligodendrocytes.

An analysis of the early events when oligodendrocyte precursor cells are triggered to differentiate by thyroid hormone, retinoic acid, or PDGF withdrawal

Oligodendrocyte precursor cells withdraw from the cell cycle and terminally differentiate after a limited number of cell divisions. The timing of cell-cycle withdrawal and differentiation is controlled by an intrinsic timer, which consists of a timing component that measures elapsed time and an effector component that arrests the cell cycle and initiates differentiation. The effector component can be triggered by either thyroid hormone (TH) or retinoic acid (RA). In this study we investigate how TH and RA act to trigger differentiation. We show the following: (1) Synthetic retinoids that can inhibit AP-1 transcription factors but do not activate gene transcription cannot trigger the effector mechanism, suggesting that TH and RA do not act only by inhibiting AP-1 activity as previously suggested. (2) Both TH and RA induce a transcriptionally dependent antigenic change in purified precursor cells within 2-4 h. (3) Unexpectedly, even before they differentiate, the precursor cells express ceramide galactosyltransferase (CGT), the enzyme that catalyzes the final step in the synthesis of galactocerebroside, an early marker of oligodendrocyte differentiation. (4) Neither TH nor RA directly activates the transcription of the CGT gene, a number of immediate early genes, or the genes that encode any of the known cyclin-dependent kinase inhibitors. (5) The withdrawal of the mitogen platelet-derived growth factor (PDGF), but not TH or RA treatment, causes a rapid decrease in c-fos, NGFI-A/Krox-24, and cyclin D2 mRNA, even though all three treatments trigger cell-cycle arrest and differentiation. (6) PDGF withdrawal and TH treatment, but not RA treatment, induce an increase in cyclin D3 mRNA within 4 h. Thus, we have not found any early changes in gene expression that occur with all three treatments that trigger oligodendrocyte differentiation.

Primary structure of the antigen-binding domains of a human oligodendrocyte-reactive IgM monoclonal antibody derived from a patient with multiple sclerosis

Several murine IgM monoclonal antibodies (mAbs) promoting remyelination in mice were shown to be germline gene-encoded natural autoantibodies that react with oligodendrocytes and intracellular antigens. Here, we show that human oligodendrocyte-reactive IgM mAb DS1F8 derived from a patient with multiple sclerosis targets microtubule-like structures similar to the murine mAbs. Sequencing of the cDNAs of the variable regions revealed that the antigen-binding domains are also encoded by germline genes. These similarities of mAb DS1F8 to the murine mAbs promoting remyelination suggest that this human mAb is a natural autoantibody. This may imply that the engineering of human autoantibodies for therapy of demyelinating diseases is feasible.

Atypical glial cells in demyelinated and hypomyelinated mouse brains

Bis-cyclohexanone oxalyldihydrazone (cuprizone) was administered to young adult mice in order to investigate the hypothesis that the differentiation of oligodendrocyte progenitors or precursors, or of immature oligodendrocytes, might be interrupted during cuprizone intoxication. Vibratome sections were prepared from brains from control mice, mice that were fed cuprizone for 27 days and mice that were fed cuprizone for 25 days, followed by normal diet for 2 days, and the sections were immunostained with monoclonal antibodies: MAbO4, which is directed against galactocerebroside sulfate (sulfatide); and RMAb, which is directed against galactocerebroside (GC). Process-bearing RMAb+/O4- cells were abundant in the brains of mice that had consumed cuprizone for 27 days, and the numbers of O4-positive cells were subnormal. Two days after refeeding the normal diet the RMAb+/O4- structures were less abundant and O4-positive cells more numerous. Moreover, the numbers of O4-positive cells were only approximately 20% of normal in the brains of hypomyelinating jimpy mutant mice, while the numbers of RMAb-positive cells were approximately 80% of normal, and the processes of the latter were associated with axons. It is suggested that RMAb+/O4- cells in the affected brains may be process-bearing oligodendrocyte precursors containing unsulfated GC or a toxic galactolipid.

Tenascin-R interferes with integrin-dependent oligodendrocyte precursor cell adhesion by a ganglioside-mediated signalling mechanism

Oligodendrocyte (OL) lineage progression is characterized by the transient expression of the disialoganglioside GD3 by OL precursor (preOL) cells followed by the sequential expression of myelin-specific lipids and proteins. Whereas GD3+ preOLs are highly motile cells, the migratory capacity of OLs committed to terminal differentiation is strongly reduced, and we have recently shown that the extracellular matrix protein tenascin-R (TN-R) promotes the stable adhesion and differentiation of O4+ OLs by a sulphatide-mediated autocrine mechanism (O4 is a monoclonal antibody recognizing sulphatides/seminolipids expressed by OLs and in myelin). Using culture conditions that allow the isolation of mouse OLs at distinct lineage stages, here we demonstrate that TN-R is antiadhesive for GD3+ preOLs and inhibits their integrin-dependent adhesion to fibronectin (FN) by a disialoganglioside-mediated signalling mechanism affecting the tyrosine phosphorylation of the focal adhesion kinase. This responsive mechanism appears to be common to various cell types expressing disialogangliosides as: (i) disialogangliosides interfered with the inhibition of cell adhesion of different neural and non-neural cells on substrata containing TN-R and FN or RGD-containing FN fragments. TN-R interacted specifically with disialoganglioside-expressing cells or immobilized gangliosides, and ganglioside treatment of TN-R substrata resulted in a delayed preOL cell detachment as a function of time. We conclude that OL response to one and the same signal in the extracellular matrix critically depends on the molecular repertoire expressed by OLs at different lineage stages and could thus define their final positioning.

Fetal human cortical neurons grown in culture: morphological differentiation, biochemical correlates and development of electrical activity

Cultured fetal human cortical neurons derived from second trimester human fetal cortex were analyzed with regard to their morphological differentiation and expression of cell-specific markers. The culture method was adapted from standardized protocols originally developed for the isolation and culture of rodent oligodendrocytes and astrocytes. This technique takes advantage of the different adhesive properties and stratification of central nervous system cells in vitro. Under these culture conditions fetal human cortical neurons underwent morphological differentiation, expressed neuron-specific markers and voltage- and ligand-gated ion channels. Highly enriched cultures of microglia and astrocytes generated from the same starting material also expressed cell-type specific markers. These cultures serve as a valuable tool for the establishment of normative data and as experimental models for neurodevelopmental and neurodegenerative studies.

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.

Oligodendrocyte apoptosis mediated by caspase activation

Treatment with NGF causes long-term cultures of oligodendrocytes to die via a yet undefined mechanism mediated by the p75 neurotrophin receptor. The p75 receptor belongs to the TNF receptor superfamily of molecules, which includes Fas and p55 TNF receptors. The Fas and TNF receptors use adaptor molecules to recruit and activate caspase-8 to the receptor. Using a combination of immunohistochemical and Western blotting assays, we have examined caspase activity during NGF-induced apoptosis. Interestingly, although caspase-1 [interleukin-1beta-converting enzyme (ICE)], caspase-2, caspase-3, and caspase-8 were expressed in oligodendrocytes, only caspase-1, -2, and -3 were activated after NGF treatment, whereas caspase-8 was not. These data suggest that the mechanism of apoptosis by NGF through the p75 receptor is different from TNF and Fas-mediated killing. gamma Radiation of oligodendrocytes also activated a similar subset of caspases as NGF, indicating that NGF-induced oligodendrocyte apoptosis uses a similar cell death execution mechanism as injury models. This consolidates a potential role of the p75 neurotrophin receptor during stress and inflammatory conditions.

Inducible nitric oxide synthase expression in cultures enriched for mature oligodendrocytes is due to microglia

Expression of inducible nitric oxide (NO) synthase (NOS-2) occurs during inflammation in the central nervous system (CNS) and has been linked to demyelination accompanying certain CNS inflammatory diseases. Although astrocytes and microglia are thought to be the major sources of NOS-2 expression in the CNS in vivo, recent evidence suggested that the myelin-producing oligodendrocytes (OLs) themselves can express NOS-2 in culture. Given the potentially important pathological implications of this finding, the purpose of this study was to examine further the expression of NOS-2 by OLs in vitro. After exposure to lipopolysaccharide (LPS) and interferon-gamma (IFNgamma), primary cultures enriched for mature OLs released NO in a time-dependent manner, although the amount varied considerably between different culture preparations. Increased NO production was accompanied by expression of NOS-2 mRNA and protein, as determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. Immunofluorescence analysis revealed that the cell-type expressing NOS-2 in these cultures was galactocerebroside (Gal C)-negative but CD11b-positive. Further, NO production could be attenuated in cultures treated with the microglial/macrophage toxin, leucine methyl ester, prior to LPS/IFNgamma stimulation. Thus, microglia were the source of NOS-2 catalytic activity in these cultures. The present results indicate that LPS and IFNgamma are not effective stimuli for induction of NOS-2 in OLs in primary cell culture.

Proteolipid protein gene product can be secreted and exhibit biological activity during early development

A gene encoding myelin proteolipid protein (PLP) and its smaller isoform DM20 is expressed at least 1 week before myelination. Mutations within the gene cause abnormalities in the development of premyelinating oligodendrocytes, resulting in hypomyelinating disorders. These findings suggest a premyelinating function of the PLP gene products. We previously demonstrated that PLP gene expression is directly associated with secretion of a factor that increases the number of oligodendrocytes. Here we show that this activity is mediated by a secreted fragment containing the C-terminal portion of PLP. This factor increased the bromodeoxyuridine incorporation rate in both oligodendrocyte and astrocyte lineage cells; a synthetic peptide (PLP 215-232) exhibited a similar activity. Dose-response curves of PLP and PLP peptide showed maximum activities at a concentration in the picomolar range, which decreased at higher concentrations. These observations demonstrate that a secreted PLP gene product exerts biological activity at a premyelinating stage before the major induction of the gene.

SNARE complex proteins, including the cognate pair VAMP-2 and syntaxin-4, are expressed in cultured oligodendrocytes

Myelin membrane synthesis in the CNS by oligodendrocytes (OLs) involves directed intracellular transport and targeting of copious amounts of specialized lipids and proteins over a relatively short time span. As in other plasma membrane-directed fusion, this process is expected to use specific trafficking and vesicle fusion proteins characteristic of the SNARE model. We have investigated the developmental expression of SNARE proteins in highly enriched primary cultures of OLs at discrete stages of differentiation. VAMP-2/synaptobrevin-2, syntaxin-2 and -4, nsec-1/munc-18-1, Rab3a, synaptophysin, and synapsin were expressed. During differentiation, expression of the vesicular SNARE VAMP-2, the small GTP-binding protein Rab3a, and the target SNARE syntaxin-4 were up-regulated. VAMP-2 and Rab3 proteins detected immunocytochemically in cultured OLs were localized within the developing process network; in situ anti-VAMP-2 antibody stained the perikarya of rows of cells with the distribution and appearance of OLs. We discuss the potential involvement of SNARE complex proteins in a plasma membrane-directed transport mechanism targeting nascent myelin vesicles to the forming myelin sheath.

New insights into the interaction between the gp120 and the HIV receptor in human sperm (human.sperm/gp120/galactoglycerolipid/antigalactosylceramide/seminolip id/spermatogonia)

The human immunodeficiency virus (HIV) can infect some cell types which lack CD4. Galactosylceramide, a glycolipid present in the nervous system and colonic epithelial cells, has been implicated in the virus entry in these cells. Our data demonstrate that the HIV surface glycoprotein gp120 binds to the galactosyl-alkyl-acylglycerol (GalAAG), a glycolipid structurally related to galactosylceramide present on the surface membrane of the spermatozoa. In this paper, we review our previous data and further confirm the specificity of the interaction between this galactoglycerolipid and the gp120. Consistent with the structural similarity to galactosylceramide, the sperm GalAAG is capable of specifically binding the gp120. The specificity of the binding of antibodies antigalactosylceramide and the gp120 to the sperm extract and to the purified GalAAG fraction prepared from the same extract has been demonstrated utilizing an ELISA assay which favors sensitivity and specificity. Immunofluorescence and immunoelectron microscopy data show a different localization for the GalAAG and its sulfated form the seminolipid (SGalAAG). The GalAAG is preferentially localized in the equatorial segment and the middle piece of the sperm tail, while the seminolipid is widely distributed on the membrane of the spermatozoa. These data indicate that human sperm express on their surface membrane a glycolipid similar in structure to galactosylceramide, the receptor for HIV identified in the CD4 cells, that could function as a HIV receptor and possibly be implicated in its transmission.

Thyroid hormone receptor isoforms are sequentially expressed in oligodendrocyte lineage cells during rat cerebral development

In the mammalian brain, thyroid hormones regulate myelination. Their actions are mediated by interactions with nuclear receptors that function as ligand-regulated transcription factors. Two genes, alpha and beta, encode different isoforms, of which only the beta and alpha1 isoforms are authentic nuclear triiodothyronine (T3)-receptors (NT3R). In agreement with the important role of T3 on myelination and oligodendrocyte generation, the presence of NT3Rs has been reported in oligodendrocytes and their precursors. We and others have shown that both progenitors and oligodendrocytes in vitro express the alpha1 and alpha2 isoforms, but the expression of the beta1 isoform is confined to differentiated oligodendrocytes, suggesting that they have different functions. To establish if this is the case during development in vivo, we have studied NT3R isoform expression in glial cells isolated by density gradient centrifugation from rat brains of various ages. We report the presence of the alpha1 NT3R and its variant alpha2, but not that of the beta1 isoform, in newborn rat glial progenitors. The pattern of expression of beta1, both at the level of mRNA and protein, parallels the increase in the number of oligodendrocytes. We found a significant change in the kinetic parameters of [125I]-T3 binding to NT3Rs in these cells during the first month of life, consisting of an increase in the binding capacity that peaks with myelination, and a significative decrease in Kd that coincides with the switch from the alpha to the beta1 isoform. Thus, the expression of NT3R isoforms in the rat oligodendrocyte lineage changes radically from the alpha to the beta1 isoform during the period when oligodendrocytes differentiate from progenitors.

Glycolipids as potential binding sites for HIV: topology in the sperm plasma membrane in relation to the regulation of membrane fusion

Although human sperm cells can bind human immunodeficiency virus (HIV-1), they lack CD4, galactoceramides (GalCer) and sulfogalactoceramides (SGalCer) as gp120 receptors. However, sperm specific glycolipids (sulfogalactosylalkylacylglycerol (SGalAAG) and galactosylalkylacylglycerol (GalAAG)) are structurally closely related to SGalCer and GalCer as predicted by computer simulated molecular modelling. SGalAAG and GalAAG are exclusively localized in the outer leaflet of the human sperm plasma membrane, and therefore we tested whether they could serve as alternative receptors for the gp120. Purified SGalAAG and GalAAG had similar affinities to recombinant gp120 as the hydroxy fatty acid (HFA) SGalCer and HFA-GalCer respectively. However, nonhydroxy fatty acid forms of (S)GalCer, galactosyldiacylglycerol and the deacylated (sulfo)galactosyllipids did not recognize recombinant gp120. Data obtained by surface pressure experiments revealed that the lipid monolayers that contained HFA-GalCer or GalAAG resulted in a similar significant penetration of recombinant gp120 in the monolayer. The penetration was a factor of two lower in monolayers with HFA-SGalCer or SGalAAG. The binding of recombinant gp120 to human sperm cells colocalized with GalAAG and could be blocked with monoclonal antibodies against galactolipids. The possible relevance of gp120 binding to glycolipids for HIV entry in sperm cells is discussed.

Targeting of IgMkappa antibodies to oligodendrocytes promotes CNS remyelination

We previously identified the remyelinating activity of a natural IgMkappa oligodendrocyte-reactive autoantibody (SCH94.03), using a virus-induced murine model of multiple sclerosis. We now describe a second mouse IgMkappa monoclonal antibody (mAb) (SCH79.08) raised against normal mouse spinal cord homogenate, which reacts with myelin basic protein and also promotes remyelination. Because these two mAbs recognize different oligodendrocyte antigens, several previously identified oligodendrocyte-reactive IgMkappa mAbs (O1, O4, A2B5, and HNK-1), each with distinct antigen specificities, were evaluated and found to promote remyelination. In contrast, IgMkappa mAbs that did not bind to oligodendrocytes showed no remyelination. One of these, CH12 IgMkappa mAb, which shares variable region cDNA sequences with SCH94.03 except for amino acid differences in the complementarity-determining region 3 in both heavy and light chains, did not bind to oligodendrocytes and did not promote remyelination. The fact that multiple oligodendrocyte-reactive antibodies with distinct antigen reactivities induce remyelination argues against direct activation by a unique cell surface receptor. These findings are most consistent with the hypothesis that the binding of mAbs to oligodendrocytes in the lesions induces myelin repair via indirect immune effector mechanisms initiated by the mu-chain. Importantly, these studies indicate that oligodendrocyte-reactive natural autoantibodies may provide a powerful and novel therapeutic means to induce remyelination in multiple sclerosis patients.

Posttranscriptional regulation of oligodendroglial thyroid hormone (T3) receptor beta 1 by T3

3,5,3'-triiodo-L-thyronine interacts with the genome by binding and activating nuclear 3,5,3'-triiodo-L-thyronine receptors. To determine how in secondary oligodendrocyte cultures, exogenous 3,5,3'-triiodo-L-thyronine influences the expression of different 3,5,3'-triiodo-L-thyronine receptor isoforms, we studied the regulation of alpha 1, alpha 2 and beta 1 3,5,3'-triiodo-L-thyronine receptor mRNAs. In culture, we find that beta 1, 3,5,3'-triiodo-L-thyronine receptor mRNA, but not alpha 1 and alpha 2 3,5,3'-triiodo-L-thyronine receptor mRNAs, is up-regulated by 3,5,3'-triiodo-L-thyronine in a time and dose dependent manner. In addition, we present evidence indicating that beta 1 3,5,3'-triiodo-L-thyronine receptor expression is posttranscriptionally regulated by 3,5,3'-triiodo-L-thyronine. Previous studies from our laboratory and others have shown that in the rat oligodendrocyte lineage, 3,5,3'-triiodo-L-thyronine receptors alpha 1 and alpha 2 were expressed in both early progenitor cells and mature oligodendrocytes. In contrast, beta 1 3,5,3'-triiodo-L-thyronine receptor was found to be expressed only in mature oligodendrocytes. This suggests that thyroid hormone may influence oligodendrocyte differentiation and maturation via 3,5,3'-triiodo-L-thyronine receptor beta 1, which is expressed only in oligodendrocytes and not in progenitor cells. We therefore show that this effect is indirect and is mediated by 3,5,3'-triiodo-L-thyronine which acts posttranscriptionally on the 3,5,3'-triiodo-L-thyronine receptor beta 1 gene.

Monoclonal antibody 14F7, which recognizes a stage-specific immature oligodendrocyte surface molecule, inhibits oligodendrocyte differentiation mediated in co-culture with astrocytes

Cells at an intermediate stage of oligodendrocyte lineage are not only well characterized by biochemical studies but also are likely to relate to the outcome of physiological events. To elucidate the molecular events leading to the development of oligodendrocyte lineage cells, we have raised monoclonal antibodies against stage-specific immature oligodendrocytes, which have previously been isolated by a novel oligodendrocyte-lineage cell culture technique (Sakurai et al.: J Neurosci Res 52:17-26, 1998). We have isolated a mouse monoclonal antibody termed 14F7 which predominantly labels stage-specific immature oligodendrocytes and have found that the expression of 14F7 immunoreactivity in the developing neonatal rat forebrain is closely associated with cells expressing the oligodendrocyte progenitor marker A2B5 and to immature oligodendrocyte expressing O4 antigen. 14F7+ cells were distributed in the ventricular and subventricular zone and the nearby forming corpus callosum as non-myelinating cells. In contrast to cell culture observations, 14F7+ cells were seen only in oligodendrocyte lineage cells. For instance, dissociated cell culture studies indicated that 14F7 labels a cell surface molecule, and its cellular distribution is coincident with all of O4+ cells and A2B5+ cells, and even A2B5- cells. By contrast, 14F7-positive cells did not label astrocytes and, furthermore, did not label myelin basic protein (MBP)-positive oligodendrocytes. 14F7 recognized a 48-kDa protein on sodium dodecyl sulfate polyacrylamide gel electrophoresis. 14F7 immunoreactivity was detectable in rat brain as early as embryonic day 18. Furthermore, in these cells, the total time for differentiation was extended, and on maturation, these cells subsequently expressed an array of myelin-specific proteins, which normally occurs by direct contact with type-1 astrocytes. However, in the presence of 14F7, stage-specific oligodendrocytes co-cultured with astrocytes completely failed to express MBP. These data suggest that the 14F7 antigen is a novel cell surface molecule that is expressed in the intermediate stage of oligodendrocyte-lineage cells, and it is expected that it regulates the differentiation of oligodendrocyte throughout development.

Maturation-dependent vulnerability of oligodendrocytes to oxidative stress-induced death caused by glutathione depletion

Death of oligodendrocyte (OL) precursors can be triggered in vitro by cystine deprivation, a form of oxidative stress that involves depletion of intracellular glutathione. We report here that OLs demonstrate maturation-dependent differences in survival when subjected to free radical-mediated injury induced by glutathione depletion. Using immunopanning to isolate rat preoligodendrocytes (preOLs), we generated highly enriched populations of preOLs and mature OLs under chemically defined conditions. Cystine deprivation caused a similar decrease in glutathione levels in OLs at both stages. However, preOLs were completely killed by cystine deprivation, whereas mature OLs remained viable. Although the glutathione-depleting agents buthionine sulfoximine and diethylmaleate were more potent in depleting glutathione in mature OLs, both agents were significantly more toxic to preOLs. Glutathione depletion markedly increased intracellular free radical generation in preOLs, but not in mature OLs, as indicated by oxidation of the redox-sensitive probe dihydrorhodamine 123. The antioxidants alpha-tocopherol, idebenone, and glutathione monoethylester prevented the oxidation of dihydrorhodamine in cystine-depleted preOLs and markedly protected against cell death. When the intracellular glutathione level was not manipulated, preOLs were also more vulnerable than mature OLs to exogenous free radical toxicity generated by a xanthine-xanthine oxidase system. Ultrastructural features of free radical-mediated injury in glutathione-depleted preOLs included nuclear condensation, margination of chromatin, and mitochondrial swelling. These observations indicate that preOLs are significantly more sensitive to the toxic effects of glutathione depletion and that oligodendroglial maturation is associated with decreased susceptibility to oxidative stress.

Tenascin-R is antiadhesive for activated microglia that induce downregulation of the protein after peripheral nerve injury: a new role in neuronal protection

Microglial activation in response to pathological stimuli is characterized by increased migratory activity and potential cytotoxic action on injured neurons during later stages of neurodegeneration. The initial molecular changes in the CNS favoring neuronofugal migration of microglia remain, however, largely unknown. We report that the extracellular matrix protein tenascin-R (TN-R) present in the intact CNS is antiadhesive for activated microglia, and its downregulation after facial nerve axotomy may account for the loss of motoneuron protection and subsequent neurodegeneration. Studies on the protein expression in the facial and hypoglossal nucleus in rats demonstrate that TN-R is a constituent of the perineuronal net of motoneurons and 7 d after peripheral nerve injury becomes downregulated in the corresponding motor nucleus. This downregulation is reversible under regenerative (nerve suture) conditions and irreversible under degenerative (nerve resection) conditions. In short-term adhesion assays, the unlesioned side of brainstem cryosections from unilaterally operated animals is nonpermissive for activated microglia, and this nonpermissiveness is almost abolished by a monoclonal antibody to TN-R. Microglia-conditioned media and tumor necrosis factor-alpha downregulate TN-R protein and mRNA synthesis by cultured oligodendrocytes, which are one of the sources for TN-R in the brainstem. Our findings suggest a new role for TN-R in neuronal protection against activated microglia and the participation of the latter in perineuronal net destruction, e.g., downregulation of TN-R.

Human oligodendroglial development: relationship to periventricular leukomalacia

Periventricular leukomalacia in the premature infant is a lesion of cerebral white matter with its greatest period of risk when white matter is immature, that is, when oligodendrocyte precursors are proliferating and differentiating, and before myelin sheaths are actively synthesized. Although the pathogenesis of perinatal cerebral white matter damage involves multiple factors, the correlation of the timing of the lesion with dominance of oligodendrocyte precursors in cerebral white matter suggests that intrinsic factors related to oligodendrocyte precursors are critical. Ischemia and infection have both been implicated as causes of perinatal white matter damage. Major mechanisms underlying oligodendrocyte injury in ischemia include glutamate toxicity, free-radical injury, and cytokine damage mediated by macrophages accompanying ischemia-induced inflammation. Factors related to a vulnerability of immature oligodendrocytes to ischemia potentially include a developmental lack of antioxidant enzymes to mediate oxidative stress. Cytokine-mediated injury to oligodendrocytes is also potentially important. A complete understanding of the role of immature white matter in the pathogenesis of periventricular leukomalacia is essential for developing strategies to prevent it.

Binding of cholera toxin B subunit: a surface marker for murine microglia but not oligodendrocytes or astrocytes

GM1 ganglioside is a receptor for the B subunit of cholera toxin. In lymphocytes, B subunit elicits an influx of extracellular Ca++ (Dixon et al., 1987). To investigate this signaling pathway in glia, we assessed the presence of GM1 ganglioside on the surface of cultured murine central nervous system (CNS) glia by binding of fluorescein-labeled B subunit. B subunit binding was compared to binding of peanut agglutinin, wheat germ agglutinin, and Bandeiraea (Griffonia) simplicifolia lectin (BSL)I, a microglial marker. Antibodies to glial fibrillary acidic protein, A007/O4 antigens, and galactocerebroside were used to identify astrocytes, immature oligodendrocytes (OLs) and mature OLs, respectively. Binding patterns differed based on cell type and developmental stage. Wheat germ and peanut agglutinins bound to the surface of microglia, astrocytes, and immature OLs; neither lectin bound to any significant extent to the surface of membrane sheets of mature OLs, although wheat germ agglutinin was rapidly endocytosed. Cells identified as microglia by BSL I binding and morphology were the only cells to stain brightly on the surface with B subunit. Thus, surface GM1 ganglioside appears to be a highly enriched marker for microglia in these mixed glial cultures. The effects of B subunit on intracellular Ca++ were examined by laser cytometry in glial cultures loaded with Indo-1. No Ca++ responses were observed in microglia. Mature OLs were examined for Ca++ responses to B subunit before and after surface levels of GM1 ganglioside were increased by incubation with exogenous GM1 ganglioside. Again, no Ca++ responses were observed. Thus, cultured microglia and mature OLs do not have the GM1-mediated signal transduction pathway seen in lymphocytes. However, the presence of GM1 ganglioside on microglia may play a role in giving rise to antibodies to this glycolipid in some CNS inflammatory diseases.

Growth and fate of PSA-NCAM+ precursors of the postnatal brain

Oligodendrocyte-type 2 astrocyte (O-2A) lineage cells are derived from multipotential stem cells of the developing CNS. Precursors of O-2A progenitors express the polysialylated (PSA) form of the neural cell adhesion molecule (NCAM) and are detected in neonatal rat brain glial cultures. It is unclear how such PSA-NCAM+ "pre-progenitors" are related to neural stem cells and whether they still have the potential to differentiate along several neural lineages. Here we isolated PSA-NCAM+ pre-progenitor cells from glial cultures by immunopanning and found that most of these cells expressed nestin and PDGF-receptor-alpha but not O-2A antigens. PSA-NCAM+ cells synthesized transcripts for fibroblast growth factor (FGF) receptors 1, 2, and 3 and responded to FGF2 by survival and proliferation, growing into large clusters resembling neural spheres. FGF2-induced proliferation of PSA-NCAM+ pre-progenitors was significantly enhanced by thyroid hormone (T3), which on its own did not increase cell survival or mitosis. After adhesion and withdrawal of the mitogen, spheres generated mostly oligodendrocytes and astrocytes but very rarely neurons. PSA-NCAM immunopanned cells grown in epidermal growth factor (EGF) also adopted a mostly glial fate after differentiation. In contrast, PSA-NCAM-negative cells and striatal neonatal stem cells, grown in EGF or FGF2, generated the three CNS cell types. Like neural stem cells, PSA-negative cells generated more oligodendrocytes and fewer neurons when expanded in FGF2 and T3. Thus emergence of PSA-NCAM at the surface of neonatal brain precursors coincides with their restriction to a glial fate. T3 modulates these events by enhancing PSA-NCAM+ pre-progenitor growth in FGF2 and favoring an oligodendrocyte fate.

rMAL is a glycosphingolipid-associated protein of myelin and apical membranes of epithelial cells in kidney and stomach

rMAL, the rat myelin and lymphocyte protein, is a small hydrophobic protein of 17 kDa with four putative transmembrane domains and is expressed in oligodendrocytes and Schwann cells, the myelinating cells of the nervous system. In addition, transcript expression has been found in kidney, spleen, and intestine. Confocal microscopy and immunoelectron microscopy with an affinity-purified antibody localized rMAL to compact myelin in a pattern similar to the structural myelin proteins: myelin basic protein and proteolipid protein. In kidney and stomach epithelia, rMAL is located almost exclusively on the apical (luminal) membranes of the cells lining distal tubuli in kidney and the glandular part of the stomach. Biochemical analysis of plasma membranes isolated from spinal cord and kidney demonstrated that rMAL is a proteolipid that is present in detergent insoluble complexes typical for proteins associated with glycosphingolipids. Lipid and protein analysis showed a co-enrichment of glycosphingolipids and rMAL protein within these complexes, indicating a close association of rMAL to glycosphingolipids in myelin and in kidney in vivo. We conclude that specific rMAL-glycosphingolipid interactions may lead to the formation and maintenance of stable protein-lipid microdomains in myelin and apical epithelial membranes. They may contribute to specific properties of these highly specialized plasma membranes.

Differential uptake of dextran beads by astrocytes, macrophages and oligodendrocytes in mixed glial-cell cultures from brains of neonatal rats

The present study addresses a controversy over the abilities of astrocytes to perform phagocytosis. Primary glial-cell cultures were prepared from the brains of neonatal rats and were incubated with fluorescently-labeled dextran beads (molecular weights approximately 10 and approximately 40 kDa). Astrocytes and oligodendrocytes were double-labeled by immunofluorescence staining of cell-specific markers, and microglia by lectin histochemistry. Cells were permitted to take up beads for 1 h, fixed, and incubated with primary antibodies, followed by fluorescent secondary antibodies or fluorescently-labeled lectin. Macrophages and astrocytes internalized beads of both sizes. In astrocyte processes the beads appeared to line up along glial filaments. The results, which provide direct evidence for uptake of beads by astrocytes in vitro and against equally rapid, if any, uptake by oligodendrocytes, bear upon issues of acid/base balance and glial cell development and are relevant to neuropathological observations in human disease.