The ultrastructure of isolated rat oligodendroglial cell cultures

Connexin and Pannexin-Based Channels in Oligodendrocytes: Implications in Brain Health and Disease

Oligodendrocytes are the myelin forming cells in the central nervous system (CNS). In addition to this main physiological function, these cells play key roles by providing energy substrates to neurons as well as information required to sustain proper synaptic transmission and plasticity at the CNS. The latter requires a fine coordinated intercellular communication with neurons and other glial cell types, including astrocytes. In mammals, tissue synchronization is mainly mediated by connexins and pannexins, two protein families that underpin the communication among neighboring cells through the formation of different plasma membrane channels. At one end, gap junction channels (GJCs; which are exclusively formed by connexins in vertebrates) connect the cytoplasm of contacting cells allowing electrical and metabolic coupling. At the other end, hemichannels and pannexons (which are formed by connexins and pannexins, respectively) communicate the intra- and extracellular compartments, serving as diffusion pathways of ions and small molecules. Here, we briefly review the current knowledge about the expression and function of hemichannels, pannexons and GJCs in oligodendrocytes, as well as the evidence regarding the possible role of these channels in metabolic and synaptic functions at the CNS. In particular, we focus on oligodendrocyte-astrocyte coupling during axon metabolic support and its implications in brain health and disease.

Gap junctions couple astrocytes and oligodendrocytes

In vertebrates, a family of related proteins called connexins form gap junctions (GJs), which are intercellular channels. In the central nervous system (CNS), GJs couple oligodendrocytes and astrocytes (O/A junctions) and adjacent astrocytes (A/A junctions), but not adjacent oligodendrocytes, forming a "glial syncytium." Oligodendrocytes and astrocytes each express different connexins. Mutations of these connexin genes demonstrate that the proper functioning of myelin and oligodendrocytes requires the expression of these connexins. The physiological function of O/A and A/A junctions, however, remains to be illuminated.

Acid hydrolase activity of cultured bovine oligodendrocytes

We measured the activity of several acid hydrolases of cultured oligodendrocytes prepared from adult bovine brain white matter to clarify the biochemical basis of bovine oligodendrocytes in vitro. Lysosomal enzyme activities were assayed by using 4-methylumbelliferyl glycosides as substrates. Lysosomal enzyme activities became higher at 8-11 days in vitro (DIV) than 4 DIV. The enrichment in acid hydrolase specific activities in oligodendrocytes may be associated with lysosomal origin of myelin-like membranes.

Microtubule organization and stability in the oligodendrocyte

The oligodendrocyte is the glial cell responsible for the formation and maintenance of CNS myelin. Because the development of neuronal morphology is known to depend on the presence of highly organized microtubule arrays, it may be hypothesized that the properties of microtubules influence the form and function of oligodendrocytes. The goals of the present study were to define the physical attributes of microtubules in oligodendrocytes maintained in vitro. The results of electron and confocal microscopy indicate that microtubules are present throughout the cell bodies and large and small processes of oligodendrocytes and are rarely associated with discrete microtubule-organizing centers. A modified "hooking" protocol demonstrated that the polarity orientation of microtubules is uniformly plus-end distal in small oligodendrocyte processes, compared with a nonuniform, predominantly plus-end distal orientation in large processes. Oligodendrocytes were exposed to the microtubule-depolymerizing drug nocodazole to examine microtubule stability in these cells. The results suggest that oligodendrocyte microtubules can be resolved into at least three distinct microtubule populations that differ in their kinetics of depolymerization in the presence of nocodazole. These findings suggest that the properties of the oligodendrocyte microtubule array reflect the functions of the different regions of this highly specialized cell.

Heterogeneous morphology and tracer coupling patterns of retinal oligodendrocytes

The present study characterizes the morphology and tracer coupling patterns of oligodendrocytes in the myelinated band of the rabbit retina, as revealed by intracellular injection of biocytin or Lucifer yellow in an isolated superfused preparation. Based on the observed heterogeneity in morphology, we have grouped the presumptive oligodendrocytes into three categories termed 'parallel', 'stratified' and 'radial'. Most parallel oligodendrocytes were tracer coupled to nearby oligodendrocytes and astrocytes, whereas the stratified and radial oligodendrocytes rarely showed coupling. We conclude that the different categories of oligodendrocytes may be stages in a developmental series, with radial oligodendrocytes being premyelinating cells, parallel oligodendrocytes being mature myelinating cells and the stratified cells representing a transition between these categories.

Development of cell-cell coupling among cells of the oligodendrocyte lineage

The development of functional gap-junctional communication was studied in cells of the oligodendrocyte lineage. The presence of cell-cell coupling was determined by the passage of current between cells using a double whole-cell patch-clamp system or by injecting the low molecular weight dye Lucifer yellow into individual cells via a patch pepette and observing the diffusion of the dye into adjacent cells. The developmental stage of the cells under study was determined using antibodies to specific surface markers (04, 01, and 010) that characterize cells of sequential maturity along the oligodendrocyte lineage (Kuhlmann-Krieg et al., 1988; Sommer and Schachner, 1981; 1982). Both stages of precursor cells of this lineage, O4+ and O4-, almost never showed dye or electrical coupling, even though they were in close physical contact with other cells. The O1-positive oligodendrocytes with simple morphology were also noncoupled, with only few exceptions. In contrast, more than 40 percent of more mature, O10-positive oligodendrocytes showed cell-cell coupling detectable with both dye and current injection. Thus, the formation of gap junctions between cells of the oligodendrocyte lineage does not occur with, but some time after, the commitment of the cell to becoming an oligodendrocyte.

Composition of the phospholipids and their fatty acids in the ROC-1 oligodendroglial cell line

ROC-1 cells are a hybrid of C-6 rat glioma and rat oligodendroglia cells. Biochemically these cells resemble the oligodendroglia parent, but their lipid composition is unknown. The phospholipid composition in mole % was: cardiolipin, 1.0; phosphatidylglycerol, 1.2; ethanolamine glycerophospholipids, 27.6; phosphatidylinositol, 5.8; lysophosphatidylethanolamine, 0.8; phosphatidylserine, 5.6; choline glycerophospholipids, 43.7; sphingomyelin, 13.7; phosphatidylinositol-4-monophosphate, 0.8; and lysophosphatidylcholine, 0.6. The choline and ethanolamine plasmalogens made up 7.2 and 18.4% of the total phospholipids, respectively. The phospholipid composition reflects that of both parental cells. The cells had moderate to high levels of 20:3n-9 indicating n-6 series fatty acid deficiency. The phosphatidylinositol had very high 20:3n-9 levels with a 20:3n-9/20:4n-6 ratio of 2.1 compared to 0.44 and 0.58 for ethanolamine glycerophospholipids (EtnGpl) and choline glycerophospholipids (ChoGpl) respectively. The saturated/polyenoic fatty acid ratios were 0.40 for EtnGpl, 3.38 for ChoGpl and 1.48 for phosphatidylinositol.

An ultrastructural study of oligodendrocytes in zitter rat: a new animal model for hypomyelination in the CNS

The ultrastructural alterations in oligodendrocytes in the zitter rat--a newly established animal model of CNS hypomyelination--were studied in animals between 3 and 37 weeks of age. Throughout the observation period, the most characteristic alteration was the presence of abnormal structures associated with nuclear membranes and resembling split myelin lamellae. The structures consisted of stacks of membranes continuous with the inner and outer membranes of the nuclear envelope or with plasma membranes. This abnormality was prominent at 3 weeks of age, when initial myelination was probably still occurring. It was decreased transiently at 12 weeks of age and then again increased slightly with advancing age. However, the mean incidence of this abnormality was approximately 5% of the oligodendrocyte nuclei in randomly examined electron micrographs. As the small membranous abnormalities were observed in a few serial sections, the actual incidence is believed to be several times higher than 5%. Such membranous abnormalities were also detected in some mitochondria, in some cisternae of rough endoplasmic reticulum, and in some lysosomes, all at lower incidence rates. Some aberrant or elongated myelin sheath formations, an increased number of radial components and intracytoplasmic inclusions in oligodendrocytes were also observed. These findings suggest that functional abnormalities in membrane biosynthesis including the myelin sheaths and the nuclear membranes in oligodendrocytes are closely related to the hypomyelination in the CNS and may be related to the genetic abnormality in the zitter rat.

Distribution of mRNAs coding for liver and heart gap junction proteins in the rat central nervous system

The present study examined the distributions of connexin43 mRNA and connexin32 mRNA in the central nervous system (CNS) of the rat by using in situ hybridization histochemistry. These connexins are the best studied gap junction proteins; connexin32 forms direct cell-cell channels in the liver, as does connexin43 in the heart. There was a differential distribution of cells containing connexin32 mRNA compared with the population of cells which contained connexin43 mRNA, thus implying a regional specificity in the expression of connexins in the CNS. Cells containing connexin43 mRNA were uniformly distributed throughout the gray matter of the neuraxis. Several areas had a higher concentration of cells that express connexin43, such as layer IA of the piriform cortex, supraoptic and paraventricular nuclei of the hypothalamus, anterior cortical amygdaloid nucleus, the reticular part of the substantia nigra, lateral habenula, mesencephalic trigeminal nucleus. Purkinje cell layer of the cerebellum, facial nucleus, prepositus hypoglossal nucleus, and dorsal cochlear nucleus. The pattern of connexin43 hybridization and the morphology of connexin43 mRNA containing cells suggest that this gap junction forming protein is found predominantly in astrocytes. Connexin32 mRNA was detected in discrete cell groups of the gray matter that appeared to be neurons, including cells in layer 2 of the neocortex, layer II of the piriform cortex, pyramidal cell layer of the hippocampus, granule and polymorphic cell layers of the dentate gyrus, islands of Calleja, olfactory tubercle, lateral thalamic nuclei, lateral habenula, and Purkinje cell layer of the cerebellar cortex. A large population of cells in white matter tracts that were labelled with the connexin32 riboprobe appeared to be oligodendrocytes. These studies suggest that neurons and glial cells express connexin32 mRNA, but only astrocytes express connexin43 mRNA. Many of the areas in which connexin mRNAs were demonstrated have electrically coupled cells, morphologically distinct gap junction plaques, and/or have immunocytochemically identifiable connexin proteins. These results indicate that cells with mRNAs coding for intercellular channels have a widespread distribution in the mammalian CNS.

Neurobiology of human oligodendrocytes in culture

Enriched populations of oligodendrocytes were isolated from adult human brains of 3-15 hours postmortem using the trypsinin digestion-Percoll density gradient method and were cultured for an extended period of time up to 6 months. Cell type specific antigens that were expressed by oligodendrocytes were galactocerebroside, myelin basic protein, proteolipid protein, 2'3'-cyclic nucleotide 3'-phosphohydrolase and myelin-associated glycoprotein. In addition, HLA-A,B,C and HLA-DR, respectively, Class I and Class II antigens of the major histocompatibility complex, were demonstrated on oligodendrocytes. Three classes of gangliosides, GM1, GM4, and GD3, were also demonstrated on oligodendrocytes, while GM1 and GM4 gangliosides were detected on the surface of astrocytes. The presence of "transitional" or "bipotential" glial cells that were derived from oligodendrocytes and that expressed both oligodendroglial and astrocytic phenotypes was demonstrated. Treatment of the cells by cyclic AMP and its derivatives reversed this dual phenotypic expression back to the oligodendroglial trait. Electron microscopic examination of oligodendrocytes indicated that they were capable of synthesizing and assembling myelin sheaths in culture in the absence of any neuronal signal input.

Ultrastructural and biochemical findings in brain cell cultures infected with canine distemper virus

To study the pathomechanism of demyelination in canine distemper (CD), dog brain cell cultures were infected with virulent A75/17-CD virus (CDV) and examined ultrastructurally. Special attention was paid to the oligodendrocytes, which were specifically immunolabelled. In addition, cerebroside sulfotransferase (CST), an enzyme specific for oligodendrocyte activity was assayed during the course of the infection. Infection and maturation as well as CDV-induced changes were found in astrocytes and brain macrophages. Infection of oligodendrocytes was rarely seen, although CST activity of the culture markedly decreased and vacuolar degeneration of these cells occurred, resulting in their complete disappearance. We concluded that the degeneration of oligodendrocytes and demyelination is not due to direct virus-oligodendrocyte interaction, but due to CDV-induced events in other glial cells.

Electrical coupling, without dye coupling, between mammalian astrocytes and oligodendrocytes in cell culture

Evidence of electrical and dye coupling between oligodendrocytes and astrocytes was sought in cultures of mouse spinal cord. Cell identity was verified using cell specific antigenic markers. In most experiments current was injected into oligodendrocytes while recording voltage in nearby astrocytes. Nine of 17 oligodendrocyte-astrocyte cell pairs showed weak electrical coupling; the average estimated coupling ratio was 0.03 +/- 0.06 (cf. 0.11 for oligodendrocyte-oligodendrocyte and 0.44 for astrocyte-astrocyte pairs; Kettenmann and Ransom: Glia, 1: 64-73, 1988). Application of 0.5 mM BaCl2 or 44.6 mM CsCl depolarized astrocytes and oligodendrocytes and was estimated to increase the coupling ratio between these cells 3-5-fold; these effects were rapid in onset and completely reversible. In 5 of 7 cases, oligodendrocyte-astrocyte pairs that appeared uncoupled in normal solution exhibited coupling during Ba++ or Cs+ exposure. The actions of these cations are believed to be mediated by blockade of glial K+ channels. Depolarization, per se, as induced by increasing [K+]o, did not increase coupling ratio. The fluorescent dye lucifer yellow (LY) was injected into 10 oligodendrocytes, 8 of which were electrically coupled to nearby astrocytes, and never passed into astrocytes in detectable quantities. Likewise, astrocytes injected with LY stained other astrocytes, but never oligodendrocytes. These findings document the presence of weak electrical coupling between astrocytes and oligodendrocytes, in the absence of dye coupling. Weak coupling of this sort could subserve metabolic interactions between these cells mediated by the passage of small but important molecules such as cyclic AMP, but would not allow strong electrical interactions. If such coupling among glial cells is widespread, it would constitute a "metabolic syncytium" that could serve to coordinate glial behavior.

Ultrastructural features of cultured oligodendrocytes expressing stage-specific cell-surface antigens

This study was designed to correlate cytological features that had previously been established for oligodendrocytes at different developmental stages in vivo and cytological criteria in vitro with the expression of stage-specific cell-surface antigens of cultured oligodendrocytes. Cells obtained from the corpus callosum of 10-day-old C57BL/6J mice were maintained in monolayer cultures and stained with monoclonal antibodies 01 through 012 by indirect immunofluorescence or immunoperoxidase methods. 0 antigen-positive cells were classified according to two criteria: (a) cell shape (type I-III); and (b) cytoplasmic features at the ultrastructural level (class 1-3). Approximately 95% of all 0 antigen-positive cells could be identified as oligodendrocytes by established cytological criteria, thus supporting previous evidence of their glial character from electrophysiological and cell type-specific marker studies. After 12 days in vitro approximately 90% of all morphologically identified oligodendrocytes expressed antigens 03, 04, 05 or 06, which are the first to appear during development in vivo, whereas only 30-40% expressed antigens 011 or 012 which are the last to appear during development in vivo. 01 through 010 antigen-positive oligodendrocytes belong to 3 morphologically distinct cell types: (1) with small (approximately 10 micron in diameter) round cell bodies and few slender processes; (2) with 'hairy eyeball' morphology with a network of processes; and (3) with large, sometimes bipolar cell bodies (up to 30 micron in diameter) surrounded by high amounts of membranous material devoid of cytoplasm. By cytoplasmic criteria at least 90% of all 0 antigen-positive cells fit the description by Mori and Leblond of 'light' to 'medium' oligodendrocytes in vivo, although a clear-cut correlation with expression of early or late appearing 0 antigens was not observed. Typically 'dark' oligodendrocytes were rarely seen in our cultures. 011 and 012 antigen-positive cells are restricted to the group of large oligodendrocytes with high amounts of membranous material, often organized in more or less compact structures (type III). In contrast to the more uniform localization of antigens 01 through 010 over the whole cell surface, antigens 011 and 012 are less strongly detectable on cell bodies than on processes and membranous whirls.

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.

A chemically defined medium for the culture of mature oligodendrocytes

A new chemically defined medium consisting of equal parts of Dulbecco modified Eagle's and Ham's F-12 media supplemented with insulin, sodium selenite, putrescine, and D+ galactose, which allows the long-term survival of mature oligodendrocyte pure cultures, is described. Immunohistochemical staining has shown that over 90% of the cells become positive for myelin proteins shortly following subculture. Contaminating astrocytes (2%) do not survive in this medium. Biochemical data have indicated that these purified oligodendrocytes express 2'3'-cyclic nucleotide 3'-phosphohydrolase and UDP-galactose ceramide galactosyltransferase activities. Electron microscopical examination revealed that the oligodendrocytes were mostly of medium-dark type and appeared to be identical to cells cultured in serum-containing medium. The ability to maintain pure oligodendrocyte cultures in such a defined medium will allow investigations concerning exogenous and endogenous factors involved in oligodendrocyte metabolism.

MHC antigen expression on bulk isolated macrophage-microglia from newborn mouse brain: induction of Ia antigen expression by gamma-interferon

Macrophage-microglia were isolated from primary mixed brain cell cultures of normal newborn mice. They were successfully maintained in vitro for at least 8 weeks. Purity of the cultures was 97-100%, as determined by endocytosis of latex beads, non-specific staining through Fc receptors, EA and EAC rosette formation. These cells were non-specific esterase-positive, but peroxidase-negative. Electron-microscope observations revealed morphological similarities to mature macrophages. Isolated macrophage-microglia seldom incorporated [3H]thymidine in vitro. By means of 51Cr release assay, using monoclonal antibodies against mouse major histocompatibility complex (MHC) antigens and complement, we detected class I MHC (H-2) antigen on unstimulated macrophage-microglia, and both class I and class II (Ia) antigens on gamma-interferon-treated cells. These observations suggest possible immunoregulatory functions of macrophage-microglia in the central nervous system, as is characteristic of other cells of monocyte lineage.

Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. I. Immunocytochemical and ultrastructural studies

The technique of organotypic tissue culture offers an opportunity to observe in vitro complex interactions among glial cells and neurons, leading to the formation of myelin. In the present and accompanying work a combined ultrastructural, immunocytochemical and autoradiographic approach was used in a detailed study of the process of gliogenesis. Using immunocytochemical and ultrastructural criteria, differentiation along the oligodendroglia cell line is seen to be initiated a few days later than along the astroglial line. The sequence and timing of oligodendroglial differentiation both ultrastructurally and chemically follow those described in vivo. Formation of myelin has been demonstrated only by oligodendrocytes in which there is continuity between the perikaryal plasmalemma and myelin membranes. Oligodendroglial maturation culminated with the formation of light, medium and dark oligodendrocytes. The periodic acid Schiff-positive, glial fibrillary acidic protein (GFAP)-negative process of radial glial cells at explantation become GFAP-positive within 3 days, as described in vivo. Many of the astrocytes appear to have been derived from radial glial cells. Large numbers of dark glial cells, similar to the so-called 'intermediate glial cells', were seen. These were found to be astrocytes whose appearance probably reflected reaction to explantation-induced injury.

Cell shape and motility of oligodendrocytes cultured without neurons

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), were cultured from newborn rat brain and optic nerve to study how they differentiate in vitro in the absence of neurons. By use of galactocerebroside (GC) as a reference marker, the development of the cell phenotype was studied with video-enhanced differential interference contrast microscopy, immunofluorescence and electron microscopy. After a few days in culture, oligodendrocytes extend 5 to 10 main processes that are very rich in microtubules, but they did not stain with a monoclonal antibody reacting with all known classes of intermediate filaments. The number of processes can vary with the substrate on which the cells are grown; fewer processes form on laminin than on polylysine coated glass. Oligodendrocytes, in a fashion similar to that of neurons appear to keep their body immobile while the long processes grow. However, while neurons display motile activities mostly at the end of the cell processes called growth cones, the oligodendrocytes display motile, actin rich filopodia and lamellipodia along the entire length of all processes. The outgrowth of motile processes from oligodendrocytes sometimes occurs preferentially towards neighboring astrocytes. Oligodendrocyte processes display intense bidirectional movement of cytoplasmic organelles. Movement of surface components also occurs since GC molecules cross-linked by antibodies move from the processes towards the cell body. Thus, oligodendrocytes cultured without neurons develop on schedule a complex phenotype similar to their in vivo counterpart. In addition, their processes are capable of specific motile activities which may function in vivo to find the target axon and to transport myelin membrane components at the site of myelin assembly.

Cell-cell junctional interactions and characteristic plasma membrane features of cultured rat glial cells

Mixed cultures of astrocytes and oligodendrocytes derived from cerebral hemispheres of 18-19 day old rat fetuses were studied with the freeze-fracture technique. The plasma membranes of cultured astrocytes and oligodendrocytes differ substantially in their intramembrane particle profiles, and they can be positively identified consistently. Orthogonal small particle assemblies and numerous isolated globular particles characterize astrocytic plasma membranes, whereas the plasma membranes of oligodendrocytes show numerous elongated particles and fewer large and small globular particles similar to those seen in situ. Using these distinct differential features, we can identify partners of glial cell junctions. We can identify numerous interastrocytic gap junctions, as well as heterologous astrocyte-to-oligodendrocyte gap junctions. The plasma membranes of adjacent oligodendrocytes form numerous tight junctions consisting of linear P face strands and/or rows of particles interrupted by short segments of grooves, the complementary features on the E face. "Reflexive" type tight junctions seen in situ are also observed. In addition to intercellular junctions, glial cells develop special plasma membrane structural domains. Astrocytic plasma membranes often contain groups of plasmalemmal vesicles (caveolae), a distinctive feature of astrocytes in situ. Oligodendrocytes form flattened velate processes with cytoplasm restricted to finger-like channels resembling myelin lamellae in situ. Cultured astrocytes and oligodendrocytes develop the entire range of plasma membrane structural specializations seen in situ in the absence of the normal brain tissue framework. Thus, primary glial cell cultures allow experimental study of many glial cell properties, including their plasma membrane specializations.

Electrical properties of oligodendrocytes in culture

The electrical properties of immunocytologically identified oligondendrocytes from embryonic mouse spinal cord maintained in culture for 3 to 6 weeks studied by passing current and recording potential changes with two separate intracellular electrodes. The average input resistance was 3.3 M omega and ranged from 0.7 to 16 M omega (n = 35). The input resistance increased by 19% with depolarization and decreased by 9% with hyperpolarization of 25 mV. The membrane time constant determined from the slope of the late exponential tail was 3.45 +/- 2.5 ms SD (n = 15). The specific membrane resistance of three cells was determined by a simplified square pulse analysis combined with measurement of membrane area. Membrane area was estimated from photomicrographs of cells injected with Lucifer Yellow CH and stained with the cell surface-reactive antibody 04 and from electron micrographs. An average specific membrane resistance of 1.3 X 10(3) omega cm2 and specific capacitance of 1.7 mu F/cm2 were calculated. Increasing [K+]o depolarized the cells and decreased the input resistance and the time constant.

Detection of Wolfgram W1 protein, myelin basic proteins and proteolipids in cultured oligodendrocytes by the electro-immunoblotting method

Oligodendrocytes, mechanically obtained from primary cultures of newborn rat brain, were investigated by a sensitive electro-immunoblotting method for the presence of the characteristic myelin proteins: Wolfgram W1 protein, basic proteins and proteolipids. These three major myelin protein types were detectable from the 20th to the 40th day in culture. The present biochemical findings are in accordance with previous immunohistochemical data and provide additional evidence that oligodendrocytes in culture are capable of synthesizing every myelin and oligodendrocyte marker found in vivo, in the absence of neurones.