Characterization of a subset of oligodendrocytes separated on the basis of selective adherence properties

Rapid and Specific Immunomagnetic Isolation of Mouse Primary Oligodendrocytes

The efficient and robust isolation and culture of primary oligodendrocytes (OLs) is a valuable tool for the in vitro study of the development of oligodendroglia as well as the biology of demyelinating diseases such as multiple sclerosis and Pelizaeus-Merzbacher-like disease (PMLD). Here, we present a simple and efficient selection method for the immunomagnetic isolation of stage three O4⁺ preoligodendrocytes cells from neonatal mice pups. Since immature OL constitute more than 80% of the rodent-brain white matter at postnatal day 7 (P7) this isolation method not only ensures high cellular yield, but also the specific isolation of OLs already committed to the oligodendroglial lineage, decreasing the possibility of isolating contaminating cells such as astrocytes and other cells from the mouse brain. This method is a modification of the techniques reported previously, and provides oligodendrocyte preparation purity above 80% in about 4 h.

Isolation, Expansion, and Maturation of Oligodendrocyte Lineage Cells Obtained from Rat Neonatal Brain and Optic Nerve

Oligodendrocytes are the myelin-forming cells in the central nervous system (CNS) and their loss or dysfunction is a hallmark of CNS demyelinating diseases, such as multiple sclerosis (MS), hypoxic-ischemic demyelination, or spinal cord injury. In the rodent CNS, oligodendrocyte progenitor cells (OPCs) arise in multiple ventral and dorsal locations of the forebrain during late embryogenesis and early postnatal periods. OPCs migrate out from these germinal zones and disperse throughout the CNS, to populate the developing white and gray matter. There, OPCs can begin to mature through a series of intermediate states characterized by the expression of stage-specific proteins until completely differentiated into postmitotic myelinating oligodendrocytes. Elucidating the cellular and molecular mechanisms that control oligodendrocyte maturation requires isolating OPCs and premyelinating oligodendrocytes by rapid and reliable methods that provide high yield, pure and viable culture, being a powerful tool to characterize their differentiation and potential capacity for myelin repair after injury. This chapter describes in detail two simple and efficient protocols for the preparation of highly enriched rat OPC populations and immature oligodendrocytes derived from mixed glial cultures and optic nerves, respectively. Functional oligodendrocytes obtained with these protocols can be cocultured with primary neurons to study myelination.

CNS myelin sheath is stochastically built by homotypic fusion of myelin membranes within the bounds of an oligodendrocyte process

Myelin - the multilayer membrane that envelops axons - is a facilitator of rapid nerve conduction. Oligodendrocytes form CNS myelin; the prevailing hypothesis being that they do it by extending a process that circumnavigates the axon. It is pertinent to ask how myelin is built because oligodendrocyte plasma membrane and myelin are compositionally different. To this end, we examined oligodendrocyte cultures and embryonic avian optic nerves by electron microscopy, immuno-electron microscopy and three-dimensional electron tomography. The results support three novel concepts. Myelin membranes are synthesized as tubules and packaged into "myelinophore organelles" in the oligodendrocyte perikaryon. Myelin membranes are matured in and transported by myelinophore organelles within an oligodendrocyte process. The myelin sheath is generated by myelin membrane fusion inside an oligodendrocyte process. These findings abrogate the dogma of myelin resulting from a wrapping motion of an oligodendrocyte process and open up new avenues in the quest for understanding myelination in health and disease.

Isolation and long-term expansion of functional, myelinating oligodendrocyte progenitor cells from neonatal rat brain

Oligodendrocytes are the myelinating cells of the central nervous system (CNS). The isolation of purified oligodendrocyte progenitor cells (OPCs) in large numbers has been sought after as a source of cells for repair following CNS-demyelinating diseases and injuries, such as multiple sclerosis (MS) and spinal cord injury (SCI). Methods for isolation of OPCs from rodent neonatal brains are well established and have formed the basis for research in myelin repair within the CNS for many years. However, long-term maintenance of OPCs has been a challenge owing to small cellular yields per animal and spontaneous differentiation within a short period of time. Much effort has been devoted to achieving long-term culture and maintenance of OPCs, but little progress has been made. Here, protocols are presented for preparation of highly enriched rat OPC populations and for their long-term maintenance as oligospheres using mixed-glial-conditioned medium. Functional myelinating oligodendrocytes can be achieved from such protocols, when co-cultured with primary neurons. This approach is an extension of our normal shaking method for isolating OPCs, and incorporates some adaptations from previous OPC culture methods.

Finding degrees of separation: experimental approaches for astroglial and oligodendroglial cell isolation and genetic targeting

The study of CNS glial cell function requires experimental methods to detect, purify, and manipulate each cell population with fidelity and specificity. With the identification and cloning of cell- and stage-specific markers, glial cell analysis techniques have grown beyond physical methods of tissue dissociation and cell culture, and become highly specific with immunoselection of cell cultures in vitro and genetic targeting in vivo. The unique plasticity of glial cells offers the potential for cell replacement therapies in neurological disease that utilize neural cells derived from transplanted neural stem and progenitor cells. In this mini-review, we outline general physical and genetic approaches for macroglial cell generation. We summarize cell culture methods to obtain astrocytes and oligodendrocytes and their precursors, from developing and adult tissue, as well as approaches to obtain human neural progenitor cells through the establishment of stem cells. We discuss popular targeting rodent strains designed for cell-specific detection, selection and manipulation of neuroglial cell progenitors and their committed progeny. Based on shared markers between astrocytes and stem cells, we discuss genetically modified mouse strains with overlapping expression, and highlight SOX-expressing strains available for targeting of stem and progenitor cell populations. We also include recently established mouse strains for detection, and tag-assisted RNA and miRNA analysis. This discussion aims to provide a brief overview of the rapidly expanding collection of experimental approaches and genetic resources for the isolation and targeting of macroglial cells, their sources, progeny and gene products to facilitate our understanding of their properties and potential application in pathology.

Lipid synthesis by oligodendrocytes from adult pig brain maintained in long-term culture

Oligodendrocytes were isolated from adult pig brain and cultivated for 18-24 days. [(14)C]acetate, [(3)H]galactose or [(35)S]sulfate were added to the medium for an additional 24 h. Lipids were extracted and separated by high-performance thin-layer chromatography. The labeled lipids were studied by fluorography and scintillation counting. [(14)C]acetate was incorporated in decreasing order into neutral lipids, phosphatidylcholine, ethanolamine phosphatides, galactocerebrosides, phosphatidylinositol, phosphatidylserine, sulfatides and sphingomyelin. From the [(14)C]acetate incorporated into ethanolamine and choline phosphatides, 71.6 and 14.8%, respectively, were found in plasmalogens. Among neutral lipids, [(14)C]acetate labeled not only cholesterol but also large amounts of triglycerides. No cholesterol esters were synthesized. [(3)H]galactose primarily labeled galactocerebrosides, sulfatides, and monogalactosyl diglyceride. [(35)S]sulfate incorporation was restricted to sulfatides. Together with our previous results concerning proteins, these data show that: (1) oligodendrocytes remain highly differentiated in long-term cultures; (2) they are able to synthesize the major components of myelin; (3) they synthesize surprisingly high amounts of triglycerides and of monogalactosyl diglyceride, a marker for myelination.

Isolation and culture of rat and mouse oligodendrocyte precursor cells

The ability to isolate oligodendroglial precursor cells (OPCs) provides a powerful means to characterize their differentiation, properties and potential for myelin repair. Although much knowledge is available for isolation of OPCs from the rat central nervous system, preparation and maintenance of mouse OPCs has been until recently a challenge owing to difficulties in obtaining a sufficient quantity of purified OPCs. Here, we describe protocols to prepare highly enriched rat OPCs and nearly homogenous mouse OPCs. The mouse method generates predominantly OPCs from cortical neural progenitor cells as clonal aggregates called "oligospheres" by taking advantage of molecular genetic tools. Isolated OPCs can be further differentiated into oligodendrocytes. Collectively, we describe simple and efficient methods for the preparation and in vitro maintenance of enriched OPCs from rats and mice. Isolation and culture of a large, homogenous population of rodent OPCs should significantly facilitate studies on OPC lineage progression and their utility in myelin repair after injury.

Oligodendrocytes express an alpha/beta-interferon-susceptible Mx gene: molecular characterization of the encoded protein

Interferons (INFs) are cytokines that exert a plethora of biological activities by interacting with receptors and inducing the synthesis of effector proteins. One such effector is an antiviral protein called Mx, widely distributed among vertebrates. Mx proteins belong to the superfamily of large GTPases, which includes dynamins, products of the Drosophila shibire gene, the yeast Vps1p, and others. Oligodendrocytes (OLGs) synthesize, assemble, and maintain myelin in the central nervous system. In an effort to identify genes implicated in establishing a myelinogenic phenotype, we isolated a 2,494 bp cDNA that encodes a protein of 654 amino acids, which are 95% identical to ovine Mx (from endometrium of pregnant sheep) and 83% similar to human MxA. Cultured OLGs express the mRNA and protein. We present structural features of this protein and speculate on the origin of its induction and the functional significance of its presence in OLGs. The finding that OLGs express an IFN-susceptible protein raises interesting possibilities in light of the evidence that beta-IFN is therapeutically beneficial for the treatment of multiple sclerosis.

Oligodendrocyte Transmembrane Protein: A Novel Member of the Glutamate-Binding Protein Subfamily

Oligodendrocytes (OLGs) are cells from the central nervous system that synthesize, assemble, and maintain myelin, the multilamellar membrane that surrounds axons and facilitates the fast conduction of nerve impulses. We have shown that OLGs initiate their myelinogenic phenotype upon adhesion to GRASP, a heparin-binding glycoprotein that we purified from horse serum. In an attempt to identify the genes implicated in establishing this phenotype, we isolated a novel 3500 bp cDNA related to, but distinct from, a subfamily of glutamate-binding proteins (GBP). The cDNA encodes a protein of 511 amino acids, whose predicted sequence can be modeled as a tetrahelical integral protein with a large external loop and with the N- and C-termini located inside the cell. We have named this protein oligodendrocyte transmembrane protein (OTMP). Transcription of the message is induced upon OLG acquiring a myelinogenic phenotype (i.e., upon adhesion). The temporal expression in conjunction with the structural and biochemical features of OTMP is suggestive of a signaling receptor with a role in myelinogenesis.

Differentiation/regeneration of oligodendrocytes entails the assembly of a cell-associated matrix

Oligodendrocytes assemble and maintain CNS myelin. We have shown that adhesion of ovine oligodendrocytes to the substratum, GRASP - a novel, horse serum heparin-binding glycoprotein - initiates their myelinogenic phenotype. Synthesis and vectorial transport to the plasma membrane of heparan sulfate proteoglycans is one of the many events that ensue upon adhesion. Proteoglycans play key roles in defining the line of communication between cells and their microenvironment. The nature of their association with cells varies. Often, proteoglycans are part of a complex extracellular network that either surrounds cells or is restricted to smaller areas of their surface. Such extracellular matrices form an integral part of the machinery that regulates cell function. As part of an effort to delineate the events and identify the molecules involved in the adhesion-induced-regeneration and possibly in differentiation of OLGs, we have undertaken to define the full repertoire of OLG proteoglycans. Oligodendrocytes express surface-associated proteoglycans and also secrete them to the medium. However, we observed a clear distinction between secreted and surface-associated proteoglycans in terms of types, temporal regulation and spacial distribution. Oligodendrocytes secrete chondroitin sulfate proteoglycans and keratan sulfate proteoglycans but have only heparan sulfate proteoglycans associated with their surface. Secreted proteoglycans are temporally modulated but adhesion-independent, whereas surface-associated proteoglycans are adhesion-induced. Herein, we present the biochemical characterization of oligodendrocyte proteoglycans. We report that a significant fraction of the surface-associated heparan sulfate proteoglycans are assembled into a cell-associated matrix. This finding is important. First, it reveals a closer parallel than hitherto documented with events that signal Schwann cell myelination. Second, it implicates HSPGs in the establishment of OLG differentiated phenotype. Third, it brings OLGs in tune with other cell types where the ECM (broadly defined) is critical for the orchestration of cues that generate tissue-specific gene expression and phenotypes.

GRASP: a novel heparin-binding serum glycoprotein that mediates oligodendrocyte-substratum adhesion

Cell-substratum adhesion plays a crucial part in the cascade of events that control growth or turn on and consummate a differentiation program. We are investigating the molecular basis of oligodendrocyte (OLG) cytodifferentiation, employing pure cultures of OLGs isolated from postmyelination brains. We have shown that such OLGs will regenerate in vitro and reenact the ontogenic development of myelin, but to do so they need a signal. Adherence to a polylysine surface in the presence of 20% horse serum generates such a signal. Among the events that are turned on upon OLG adhesion is the phosphorylation of myelin basic protein; no such phosphorylation takes place in the non-adhered cell. We postulated that horse serum provides an adhesion molecule. Laminin, fibronectin, collagen and native vitronectin failed to replace horse serum. Hence, we set out to fractionate horse serum by screening with an adhesion assay. We report here the identification, purification and partial characterization of a novel, heparin-binding horse serum glycoprotein that we have termed Glycine-Rich Adhesion Serum Protein--GRASP--to stress the fact that this protein has a high content of glycine and functions, in vitro, as an adhesion molecule for OLGs. There is 61% similarity at the N-terminus between GRASP and histidine-rich glycoprotein precursor (HRGP), an alpha 2-glycoprotein from human plasma. However, our data suggest that GRASP is not the horse serum homolog of HRGP. First, the two Gps are functionally distinct: HRGP does not promote the adhesion of OLGs. Second, the amino acid compositions differ significantly, e.g., GRASP is not histidine- but rather glycine-rich. Third, the region of sequence similarity between GRASP and HRGP is conserved throughout the cystatin superfamily. Fourth, anti-Gp55 polyclonal Abs recognize a similar set of polypeptides--save for slight differences in M(r)-in human serum as in horse serum, indicating that HRGP and GRASP are two distinct but related proteins and are both present in human and horse sera. GRASP is a dimer trimer of seemingly identical subunits of M(r) approximately 55,000 ; the native protein has an M(r) x 10(-3) approximately 120-140, of which 24-27% is contributed by carbohydrate. Using GRASP as a substratum allows the growth of OLGs in serum-free medium. GRASP is as good an effector of myelin basic protein phosphorylation as 20% horse serum. We conjecture that the mechanism of GRASP function features: 1) exposure of a cryptic sequence--after a change in conformation induced upon binding to polylysine--with affinity for an OLG signal-transducing receptor; and 2) interaction of its heparin-binding domain with OLG surface heparin sulfate proteoglycans and/or the aforementioned receptor.

Depolarizing agents and tumor necrosis factor-alpha modulate protein phosphorylation in oligodendrocytes

Membrane depolarization and changes in ionic fluxes have been implicated in the signaling mechanisms between neurons and glial cells. We report here that K(+)-induced depolarization of cultured ovine oligodendrocytes (OLGs) decreases the phosphorylation of myelin basic protein (MBP) and 2'3'-cyclic nucleotide phosphohydrolase (CNPase). Membrane depolarization and decrease in phosphorylation of MBP and CNPase can also be elicited by inhibition of the inward rectifier with Ba2+ but not by inhibition of outward K+ channels with 4-aminopyridine or tetraethylammonium. These findings demonstrate that modulation of K+ currents can influence phosphorylation states of OLG proteins. Tumor necrosis factor-alpha (TNF-alpha), an immune peptide implicated in autoimmune demyelinating diseases, also inhibits the phosphorylation of these proteins. In contrast to elevated [K+]o, TNF-alpha does not decrease the stimulatory effect of protein kinase C activators or phosphatase inhibitors on MBP and CNPase phosphorylation, suggesting that depolarizing agents and TNF-alpha act via distinct mechanisms. We postulate that the presence of elevated extracellular K+ and/or cytokines under certain pathological conditions can perturb OLG function by altering the phosphorylation states of their proteins and perhaps affect myelin maintenance, contributing to demyelination.

Myelination by mature ovine oligodendrocytes in vivo and in vitro: evidence that different steps in the myelination process are independently controlled

The ability of isolated mature post-myelination ovine oligodendrocytes to myelinate was investigated in tissue culture and in vivo. In culture, although the cells adhered preferentially to rat dorsal root ganglia (DRG) axons, sent out processes that encircled and wrapped them, proliferated, and synthesised myelin proteins (MBP), no myelination was found. This failure to find myelination occurred despite the fact that the oligodendrocytes both in the present experiments and in previous studies elaborated membranous structures that have been shown chemically and structurally to be similar to normal central nervous system myelin. These findings contrasted with those seen when neonatal rodent glial cells were added to similar DRG neuron cultures, in which myelination readily occurred. When the same adult ovine oligodendrocytes were transplanted into the brains of Shiverer mice, normal compact myelin was formed, proving that the cells were capable of myelination and suggesting that cross-species incompatibility was probably not a major factor in the lack of myelination in vitro. It is possible that the failure of ovine oligodendrocytes to myelinate DRG axons is due either to the relatively low number of supporting glial cells, such as astrocytes or microglia which may be necessary for satisfactory myelination, or that some other factor in the microenvironment is lacking; in any event, these results point to the complexity of oligodendrocyte-axon interactions. It is clear that each of the events, from adherence to proliferation to wrapping and the myelin compaction may be under the control of a different signal and may operate through a distinct mechanism, even though each process is dependent on the other. The results also point to the potential usefulness of this model system for deciphering such signals and mechanisms.

Oligodendrocyte proteoglycans: modulation by cell-substratum adhesion

The signals that trigger the cytodifferentiation of oligodendrocytes (OLGs) are largely unknown. Using as a model system cultures of pure OLGs, we have shown that adhesion to a substratum initiates myelinogenesis (Yim SH, Szuchet S, Polak PE, J Biol Chem 261:11808-11815, 1986). It was of interest to investigate whether components such as proteoglycans (PGs) play any role in the biology of OLGs as it pertains to myelinogenesis. We set out to determine first, whether OLGs carry PGs; second, the nature of the association of these components with OLG plasma membrane; and third, if and how these PGs are modulated by OLG-substratum interaction. We compared the expression and characteristics of PGs extracted with different solvents from nonattached (B3.f) and attached (B3.fA) OLGs. B3.f and B3.fA OLG cultures were labeled with carrier-free 35SO4(2-) in serum-free medium. After removing excess label, OLGs were treated with heparin to extract susceptible components. Pellets were then exposed to 1% Triton X-100 plus 0.1 M NaCl and subsequently to 4 M guanidine-HCl plus 0.5 M NaCl. Solutions containing extracted material were characterized by size-exclusion chromatography, SDS-PAGE, and enzymatic degradation. Herein we report that (1) OLGs display [35S]PGs on their surface within 24 hr of substratum adhesion, and (2) these PGs can be operationally classified as peripheral and integral. We further show that the peripheral PGs are of high and intermediate size as assessed by size-exclusion chromatography and are segregated within the plasma membrane in such a way that the species with intermediate mass are extracted while OLGs remain adhered, whereas the high-molecular-weight species are only extracted after OLGs have been detached. Heparin also dislodges a number of sulfated proteins/Gps. Only a single class--high molecular weight--of integral PGs was identified; this PG requires guanidine-HCl for extraction. All PGs belong to the heparan sulfate class as evidenced by their degradation with heparitinase and their lack of susceptibility to chondroitinase ABC. The common theme of our findings is that these macromolecules have basal levels of expression in the nonadhered OLGs but undergo an adhesion-induced enhancement in their syntheses. We postulate that these PGs (1) play a role in OLG-substratum adhesion and hence myelinogenesis, and (2) may be determinants in establishing OLG polarity. Such polarization is the first overt sign of OLG functional differentiation and occurs prior to any morphological differentiation, e.g., extension of processes does not occur until 48 hr later when the plasma membrane is already polarized.

Biosynthesis and expression of the myelin-associated glycoprotein in cultured oligodendrocytes from adult bovine brain

The biosynthesis and expression of myelin-associated glycoprotein (MAG) were investigated in cultured oligodendrocytes isolated from adult bovine brain. Western blotting revealed two prominent MAG bands that were present in comparable amounts; the larger component electrophoresed above the 97 kD standard but was slightly smaller than the MAG band in purified bovine myelin, and the smaller component electrophoresed below the 97 kD standard. In comparison to other precursors of oligosaccharides, inorganic [35S]sulfate was a relatively specific isotope for labeling MAG relative to other glycoproteins in the cells. Sulfate labeled only the larger of the two MAG components, which contains complex N-linked oligosaccharides, but which appears to be glycosylated to a lesser extent than MAG in vivo. The smaller MAG band in the cells is a form with high-mannose oligosaccharides and was not detected in purified bovine myelin. Both the large and small MAG components were expressed on the oligodendrocyte surface as indicated by their sensitivity to neuraminidase and/or trypsin treatment of live cells. MAG was also released by the oligodendrocytes into the culture medium. The MAG in the medium was slightly smaller than that in the cells, suggesting that it may be released from the cell surface by limited proteolysis. The release of MAG by myelin-forming cells could be relevant to physiological roles that have been postulated for soluble forms of MAG and other adhesion proteins.

Oligodendrocyte-substratum adhesion activates the synthesis of specific lipid species involved in cell signaling

Ovine oligodendrocytes (OLGs) undergo biochemical and morphological changes following attachment to polylysine. Autoradiographs of two-dimensional thin-layer chromatograms of [14C]Gal-labeled OLG cultures revealed that attachment of OLGs to a polylysine substratum and their subsequent morphological differentiation is accompanied by an increased synthesis of multiple forms of galactosylceramide, sulfogalactosylceramide, and both sulfogalactosyl- and galactosyl-diglycerides, together with an array of complex sialoglycosphingolipids, predominantly GM2 ganglioside. As previously reported, overall lipid synthesis measured by [14C]acetate incorporation into glycerophosphatides, sphingomyelin, and neutral lipids also increased dramatically for up to 60 days (last time point examined) following OLG-substratum adhesion, reflecting membrane growth. Attachment was associated with a rapid augmentation in the synthesis of ethanolamine plasmalogen from 12 to 27% within 24 hr to reach a 35% plateau at 30 days and remain constant thereafter. In contrast, the plasmalogen content of phosphatidylcholine remained constant at 3-5%. This rapid increase in lipid synthesis (especially in the ethanolamine plasmalogen content following attachment) closely paralleled increased diacylglycerol (DAG) production and protein kinase C-dependent phosphorylation of both myelin basic protein and 2',3'-cyclic nucleotide phosphohydrolase. Labeling studies indicated that the major source of [3H]arachidonate-labeled DAG following attachment was from phosphatidylinositol turnover (and to a lesser extent phosphatidylcholine) rather than polyphosphoinositides or plasmalogens. Enhanced lipid synthesis is not only required for the production of membranes in these myelin-producing cells but is also a source of second messengers required in the posttranslational modification of key myelin and cellular proteins.

Exogenous GM3 ganglioside stimulates process formation and glycoprotein release by cultured bovine oligodendrocytes

Isolated adult bovine oligodendrocytes maintained in vitro for 10 days were treated for 1 day with 50 micrograms/ml of GM3 ganglioside (NeuNac alpha 2-3Gal beta 1-4Glc beta 1-1'ceramide) in serum-free culture medium. The treated oligodendrocytes had significantly longer processes with more branching than control cells in the same medium without GM3. The treatment also stimulated the release of a series of 22-100-kDa, [3H]glucosamine-labeled glycoproteins into the culture medium. Treatment of oligodendrocytes maintained in vitro for 50 days with GM3 for 1 day resulted in a thickening of the processes and the appearance of many fine branches on existing processes as well as a similar stimulation of glycoprotein release into the medium.

Tumor necrosis factor inhibits K+ current expression in cultured oligodendrocytes

The effects of tumor necrosis factor-alpha (TNF-alpha), a cytokine secreted by activated macrophages, on the electrical membrane properties of cultured adult ovine oligodendrocytes (OLGs) were investigated using the whole-cell voltage-clamp technique. Treatment with recombinant human TNF-alpha (rhTNF) for 24 to 72 hr produces (i) process retraction in some but not all OLGs, (ii) a reduction in the resting membrane potential with no significant change in membrane capacitance or input resistance over control cells and (iii) a decrease in the expression of both the inwardly rectifying and outward K+ current. The magnitude of the membrane potential change as well as K+ current inhibition was larger in cells with retracted processes. The electrophysiological effects of rhTNF were attenuated when rhTNF was neutralized with a polyclonal anti-rhTNF antibody. The binding of rhTNF to its receptor has been reported to increase GTP binding, to increase GTPase activity of a pertussis-sensitive G protein, and to produce an elevation in intracellular cAMP in other cell types. However, pretreatment of OLGs with activated pertussis toxin failed to attenuate or mimic the effects of rhTNF. Chronic exposure of OLGs to the membrane permeant analogue of cAMP, 8-bromo-cAMP, resulted primarily in an inhibition of the inwardly rectifying K+ current, an effect which was less than that produced by rhTNF alone and without any of the associated rhTNF-induced morphological changes. This indicates that the effects of rhTNF cannot be entirely accounted for by an elevation in intracellular cAMP. Cycloheximide (CHX), an inhibitor of protein synthesis, mimicked the effects of rhTNF; however, the effects of rhTNF and CHX were not additive. The finding that both ionic current expression and membrane potential were reduced in cells treated with rhTNF that appeared morphologically normal suggests that abnormal ion channel expression in OLGs precedes and may contribute to eventual myelin swelling and damage.

Isolation, culture, and characterization of adult rat oligodendrocytes

Investigation into CNS demyelinating diseases, which usually occur in adults, can be facilitated by the use of a good in vitro model. We have established a methodology whereby oligodendrocytes from adult rat CNS can be cultured in vitro, and we have characterized these cultures morphologically and immunologically. Approximately 1 g of spinal cord and brainstem per adult rat was removed and dissociated mechanically and enzymatically. After filtration of the white matter homogenate, myelin was removed by 0.9 M sucrose density centrifugation. The cells were further purified by centrifugation through a 0.3%/4% discontinuous gradient of bovine serum albumin (BSA). The pellet was resuspended and placed in an untreated 6-well culture dish overnight to allow the astrocytes to attach. The non-adherent cells were replated on poly-l-lysine-treated coverslips. Approximately 8.25 x 10(5) cells were recovered per animal. The adult oligodendrocytes initially appeared as rounded cell bodies, but after 2-5 days in vitro (DIV), the oligodendrocytes extended 6-10 thick processes. A membrane sheath between these processes was immunostainable with either anti-galactocerebroside (GC), anti-O4, anti-myelin basic protein (MBP), or anti-2'3' cyclic nucleotide 3' phosphohydrolase (CNPase) and was also evident in scanning EM. Older cultures (up to 60 DIV) maintained whorls of myelin and transmission EM revealed a major dense line distance of approximately 103 A with up to 11 concentric layers of membrane. Immunologically, the adult oligodendrocytes are GC+, O4+, MBP+, CNPase+, and GFAP-. The method described will allow adult rat oligodendrocytes to be isolated and maintained in culture; these cultures retain the characteristics of differentiated adult oligodendrocytes.

Channel-mediated and carrier-mediated uptake of K+ into cultured ovine oligodendrocytes

Uptake of radioactive K+ by mature ovine oligodendrocytes (OLGs) maintained in primary culture was measured under steady-state conditions, i.e., in cells maintained in a normal tissue culture medium (5.4 mM K+), and in cells after depletion of intracellular K+ to less than 15% of its normal value by pre-incubation in K(+)-free medium. The latter value is dominated by an active, carrier-mediated uptake (although it may include some diffusional uptake), whereas the former, in addition to active uptake, also reflects passive K+ diffusion through ion selective channels and possible self-exchange between extracellular and intracellular K+, which may be carrier-mediated. The total uptake rate was 144 +/- 10 nmol/min/mg protein, and the uptake after K+ depletion was 60 +/- 2 nmol/min/mg protein, much lower rates than previously observed in astrocytes. The uptake into K(+)-depleted cells was inhibited by about 80% in the presence of ouabain (1 mM) and about 30% in the presence of furosemide (2 mM). Activators of protein kinase C (phorbol esters) and cAMP-dependent protein kinase (forskolin) have been shown to alter the myelinogenic metabolism as well as outward K+ current in cultured OLGs. The present study demonstrates that K+ homeostasis in OLGs is modulated through similar second messenger pathways. Active uptake was inhibited by about 60% in the presence of active phorbol esters (100 nM) but was not affected by forskolin (100 nM). Forskolin likewise had no effect on total uptake, whereas phorbol esters caused a much larger inhibition than expected from their effect on carrier-mediated uptake alone, suggesting that channel-mediated uptake was also reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of myelin basic protein in intact oligodendrocytes: inhibition by galactosylsphingosine and cyclic AMP

We have previously shown that cyclic AMP (cAMP) inhibits the protein kinase C (PKC)-mediated phosphorylation of myelin basic protein (MBP) in cultured oligodendrocytes (OLGs). Recently, it has been demonstrated that the long chain base sphingosine inhibits PKC by competing PKC effectors (diacylglycerol and phorbol esters) for a binding site on the kinase (Hannun and Bell: Science 235: 670-674, 1987). In this report we define further the mechanism by which cAMP inhibits MBP phosphorylation by comparing the effects of cAMP with that of galactosylsphingosine (psychosine), a potential catabolite of galactocerebroside, the major OLG glycosphingolipid. We identify the consequences of psychosine treatment and PKC down-regulation on OLG morphology and electrophysiology and discuss their relevance. Our results in intact ovine oligodendrocytes are consistent with a mechanism in which cAMP inhibits MBP phosphorylation by interfering with the release of diacylglycerol (DAG) from phosphatidylinositol. First, the effects of cAMP on MBP phosphorylation are reversed with exogenous TPA; and second, cAMP inhibits the incorporation of 1-[14C]arachidonate into DAG and specifically inhibits the turnover (as judged by 32PO4 3-incorporation) of phosphatidylinositol. Psychosine inhibits MBP phosphorylation, and its action can be reversed by TPA suggesting a mechanism of inhibition similar to that described for other systems. In addition, psychosine has profound effects on OLG morphology; it disintegrates OLG processes while leaving the cell soma intact. Stable hyperpolarized resting potentials were obtained following psychosine treatment, but there was a 66% decrease in membrane capacitance indicating a significant decrement in membrane surface area. The morphological changes induced by psychosine are reversible and can be eliminated by removing the drug but not by the addition of TPA. Whether inhibition of PKC by psychosine plays any role in process dissolution remains an unanswered question. However, current evidence suggests that a PKC-independent mechanism may be at play. This investigation in conjunction with our previous work emphasizes a role for the interregulation of protein kinase A (PKA) and PKC in the control of OLG somal vs. myelin components. This may have significant implications for central nervous system myelin assembly.

Forskolin and phorbol esters decrease the same K+ conductance in cultured oligodendrocytes

Cultured ovine oligodendrocytes (OLGs) express a number of voltage-dependent potassium currents after they attach to a substratum and as they begin to develop processes. At 24-48 hours following plating, an outward potassium current can be identified that represents a composite response of a rapidly inactivating component and a steady-state or noninactivating component. After 4-7 days in culture, OLGs also develop an inward rectifier current. We studied the effects of forskolin and phorbol 12-myristate 13-acetate (PMA) on OLG outward currents. These compounds are known to alter the myelinogenic metabolism of OLGs. PMA, an activator of protein kinase C (PK-C), has been shown to enhance myelin basic protein phosphorylation while forskolin acting on adenylate cyclase, and thereby increasing cAMP, inhibits it. Both forskolin and PMA increase the phosphorylation of 2'3'-cyclic nucleotide phosphodiesterase, an OLG/myelin protein. We found that forskolin decreased the steady-state outward current at 120 mV by 10% at 100 nM, and by 72% at 25 microM from a holding potential of -80 mV. The time course of inactivation of the peak currents was decreased, affecting both the fast and slow time constants. There was no significant change in the steady-state parameters of current activation and inactivation. The effect of forskolin was attenuated when the adenylate cyclase inhibitor adenosine (2 mM) was present in the intracellular/pipette filling solution. The results of PMA experiments were similar to those obtained with forskolin. Whereas the amplitude of the currents in the presence of PMA was reduced by 28% at 1.5 nM and 60% and 600 nM, the decay phase of the peak currents was less affected. The PMA effect could still be seen when the intracellular Ca2+ was reduced to less than or equal to 10 nM with 5 mM BAPTA, but was inhibited when the cells were pre-exposed to 50 microM psychosine, a PK-C inhibitor. It is postulated that the potassium currents in OLG can be physiologically modulated by two distinct second-messenger systems, perhaps converging at the level of a common phosphorylated enzyme or regulatory protein.

Developmental expression of glial-specific mRNAs in primary cultures of rat brain visualized by in situ hybridization

The localization of mRNAs which encode the glial-specific marker proteins, glial fibrillary acidic protein (GFAP), glycerol phosphate dehydrogenase (GPDH, EC 1.1.1.8), and myelin basic protein (MBP), was mapped by in situ hybridization in primary cultures of 1-2-day-old rat brain in serum-supplemented medium. Developmental changes of these expressed mRNAs were examined after various times in culture ranging from 8 to 50 days and were correlated with the histological, morphological, and positional characteristics of the cells. By day 8, the culture stratified into a population of flat polygonal astrocytes covered by another population of phase-dark process-bearing cells. When counterstained with May-Grunwald histological stain, astrocytes appeared pale blue, whereas two subpopulations of phase-dark cells stained differentially; one was dark blue while the other was red and smaller. GFAP-specific sequences were abundant at day 8, increased in the astrocyte bedlayer as the culture became confluent, and plateaued at approximately day 16. A minor proportion of blue phase-dark cells contained GFAP mRNA although at a lower abundance. In contrast, GPDH mRNA positive blue phase-dark cells were seen scattered throughout the upper layer of the culture and also around the perimeter of large clumps of red phase-dark cells. These cells were infrequent at day 8 but increased in number at later time points. The expression of MBP mRNA differed from GPDH in that it was more abundant at early time points, plateaued between day 20 and day 24, and was predominantly localized in red phase-dark cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

Phorbol ester enhances morphological differentiation of oligodendrocytes in culture

The effects of phorbol esters have been attributed to the activation of the enzyme protein kinase C. While much has been described for the actions of phorbol esters on neurons and synaptic transmission, sparse data exist on the effects of phorbol esters on oligodendrocytes, the cells that make and maintain myelin in the central nervous system. In this report, we show that 10 and 100 nM of a phorbol ester, 4 beta-phorbol-12,13-dibutyrate, extensively enhanced process formation by cultured bovine oligodendrocytes. This effect was blocked by two inhibitors of protein kinase C, sodium heparin and polymixin B. We propose the hypothesis that activation of protein kinase C is an important process that leads to the differentiation of oligodendrocytes and the formation of myelin in vivo.

Oligodendrocyte substratum adhesion modulates expression of adenylate cyclase-linked receptors

The molecular mechanisms of myelin formation/reformation in the central nervous system are unknown. In previous work we have demonstrated that mature oligodendrocytes (OLG) respond to a signal(s), elicited by their adhesion to a substratum, by turning on a myelinogenic metabolism. Events occurring within 24 hr of adhesion include generation of diacylglycerol, activation of protein kinase C, phosphorylation of myelin basic protein, and enhanced synthesis of myelin lipids and proteins. To elucidate the mechanism(s) of signal transduction, we have investigated whether OLG-substratum interaction influences the level of basal cAMP and the expression of receptors coupled to adenylate cyclase. By using ovine brain OLG we have found that adhesion to a polylysine-coated surface for 24 hr increased the basal level of cAMP 2-fold and altered the expression (assessed by cAMP production) of receptors coupled to adenylate cyclase. Isoproterenol (beta-adrenergic agonist) augmented cAMP from 4 to 26 pmol/mg of protein in adhering OLG but had no such effect in nonattached OLG. Adhesion of OLG was accompanied by rapid synthesis of ethanolamine plasmalogen, a class of lipids believed to be associated with beta-adrenergic receptors. Nonattached OLG responded to prostaglandin E1 with only a 3-fold stimulation in their cAMP content; in attached OLG, 6-fold stimulation was observed. In contrast, vasoactive intestinal polypeptide elicited a 3-fold increase in cAMP in nonattached OLG but, following 24 hr of attachment, OLG did not respond to vasoactive intestinal polypeptide. The increase of cellular cAMP levels was accompanied by a 2.5-fold gain in protein kinase A. OLG-substratum adhesion resulted also in phosphorylation of the OLG/myelin protein, 2',3'-cyclic nucleotide 2'-phosphodiesterase, which proved to be a substrate for cAMP and phospholipid-, Ca2+-dependent protein kinases. These findings, in conjunction with our earlier work, implicate cAMP and diacylglycerol in signaling myelinogenesis; they suggest that phosphorylation/dephosphorylation of myelin basic protein and 2',3'-cyclic nucleotide 2'-phosphodiesterase may be key processes in the cascade of events that are initiated by adhesion of OLG to a polylysine surface (possibly acting as a surrogate for axons) and culminate in the reformation of myelin.

Oligodendrocytes with aberrant cytoplasmic processes in a human white matter disorder

Unusual ultrastructural features of oligodendrocytes with numerous aberrant cytoplasmic processes were described in the brain biopsy from a degenerative disease of the white matter. The perikaryal regions of these oligodendrocytes contained well-developed normal cytoplasmic organelles and randomly scattered microtubules. The cytoplasmic processes were tightly packed and numerous junctional complexes were observed between the plasma membranes of these processes. Aberrant myelination by focal compaction of plasma membranes without any association of axons was also observed. These features closely resembled those of cultured isolated oligodendrocytes and were interpreted as reactive changes of oligodendrocytes to the long-standing deafferentation from axons since no axons were detected in the vicinity of these oligodendrocytes.

Plasma membrane of cultured oligodendrocytes: II. Possible structural and functional domains

An oligodendrocyte plasma membrane-rich fraction, F2.2, was resolved by equilibrium density centrifugation on a linear sucrose gradient from 0.5 M to 1.3 M into three fractions, F2.2a, F.2.2b, F2.2c, and a pellet F2.2p. F2.2a and F.2.2b were enriched 1.5-fold relative to F2.2 in plasma membrane markers at the expense of F2.2c and F2.2p, which became correspondingly impoverished. This gave F2.2a and F2.2b a 42-fold and 37-fold enrichment, respectively, in plasma membrane markers relative to the initial cell homogenate. F2.2c had a sevenfold enrichment in a Golgi marker; together with F2.2p, they contained all the Golgi marker initially present in F2.2. Preliminary data indicated that the F2.2-subfractions differed from one another in their molar ratios of cholesterol to phospholipids and protein to lipids but had similar protein profiles when examined by sodium dodecylsulfate-polyacrylamide gel electrophoresis. Their content of fucosylated glycoproteins appeared also to be different. Morphologically, F2.2a and F2.2b were very similar: they contained large membrane vesicles, membrane sheets, and vesicles entrapped within other vesicles. Membrane-membrane interaction was apparent in these fractions. F2.2c had many of the same elements, but most of the membrane structures contained amorphous material. F2.2p differed morphologically from the other fractions in that it had principally electron-dense structures. It is postulated that F2.2a, F2.2b, and perhaps F2.2c represent different domains of oligodendrocyte plasma membrane. Alternatively, these fractions might correspond to the plasma membrane of oligodendrocyte subtypes.

Plasma membrane of cultured oligodendrocytes: I. Isolation, purification, and initial characterization

Oligodendrocytes generate myelin as extensions of the plasma membrane. Myelin has been well characterized, yet little is known concerning oligodendrocyte plasma membrane. We have developed a reproducible method for the isolation of an oligodendrocyte plasma membrane-rich fraction (F2.2). F2.2 has a 25-fold enrichment in K+-dependent p-nitrophenyl phosphatase, a plasma membrane marker. Impurities are composed of Golgi elements (8-12%), microsomes (4-6%), and lysosomal membranes (1-5%). Our starting material was oligodendrocytes kept in culture in a nonattached state for 3 to 5 d. After disrupting the cells and removing nuclei (P1), the supernatant (SP1) was fractionated on a self-generating 20% Percoll gradient into three bands: F1, F2, and F3. F1 had only 3% of the applied protein and was not characterized. F2, with 11% of the protein, was fivefold enriched in plasma membrane. F3 had 27% of initial protein; it consisted of a crude mitochondrial and lysosomal fraction. F2 was further purified by first washing it hypotonically, treating it with Mg2+, and then fractionating it on a Ficoll step gradient that yielded F2.2 at the interphase. Morphologically F2.2 comprises 1) membranous sheets, often with more than one membrane in close apposition; 2) membrane vesicles of various sizes and shapes frequently filled with amorphous material; 3) Golgi elements; and 4) unrecognizable profiles. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis protein profile of F2.2 reveals CNPase as a major component in agreement with the high CNPase specific activity (3,860 mumol/mgP/h) found in F2.2. Other significant polypeptides have Mr = 170,000, 135,000, 108,000, 80,000, 53,000, 38,500, 32,000, and 22,200.

Regulation of GM2 ganglioside metabolism in cultured cells

GM2-ganglioside (II3NeuAcGgOse3Cer) is a minor component of adult nervous tissue, but is probably an oncofetal antigen. Its biological role is unknown, but several lines of evidence indicate its potential role in cell adhesion both in the retina and in oligodendrocytes. The biosynthesis of GM2-ganglioside appears to be tightly regulated, since it is a key intermediate in complex ganglioside synthesis. The specific GM3: hexosaminyl-transferase is activated under conditions which activate cyclic AMP-dependent protein kinase, and cell transformation with retroviruses inactivates it. Catabolism of GM2 requires the concerted action of three gene products (alpha-chain, beta-chain and activator protein in a thermolabile alpha beta 2 AP complex referred to as HexA). Defects in either three components results in the neuronal storage of GM2 ganglioside and the manifestations of Tay-Sachs Disease in children or motor neuron disease in adults.

Oligodendrocyte adhesion activates protein kinase C-mediated phosphorylation of myelin basic protein

When isolated adult oligodendrocytes adhere to a substratum myelinogenesis occurs. Investigation of the mechanism by which this happens indicated that the oligodendrocyte-substratum interaction activated protein kinase C-dependent phosphorylation of myelin basic protein and promoted the synthesis of myelin basic protein. In addition, when agents that activate protein kinase C (second messenger diacylglycerol or a tumor-promoting phorbol ester) were added to nonattached oligodendrocytes, they mimicked the influence of the substratum by inducing phosphorylation of myelin basic protein; and reagents that increase cellular adenosine 3', 5'-monophosphate (cyclic AMP) inhibited phosphorylation of myelin basic protein. Thus, at least in vitro, the interaction between oligodendrocytes and the substratum may mediate myelinogenic events, and phosphorylation of myelin basic protein may be an early requirement in the sequence of steps that ultimately results in myelin formation.

The generation and regeneration of oligodendroglia. A short review

During postnatal development of the higher vertebrate CNS, large populations of oligodendroglia are generated from precursor cells in a very dependable way. In adult lesioned CNS tissues, local populations of oligodendroglia are replenished by proliferation of this replenishment varies from one species to another and also from one lesion type another. Studies on the developmental generation of oligodendroglia are reviewed here, delineating what is known of the early relationships between the CNS glial lineages and of what regulates this development. Contributions from recent cell biological work are considered against the background of morphological and radioautographic results. The quiescent condition of extremely slow turnover in the normal adult CNS is noted, and the dramatic effects of lesions on the neural cell environment are considered. Lesions can trigger proliferation at a much greater rate in the mature oligodendroglial population, as observed both in situ and in tissue culture; in addition to persisting stem cells, the mature cells participate in replenishing the local oligodendroglial population. This regeneration from cells already committed to the oligodendroglial lineage may minimise such disturbing effects of the lesion environment as might distort replenishment of the population from precursor cells.

Lipid and glycolipid metabolism of cultured astrocytes: a time course study

Primary cultures of astrocytes free of neurons and containing less than 1% of oligodendrocytes were examined for their ability to incorporate labeled precursors into lipids and glycolipids. At selected developmental stages cultures were double-labeled with either [3H]glycerol and [14C]acetate or with [3H]galactose and Na2[35SO4] for a total of 72 hr. Lipids were extracted with CHCl3/CH3OH, fractionated on a silicic acid column, and further resolved by two-dimensional thin-layer chromatography. It was found that cultured astrocytes actively incorporate acetate and glycerol into various phospholipids; they have very limited ability to utilize galactose and virtually lack the synthetic machinery to use Na2SO4 for the synthesis of sulfated sphingogalactolipids; and their overall lipid metabolism is very distinct from that of oligodendrocytes. It was also found that cultured astrocytes have low levels of 2',3' cyclic phosphodiesterase and glycerol phosphate dehydrogenase activities; the latter is less than one fifth of that in oligodendrocytes.

A procedure for long-term culture of oligodendrocytes

A procedure for long-term culture of oligodendrocytes is described, the starting material being 20-day-old primary mixed cultures of newborn rat brain. Cells were first incubated in a serum-free medium for 48 h before they were subcultured on poly-L-lysine coated plastic dishes. After this treatment, the oligodendrocytes developed well in Waymouth medium containing 10% (v/v) calf serum, while most of the astrocytes died. At 13 days in subculture more than 90% of the cells were identified as oligodendrocytes; the criteria for oligodendrocytes were based on their immunoreactivity to antisera against W1 Wolfgram protein, myelin basic proteins and the synthetic C-terminal hexapeptide of the major myelin proteolipid. At 13 and 19 days astrocytes were present, 7% and 20% respectively. The culture system described here may be useful to study the biochemical and immunological aspects of the oligodendrocytes.

Cell and tissue culture of the central nervous system: recent developments and current applications

A survey of methods for cell and tissue culture of the central nervous system (CNS) is given. This includes a brief historical outline and description of methods in current use. Recent methodological improvements are emphasized, and it is shown how these are applied in modern neurobiological research. Both monolayer cell cultures and three-dimensional organ culture systems are widely used, each having advantages and limitations. In recent years, there has been considerable improvement of culture for prolonged periods in chemically defined media. Brain tissue from a wide spectrum of species have been used, including different types of human brain cells which can be propagated for several months. At present, these culture systems are employed for dynamic studies of the developing, the adult and ageing brain. It is possible to select neurons and the different classes of glial cells for culture purposes. Cell culture of the CNS has given new insights into the biology of brain tumours. Culture systems for experimental tumour therapy in vitro are also available. Recently, it has been shown that organ cultures of brain tissue can be used as targets for invasive glioma cells, enabling a direct study of the interactions between tumour cells and normal tissue to take place.

A low repeat length in oligodendrocyte chromatin

The behavior of oligodendrocyte chromatin after micrococcal nuclease digestion of nuclei was assayed in brains of rats of four different ages. During oligodendrocyte differentiation, a decreasing sensitivity of the chromatin to enzymatic attack was observed. On the other hand, the nucleosomal repeat length showed a slight tendency to increase during development. It is worth noting that even the highest values reported here for "oligodendrocyte" chromatin repeat lengths are significantly lower than 200 base pairs, the value previously reported by others for "non-astrocytic glia."

Immunocytochemical demonstration of glial-neuronal interactions and myelinogenesis in subcultures of rat brain cells

Subcultures have been established from primary rat brain cell cultures and have been characterised with a range of cell-specific immunocytochemical markers. The subcultures are mainly composed of fibrous astrocytes, oligodendrocytes and neurones. The cells do not divide to any great extent giving a system where it is possible to follow culture development at the cellular level for a number of weeks. During this time oligodendrocytes colonise subpopulations of neurones, differentiate further showing the presence of myelin basic protein and elaborate myelin-like membrane; the fibrous astrocytes remain scattered uniformly throughout the cultures. Radially oriented processes emerge from the oligodendrocyte-neurone aggregates which subsequently coalesce to form fascicles that link the clusters of cells together. These fascicles react with antibodies for both neurofilament protein and myelin basic protein. The subcultures provide a straightforward system that is composed of cells derived entirely from the CNS, is free from mitotic inhibitors and yet retains a sufficiently low cell density to allow immunocytochemical identification of the cell types present. The subcultures should be useful for the study of trophic interactions between oligodendrocytes and neurones as well as the early events associated with myelinogenesis.

Detection of visna virus antigens and RNA in glial cells in foci of demyelination

Visna is a slow virus infection of sheep in which the characteristic pathological change is demyelination in foci of inflammation. The latter is thought to be the result of an immunopathological process directed against cellular and antigenic targets that have been difficult to define because of restricted viral gene expression. A new simultaneous detection assay is used to demonstrate viral RNA in cells identified unambiguously as oligodendrocytes and astrocytes. These cells were found in inflammatory foci. With a new strain of virus that causes a rapid form of visna in Icelandic sheep, viral antigens were demonstrated in cells in the inflammatory lesions. These findings are consistent with the postulated immunopathological mechanism of demyelination: cells that maintain intact myelin sheaths in the central nervous system are destroyed by the inflammatory response to viral antigens expressed in these cells.