Synthesis of a myelin-like membrane by oligodendrocytes in culture

Myelination of Neuronal Cell Bodies when Myelin Supply Exceeds Axonal Demand

The correct targeting of myelin is essential for nervous system formation and function. Oligodendrocytes in the CNS myelinate some axons, but not others, and do not myelinate structures including cell bodies and dendrites [1]. Recent studies indicate that extrinsic signals, such as neuronal activity [2, 3] and cell adhesion molecules [4], can bias myelination toward some axons and away from cell bodies and dendrites, indicating that, in vivo, neuronal and axonal cues regulate myelin targeting. In vitro, however, oligodendrocytes have an intrinsic propensity to myelinate [5, 6, 7] and can promiscuously wrap inert synthetic structures resembling neuronal processes [8, 9] or cell bodies [4]. A current therapeutic goal for the treatment of demyelinating diseases is to greatly promote oligodendrogenesis [10, 11, 12, 13]; thus, it is important to test how accurately extrinsic signals regulate the oligodendrocyte’s intrinsic program of myelination in vivo. Here, we test the hypothesis that neurons regulate myelination with sufficient stringency to always ensure correct targeting. Surprisingly, however, we find that myelin targeting in vivo is not very stringent and that mistargeting occurs readily when oligodendrocyte and myelin supply exceed axonal demand. We find that myelin is mistargeted to neuronal cell bodies in zebrafish mutants with fewer axons and independently in drug-treated zebrafish with increased oligodendrogenesis. Additionally, by increasing myelin production of oligodendrocytes in zebrafish and mice, we find that excess myelin is also inappropriately targeted to cell bodies. Our results suggest that balancing oligodendrocyte-intrinsic programs of myelin supply with axonal demand is essential for correct myelin targeting in vivo and highlight potential liabilities of strongly promoting oligodendrogenesis.

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Balance between axons and myelin production regulates its targeting in vivo

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Excess myelin is mistargeted to cell bodies

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Low, but not zero, level of mistargeting during normal development

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Potential implications for myelin-promoting therapies

The Influence of Myelin Oligodendrocyte Glycoprotein on White Matter Abnormalities in Different Onset Age of Drug-Naïve Depression

Neurophysiological mechanisms of white matter abnormalities in the earlier onset major depressive disorder (eoMDD, onset age ≤25 years old) differ from that in the later onset MDD (loMDD, onset age >25 years old). Myelin oligodendrocyte glycoprotein (MOG) is an important factor influencing white matter development. The influence of MOG on white matter in MDD of different age onset need to be explored. We compared MOG plasma concentrations and diffusion tensor imaging (DTI) data in 35 first-episode medication-naïve MDD patients (23 eoMDD, 12 loMDD), and 32 healthy controls (HC, 17 younger, 15 older). MOG was significantly higher in eoMDD and lower in loMDD compared with HC. Mean diffusivity (MD) values were significantly increased in inferior fronto-occipital fasciculus (IFOF) in eoMDD, and decreased in loMDD. In both younger and older groups, MOG correlated positively with IFOF MD values. Abnormal MOG has different influence in MDD of different age onset, which is linked to MOG's overly active effect on abnormal white matter in eoMDD and markedly weak effect in loMDD cases. Abnormal MOG would be an important factor in white matter damage in MDD; the influence of MOG differs with onset age.

On Myelinated Axon Plasticity and Neuronal Circuit Formation and Function

Studies of activity-driven nervous system plasticity have primarily focused on the gray matter. However, MRI-based imaging studies have shown that white matter, primarily composed of myelinated axons, can also be dynamically regulated by activity of the healthy brain. Myelination in the CNS is an ongoing process that starts around birth and continues throughout life. Myelin in the CNS is generated by oligodendrocytes and recent evidence has shown that many aspects of oligodendrocyte development and myelination can be modulated by extrinsic signals including neuronal activity. Because modulation of myelin can, in turn, affect several aspects of conduction, the concept has emerged that activity-regulated myelination represents an important form of nervous system plasticity. Here we review our increasing understanding of how neuronal activity regulates oligodendrocytes and myelinated axons in vivo, with a focus on the timing of relevant processes. We highlight the observations that neuronal activity can rapidly tune axonal diameter, promote re-entry of oligodendrocyte progenitor cells into the cell cycle, or drive their direct differentiation into oligodendrocytes. We suggest that activity-regulated myelin formation and remodeling that significantly change axonal conduction properties are most likely to occur over timescales of days to weeks. Finally, we propose that precise fine-tuning of conduction along already-myelinated axons may also be mediated by alterations to the axon itself. We conclude that future studies need to analyze activity-driven adaptations to both axons and their myelin sheaths to fully understand how myelinated axon plasticity contributes to neuronal circuit formation and function.

Oligodendrocyte Development in the Absence of Their Target Axons In Vivo

Oligodendrocytes form myelin around axons of the central nervous system, enabling saltatory conduction. Recent work has established that axons can regulate certain aspects of oligodendrocyte development and myelination, yet remarkably oligodendrocytes in culture retain the ability to differentiate in the absence of axons and elaborate myelin sheaths around synthetic axon-like substrates. It remains unclear the extent to which the life-course of oligodendrocytes requires the presence of, or signals derived from axons in vivo. In particular, it is unclear whether the specific axons fated for myelination regulate the oligodendrocyte population in a living organism, and if so, which precise steps of oligodendrocyte-cell lineage progression are regulated by target axons. Here, we use live-imaging of zebrafish larvae carrying transgenic reporters that label oligodendrocyte-lineage cells to investigate which aspects of oligodendrocyte development, from specification to differentiation, are affected when we manipulate the target axonal environment. To drastically reduce the number of axons targeted for myelination, we use a previously identified kinesin-binding protein (kbp) mutant, in which the first myelinated axons in the spinal cord, reticulospinal axons, do not fully grow in length, creating a region in the posterior spinal cord where most initial targets for myelination are absent. We find that a 73% reduction of reticulospinal axon surface in the posterior spinal cord of kbp mutants results in a 27% reduction in the number of oligodendrocytes. By time-lapse analysis of transgenic OPC reporters, we find that the reduction in oligodendrocyte number is explained by a reduction in OPC proliferation and survival. Interestingly, OPC specification and migration are unaltered in the near absence of normal axonal targets. Finally, we find that timely differentiation of OPCs into oligodendrocytes does not depend at all on the presence of target axons. Together, our data illustrate the power of zebrafish for studying the entire life-course of the oligodendrocyte lineage in vivo in an altered axonal environment.

Do Action Potentials Regulate Myelination?

A variety of anatomical features suggest that functional activity in the nervous system can influence the process of myelination, yet direct evidence of this is lacking. Research by Zalc and colleagues shows that myelination of optic nerve is inhibited by a neurotoxin that blocks action potential activity and is stimulated by a toxin that increases impulse activity, suggesting that impulse activity is necessary for initiating myelination during development of the optic nerve. Research by Fields and colleagues, using electrical stimulation of axons, shows that low frequency impulse activity inhibits myelination of dorsal root ganglion neurons, but high frequency impulse activity has no effect. This results from reduced expression of a cell adhesion molecule on the stimulated axons that is critical for inducing myelination. Together these studies support the conclusion that impulse activity can influence the process of myelination, probably through more than one molecular mechanism operating during discrete steps in the myelination process.

In vitro myelination by oligodendrocyte precursor cells transfected with the neurotrophin-3 gene

Oligodendrocyte precursor cells require exogenous neurotrophin-3 (NT-3) for differentiation into oligodendrocytes. We transfected precursor cells with the gene for NT-3 and looked for changes in their development into myelin-forming cells. The expression of NT-3 in transfected cells was demonstrated by reverse transcription followed by PCR as well as by Northern blots. Direct synthesis of the neurotrophin product and its release to the culture supernatants were also shown by specific ELISA. Transfection converts precursor cells into actively dividing cells that can incorporate 3H-thymidine into DNA. In the absence of growth factors, a parallel increase in the survival of the transfected cultures was also demonstrated by the MTT test. The final demonstration of biological changes in transfected versus untreated cells was a 10-fold increase in myelin basic protein production observed in Western blots and the direct observation by phase-contrast and electron microscopy of myelin membranes in cocultures with hippocampal neurons. We discuss the future use of this transfected cells in regeneration and functional recovery in experimental models of multiple sclerosis.

The oligodendrocyte-myelin glycoprotein gene is highly expressed during the late stages of myelination in the rat central nervous system

Oligodendrocyte-myelin glycoprotein (OMgp) is expressed on the surface of oligodendrocytes and neurones and is thought to inhibit axonal regeneration after brain injury in adult, like Nogo and myelin-associated glycoprotein (MAG). We previously observed that the OMgp gene locus on chromosome 17 could be associated with autism, a developmental disorder. The aim of the present study was to characterise the developmental expression of OMgp mRNA in the central nervous system. First we determined the rat OMgp gene sequence and compared it with the human and mouse sequences. Several regions, putative sites for the fixation of transcription factors, are conserved between these three species in the unique intron of this gene. Using quantitative and semi-quantitative RT-PCR, we studied OMgp gene expression in rat brain during post-natal development. We found that OMgp mRNA expression was developmentally regulated, with a peak of expression in the late stages of myelination. We observed a similar profile in oligodendrocyte cultures, in absence of neurones, suggesting that OMgp mRNA expression by oligodendrocytes was independent of axonal influence. Our observations suggest that OMgp is a late marker of myelination, which could be implicated in the arrest of oligodendrocyte proliferation, arrest of myelination or compaction of myelin.

The structure and function of myelin: from inert membrane to perfusion pump

The goal of this overview is to propose a novel structure/function model of central nervous system myelin. Although myelin is known to be a compact multilamellar structure that wraps around axons, the biologic role this structure plays in the nervous system remains an enigma. One means of ascertaining myelin's biologic role is by analyzing its structure. The recent discovery of tight junctions in myelin may be the key that unlocks the mysterious black box of myelin structure/function. Tight junctions in other cell types are invariably adjacent to adherens junctions, with both of these junctional plaques playing critical roles in paracellular barrier function, i.e., adhesion of cell membranes, signal transduction, and fluid movement between cells via aqueous pores and channels. The application of current knowledge about junctional plaques to myelin is an original concept. This knowledge, taken together with evidence from studies of normal and pathologic myelin, supports the possibility that a primary function of junctional plaques in myelin is to perfuse the periaxonal space.

Pituitary adenylyl cyclase-activating polypeptide stimulates DNA synthesis but delays maturation of oligodendrocyte progenitors

The neuropeptide pituitary adenylyl cyclase-activating peptide (PACAP) and one of its receptors (PAC(1)) are expressed in embryonic neural tube, where they appear to regulate neurogenesis and patterning. We now show that PAC(1) gene expression is also present in neonatal rats in the ventricular and subventricular zones and in the optic chiasm, areas that are rich in oligodendrocyte (OL) progenitors (OLP). Because actions of PACAP on OLP have not been reported, we examined the effects of PACAP on the proliferation of purified OLP in culture and on myelinogenesis in cerebellar slices. Northern analyses on total RNA from purified glial cell subtypes revealed an abundant 7 kb hybridizing transcript in OLP, which was confirmed to correspond to the PAC(1) receptor by reverse transcription-PCR. The presence of this receptor was also corroborated by radioligand binding and cAMP assay. In cultured OL, receptor density decreased during maturation but was partially counterbalanced by the appearance of sites that bound both PACAP and the related peptide vasoactive intestinal peptide. PACAP increased DNA synthesis in OLP cultures almost twofold and increased the bromodeoxyuridine-labeling index in O4-positive OLP. PACAP treatment also resulted in decreased sulfate incorporation into sulfatide in cultures of differentiating OL. The PACAP effect on sulfatide synthesis was fully reproduced in a cerebellar explant model. These findings indicate that PACAP may act at two stages during OL development to (1) stimulate proliferation and (2) delay maturation and/or myelinogenesis.

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.

Differentiation dependent activation of the myelin genes in purified oligodendrocytes is highly resistant to hypoglycemia

We have previously demonstrated that the developmental upregulation of myelin-specific genes in mixed glial cultures is strongly attenuated by hypoglycemia. The present study was designed to evaluate the effect of hypoglycemia on differentiation-dependent upregulation of myelin genes in purified oligodendrocyte cultures. The expression of major myelin protein genes, i.e., proteolipid protein (PLP), basic protein (BP) and myelin associated glycoprotein (MAG) were monitored by Northern blot analysis. In control cultures maintained at 6 mg/ml of glucose, the expression of all the genes upregulated rapidly, and plateaued at approximately day 4. A similar pattern of differentiation-dependent upregulation was observed for the gene encoding a lipogenic enzyme, i.e., malic enzyme (ME). In contrast to mixed glial cultures, however, this developmental gene upregulation was not significantly affected by severe hypoglycemia (approximately 0.02 mg/ml). The results indicate that the effect of glucose deprivation on oligodendrocyte genes observed in mixed glial cultures is mediated by other cells. The upregulation of the genes in differentiating oligodendrocytes was accompanied by the production of myelin-related membrane that was isolated by density gradient fractionation. In contrast to the effect on gene expression, this anabolic activity was highly dependent on glucose, as seen from a profound suppression by severe hypoglycemia.

Glucocorticoids and progestins signal the initiation and enhance the rate of myelin formation

Dexamethasone and progesterone have been found to accelerate the time of initiation and enhance the rate of myelin synthesis in Schwann cell/neuronal cocultures. The expression of mRNA for cytochrome P450scc (converts cholesterol to pregnenolone), 3beta-hydroxysteroid dehydrogenase (converts pregnenolone to progesterone), and the progesterone receptor were detected and markedly induced during peak myelin formation in the cocultures. The mRNA for the glucocorticoid receptor was detected, but was found to be constituitively expressed. In addition, the specific activity of 3beta-hydroxysteroid dehydrogenase was measured and found to increase by 10-fold. The mRNA for cytochrome P450scc and 3beta-hydroxysteroid dehydrogenase also were found to be induced during the differentiation of O-2A precursor cells to oligodendrocytes. Fibroblast growth factor and platelet-derived growth factor were found to have proliferative effects on Schwann cells, but they had no effect on the initiation or the rate of myelin formation. These results demonstrate that myelin-forming cells have inducible enzymes responsible for steroid biosynthesis and suggest a critical role for endogenous steroid hormones in signaling the initiation and enhancing the rate of myelin formation.

Role of axonal components during myelination

Myelination is a multistep ordered process whereby Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS), produce and extend membranous processes that envelop axons. Mechanisms that regulate this complex process are not well understood. Advances in deciphering the regulatory components of myelination have been carried out primarily in the PNS and although the mechanisms for triggering and directing myelination are not known, it is well established that myelination does not occur in the absence of axons or axon/neuron-derived factors. This appears to be true both in PNS and CNS. Progress in understanding CNS myelinogenesis has been relatively slow because of the unavailability of a suitable culture system, which, in turn, is partly due to complexity in the cellular organization of the CNS. Though the myelin composition differs between PNS and CNS, the regulation of myelination seems to parallel rather than differ between these two systems. This article reviews the regulatory role of axonal components during myelination. The first half consists of an overview of in vitro and in vivo studies carried out in the nervous system. The second half discusses the use of a cerebellar slice culture system and generation of anti-axolemma monoclonal antibodies to investigate the role of axonal membrane components that participate in myelination. It also describes the characterization of an axonal protein involved in myelination.

Neurological disturbances, premature lethality, and central myelination deficiency in transgenic mice overexpressing the homeo domain transcription factor Oct-6

Pit, Oct, Unc (POU) homeo domain transcription factors have been implicated in various developmental processes, including cell division, differentiation, specification, and survival of specific cell types. Although expression of the transcription factor Oct-6 in oligodendroglia is confined to the promyelin stage and is downregulated at the myelin stage of development, the effect of Oct-6 overexpression on oligodendrocyte development has not been established. Here we show that transgenic animals overexpressing Oct-6 at late oligodendrocyte development develop a severe neurologic syndrome characterized by action tremors, recurrent seizures, and premature death. Axons in the central nervous system of Oct-6 transgenics were hypomyelinated, hypermyelinated, or dysmyelinated, and ultrastructural analyses suggested that myelin formation was premature. The vulnerability of developing oligodendroglia to Oct-6 deregulation provides evidence that the POU factor may play a direct role in myelin disease pathogenesis in the mammalian CNS.

Signals that initiate myelination in the developing mammalian nervous system

The myelination of axons by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system is essential for the establishment of saltatory conduction. In the absence or destruction of the myelin sheath, as seen in demyelinating diseases, impulse conduction is impeded resulting in severe sensory and motor deficits. Axon myelination is the culmination of a sequence of events that begins with the differentiation of glial cells and proceeds to the transcription and translation of myelin genes, the elaboration of a myelin sheath, and the recognition and ensheathment of axons. This review examines the regulatory mechanisms for each of these steps and compares and contrasts the role of the axon in initiating myelination in the central and peripheral nervous system.

Selenium is required for normal upregulation of myelin genes in differentiating oligodendrocytes

The purpose of this study was to characterize the selenium requirement for the normal differentiation of oligodendrocyte lineage cells. In primary mixed glial cultures prepared from newborn rat brains, the overall growth of cultures, as seen from the total RNA yield, was not significantly affected by selenium. However, 30 nM selenium was required for the normal upregulation the proteolipid protein, basic protein, and myelin-associated glycoprotein gene expression assessed by Northern blot analysis. Selenium deprivation during initial, rapid phase of the gene upregulation irreversibly suppressed the genes, indicating the existence of a critical period in oligodendrocyte differentiation. In purified oligodendrocyte cultures prepared by mechanical dislodging of progenitor (O-2A) cells from mixed glial cultures, total cell number and total RNA yield were virtually unaffected by selenium deprivation; however, the developmental upregulation of the myelin genes was profoundly attenuated. Immunocytochemical analysis confirmed the suppressive effect of selenium deficiency on the differentiation of oligodendrocyte lineage cells, as seen from a significant decrease in the population of GalC+ and O4+ cells. Because the number of GC+ cells was more reduced than the number of O4+ cells, the results indicate that selenium deficiency may specifically inhibit the progression from immature to mature oligodendrocytes.

Developmental expression of the C1G5F2 antigen in cultured rat oligodendroglial cells

The C1G5F2 antigen is a newly described minor myelin antigen of the central nervous system. Its expression compared with that of some other main myelin protein components (Wolfgram W1 protein, myelin basic proteins (MBP) and proteolipids) was investigated in rat oligodendrocytes derived from 10-day-old primary glial cell cultures and subcultured for several days in a chemically defined medium. It was demonstrated immunocytochemically that this antigen is detected later than the major myelin markers. All cells immunoreactive with the monoclonal antibody C1G5F2 were always labeled either by W1-, MBP- or proteolipid-specific antisera. It was also shown at the electron microscopic level that this antigen is mainly expressed on the surface of the extremities of the fine oligodendroglial processes. All these observations suggest that the C1G5F2 antigen may be a useful marker for a specific step in the oligodendrocyte maturation stage.

High gelsolin content of developing oligodendrocytes

The actin-binding protein gelsolin that severs and caps the actin microfilaments under the control of the cytoplasmic free calcium and the membranous phosphatidylinositol 4,5-bisphosphate, is essentially restricted to the oligodendroglia in the central nervous system. Immunocytochemistry showed that gelsolin is an early marker of oligodendrocytes, both in vivo, in the rat cerebellum, and in vitro, in oligodendrocyte culture. We report the early appearance of gelsolin in A2B5-positive precursor oligodendrocyte cells and the specific expression of gelsolin in OL-1-, GC-, and MBP-positive oligodendrocytes in culture. The protein was distributed throughout the cell body and in the branched cell processes of cultured oligodendrocytes, but not in the MBP-positive membrane sheets. Gelsolin is thus cytosolic and not a myelin component. The quantitative study demonstrated that that the cerebellar gelsolin content changes significantly with age, with the maximal value at the age of 21 days, confirming that large amounts of gelsolin are transiently synthesized during development, especially from the first events of myelinogenesis. The results are consistent with gelsolin being involved, through its effects on the actin cytoskeleton, in the motile events occurring during the growth of the oligodendroglial processes towards the axons and the wrapping of the myelin sheaths around the axons.

Functional evidence for the role of axolemma in CNS myelination

A direct role for neurons in CNS myelination has yet to be demonstrated. CNS myelination can be examined in cerebellar slice cultures, which faithfully reproduce both synthesis and wrapping of myelin. In an attempt to demonstrate a role for axolemma in this process, we generated more than 2000 axolemma-reactive monoclonal antibodies. One clone, G21.3, repeatedly blocked myelination in cerebellar slices, as documented by both biochemistry and morphology. The antibody caused a dramatic reduction in myelin lipid and protein synthesis. CNS white matter, sciatic nerve, and neuronal cultures were positively stained with G21.3, whereas oligodendrocytes and myelin were fully negative. The antibody identified a restricted number of proteins in purified axolemma. These results suggest a direct involvement of axons in CNS myelination.

Stimulation of in vitro myelin synthesis by microglia

Central nervous system myelin is elaborated by oligodendrocytes, which have been studied extensively in cell culture. Dissociated brain cultures allow in vitro analysis of events in myelinogenesis, including cell-cell interactions. Microglia, the primary phagocytic cell of the central nervous system, appear in developing fiber tracts prior to the onset of myelination in vivo. To gain insight into potential oligodendrocyte-microglial interactions during development, these cells were co-cultured and various parameters of myelin synthesis were measured. In co-culture, microglia stimulated the synthesis of sulfatide, a myelin-specific galactolipid, in oligodendrocytes, as well as the expression of the myelin-specific proteins myelin basic protein and proteolipid protein. Activity of the oligodendrocyte cytoplasm-specific enzyme 2',3'-cyclic nucleotide 3'-phosphohydrolase was not elevated, suggesting that the effects of microglia were not due to stimulation of oligodendrocyte proliferation. This was confirmed by the inability of microglia to induce significant DNA synthesis. Conditioned medium from cultured microglia provided a similar stimulatory activity, suggesting that the increase in myelin synthesis does not require contact between oligodendrocytes and microglia. These findings suggest a stimulatory role for microglia during myelinogenesis.

Expression of a beta 1-related integrin by oligodendroglia in primary culture: evidence for a functional role in myelination

We have investigated the expression of integrins by rat oligodendroglia grown in primary culture and the functional role of these proteins in myelinogenesis. Immunochemical analysis, using antibodies to a number of alpha and beta integrin subunits, revealed that oligodendrocytes express only one detectable integrin receptor complex (alpha OL beta OL). This complex is immunoprecipitated by a polyclonal anti-human beta 1 integrin subunit antibody. In contrast, astrocytes, the other major glial cell type in brain, express multiple integrins including alpha 1 beta 1, alpha 3 beta 1, and alpha 5 beta 1 complexes that are immunologically and electrophoretically indistinguishable from integrins expressed by rat fibroblasts. The beta subunit of the oligodendrocyte integrin (beta OL) and rat fibroblast beta 1 have different electrophoretic mobilities in SDS-PAGE. However, the two beta subunits appear to be highly related based on immunological cross-reactivity and one-dimensional peptide mapping. After removal of N-linked carbohydrate chains, beta OL and beta 1 comigrated in SDS-PAGE and peptide maps of the two deglycosylated subunits were identical, suggesting differential glycosylation of beta 1 and beta OL accounts entirely for their size differences. The oligodendrocyte alpha subunit, alpha OL, was not immunoprecipitated by antibodies against well characterized alpha chains which are known to associate with beta 1 (alpha 3, alpha 4, and alpha 5). However, an antibody to alpha 8, a more recently identified integrin subunit, did precipitate two integrin subunits with electrophoretic mobilities in SDS-PAGE identical to alpha OL and beta OL. Functional studies indicated that disruption of oligodendrocyte adhesion to a glial-derived matrix by an RGD-containing synthetic peptide resulted in a substantial decrease in the level of mRNAs for several myelin components including myelin basic protein (MBP), proteolipid protein (PLP), and cyclic nucleotide phosphodiesterase (CNP). These results suggest that integrin-mediated adhesion of oligodendrocytes may trigger signal(s) that induce the expression of myelin genes and thus influence oligodendrocyte differentiation.

Myelination in cerebellar slice cultures: development of a system amenable to biochemical analysis

Myelin deposition and maintenance are critical to proper function of the mammalian nervous system. Previous investigations of myelination in the central nervous system (CNS) were hampered by the lack of an in vitro system that can faithfully reproduce in vivo events yet is amenable to biochemical investigation. We have developed a procedure, based on organotypic cultures, which permits efficient preparation of large numbers of cerebellar slice cultures that can be easily manipulated. Cultures have been examined to document myelination biochemically (by incorporation of [35S]sulfate into sulfolipids), immunohistochemically (by labeling the myelin components myelin basic protein and galactocerebroside), and morphologically (by both light and electron microscopy). We tested the effects of biologically active peptides and antibodies on myelination in the thin slices. The results indicate that the cultures provide an in vitro system that can be used to examine specific cellular events that occur during CNS myelination.

Myelin basic protein mRNA translocation in oligodendrocytes is inhibited by astrocytes in vitro

Myelin basic protein (MBP) mRNAs are translocated from cell bodies into the slender processes connecting oligodendrocyte somas with the myelin sheath in vivo. This translocation was observed in mixed glial cultures prepared from newborn mouse brains and it occurred in approximately 25% of the cells expressing the gene. However, when "enriched" oligodendrocytes were prepared by shaking them free of other glial cells, MBP mRNA translocation occurred into the processes of essentially all of the cells. When enriched oligodendrocytes were plated back onto astrocytes, MBP mRNA was observed to be confined to the cell bodies of almost all the cells, indicating a marked inhibition of translocation of the mRNA. This inhibition of mRNA translocation did not appear to be mediated through soluble factors secreted by astrocytes or by "astromatrix," but rather through physical contact between the oligodendrocytes and astrocytes. Intact, but not necessarily live, astrocytes were required for the inhibition of mRNA translocation in the oligodendrocytes. Fibroblasts and a neuroblastoma cell line, SKN-SH, did not inhibit MBP mRNA translocation in oligodendrocytes suggesting that astrocyte surface-specific components might be involved in the interaction between astrocytes and oligodendrocytes in culture. These results suggest that contact between these two cell types can influence intramolecular events related to myelinogenesis.

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.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

Epidermal growth factor affects both glia and cholinergic neurons in septal cell cultures

The effects of epidermal growth factor on high density primary cultures of fetal (embryonic day 17) rat septal cells were examined. Under serum-free conditions, the continuous exposure of these cultures to epidermal growth factor for seven days significantly decreased choline acetyltransferase (EC 2.3.1.6) activity in a dose-dependent manner. Maximal decreases were observed from 1 to 10 ng/ml epidermal growth factor. This effect was completely abolished by the addition of anti-epidermal growth factor antibodies. The epidermal growth factor-mediated decrease in choline acetyltransferase activity was culture-time dependent, being first detectable after five days of factor application and may likely represent an inhibition of the spontaneous increase in enzyme activity that occurs with time in culture. Concomitant with changes in enzyme activity, epidermal growth factor produced a significant and proportional decrease in the number of acetylcholinesterase-positive neurons. This decrease in acetylcholinesterase-positive cells did not reflect a decrease in cholinergic cell survival as nerve growth factor could restore the number of acetylcholinesterase-positive neurons in epidermal growth factor-treated cultures to control levels. Furthermore, in these high-density cultures, epidermal growth factor did not affect general neuronal survival, while it did produce an increase in the number and intensity of glial fibrillary acidic protein-immunoreactive astroglia as well as in the number of macrophage-like cells. The proliferative response of these non-neuronal cells to epidermal growth factor, as assessed by [3H]thymidine incorporation, was evident after three days of epidermal growth factor application, persisted thereafter, and could be antagonized by the inclusion of the antimitotic 5-fluorodeoxyuridine. Furthermore, 5-fluorodeoxyuridine completely blocked the epidermal growth factor-mediated decrease in choline acetyltransferase activity. However, when epidermal growth factor was tested in pure glial cultures, it only directly induced proliferation of astrocytes. These results suggest that the proliferative response of either one or both of these glial cell types in the mixed cultures may be indirectly affecting cholinergic cell expression.

Myelin protein expression in dissociated superior cervical ganglia and dorsal root ganglia cultures

Schwann cells of the adult rat superior cervical ganglia (SCG) synthesize negligible levels of the major myelin glycoprotein, P0, in vivo. This suggests that the sympathetic axons of the SCG are deficient in one of more components involved in the regulation of myelin protein expression. Here we have compared the ability of neurites from neonatal rat SCG and embryonic rat dorsal root ganglia (DRG) to induce Schwann cell expression of myelin proteins after growth in culture using a serum-free medium. Steady-state P0 mRNA levels in the SCG and DRG culture paradigms were determined with a sensitive polymerase chain reaction (PCR) assay that amplified cDNA produced by reverse transcription of mRNA. This semiquantitative assay showed a linear response to increasing amounts of P0 and actin mRNA and required substantially less cellular RNA than typical hybridization techniques. Using the PCR assay, we found that SCG cultures contained significantly lower amounts of P0 mRNA than did DRG cultures. To further confirm that SCG cultures have negligible expression of myelin proteins, immunoblot analyses were done to examine the steady-state levels of both P0 and myelin basic protein. While nonmyelinating DRG cultures had readily detectable amounts of these myelin-specific proteins, neither could be demonstrated in the SCG cultures. The data indicate that SCG neurites lack one or more signals needed to induce myelin protein expression. Employing SCG and DRG cultures in comparative biochemical studies should prove useful in identifying the axonal molecule(s) involved in the regulation of myelin protein expression.

Ontogeny of glycerol phosphate dehydrogenase-positive oligodendrocytes in rat brain. Impaired differentiation of oligodendrocytes in the myelin deficient mutant rat

The ontogeny of oligodendrocytes in the myelin deficient (md) rat mutant and in control rats was explored immunohistochemically using an antiserum against the oligodendrocyte specific enzyme, glycerol phosphate dehydrogenase (GPDH), and the avidin-biotin complex technique. In control rats, GPDH was demonstrated to be expressed relatively early in oligodendrocyte differentiation, prior to either myelin basic protein or proteolipid protein expression. With development, oligodendrocytes containing GPDH increased in number, apparent staining intensity, cell soma area and process elaboration. Fewer GPDH+oligodendrocytes were observed in the brain of mutant rats than in unaffected littermates at all developmental ages, and major developmental increases in oligodendrocyte density were delayed. The density of GPDH+oligodendrocytes was reduced by about 40% in both the corpus callosum and in the cingulate cortex of P22-25 and mutants compared with control rats. The oligodendrocyte cell soma area was not influenced by the md condition, and increased 2-fold with development in rats of both genotypes. The area of coronal sections occupied by the corpus callosum increased about 2.5-fold with development, and was 30% smaller in mutant rats late in their lifespan than in unaffected littermates. The reductions in oligodendrocyte density reported here are of insufficient magnitude to fully account for biochemically measured reductions in oligodendrocyte gene expression accompanying the md trait, indicating that gene expression per oligodendrocyte is also impaired. Cell counts in control rats also revealed that oligodendrocytes are overproduced during development. Cell density and the total number of corpus callosum GPDH+oligodendrocytes per section were maximal at P22-25 and then decreased to adult values. These results suggest that glial cells, like neurons, may be generated in excessive numbers, and some subsequently die, as a normal concomitant of development.

Expression of multiple integrins and extracellular matrix components by C6 glioma cells

We have investigated the expression of integrins in C6 glioma, a chemically-induced glial tumor cell line from rat brain. Immunochemical analysis revealed that C6 cells express sets of integrin receptor complexes which immunologically and electrophoretically are indistinguishable from those expressed by normal rat skin fibroblasts. These include the well-characterized fibronectin (alpha 5 beta 1) and the multi-specific laminin, collagen and fibronectin (alpha 3 beta 1) receptors. Assay of cell adhesion indicated that C6 cells adhere to fibronectin-coated surfaces or matrix deposited by the C6 glioma cells (CGM) in an RGD- and divalent cation-dependent fashion. However, anti-fibronectin antibodies, which are able to inhibit fibroblast adhesion to fibronectin, did not inhibit adhesion of the C6 cells to fibronectin or CGM. This may reflect differences in functional properties and/or distribution patterns of integrins in C6 cells and normal fibroblasts.

Differentiation of myoblasts and CNS cells grown either separately or as co-cultures on microcarriers

Dispersed neuronal and muscular elements from fetal or neonatal origin, can organize and mature in culture when grown on positively charged cylindrical microcarriers (MCS), to a stage which simulate in vivo maturation. Cells arrange themselves on the MCS to form aggregates which remain floating in the nutrient medium. In such a tridimensional organization, the neuronal tissue is capable of regenerating a network of nerve fibers which establish synapse interconnections and undergo myelination. Oligodendrocytes organize on MCS in a tridimensional pattern and produce extensive myelin-like membranes. Myoblasts in MC-cultures fuse into polynucleated myotubes which become striated and contract spontaneously. Creatine kinase and acetylcholine receptor (AChR) are formed during myogenesis in similar quantities in MC-cultures and in monolayers. When both neuronal and muscle tissues are prepared from the same fetus (autologous nerve-muscle co-cultures) and are cultured on MCS, they interconnect to form neuro-muscular junctions. Cells from both tissues, exhibit better differentiation, for longer periods in MC-cultures than they do in monolayers. The floating functional entities are easy to sample and can be harvested for ultrastructural, immunocytochemical and biochemical analysis. In addition, MC-cultures can be used as a good tool for the study of acute and chronic exposures to toxicological agents, as well as for implantation into demyelinated, injured or dystrophic tissues. In this case the MCS in the implanted entities will serve as identifiable markers.

Laminin and s-laminin are produced and released by astrocytes, Schwann cells, and schwannomas in culture

Components of the extracellular matrix (ECM) have been implicated in the regulation of neuronal migration, axonal growth, and synaptogenesis. We have examined cultures of glial cells, Schwann cells, and schwannomas for the expression of two components of the ECM, laminin and s-laminin, using immunohistochemical and Western blot techniques. Laminin is a potent promotor of neurite outgrowth in cultures of both central and peripheral neurons, and is present in all ECMs. In contrast, s-laminin (for synaptic laminin), a recently described homolog of laminin, is highly localized at the neuromuscular synaptic cleft (Sanes and Chiu, Cold Spring Harbor Symp. Quant. Biol. 1983;48:667-678; Chiu and Sanes, Dev. Biol. 1984;103:456-467) and shows selective adhesivity for motor neurons (Hunter et al. Cell 1989;59:905-913). While the distribution of these ECM components have been well documented in situ, the sources of these extracellular molecules are unclear. We report that astrocytes cultured in serum-free medium maintain an organized ECM that only bears laminin immunoreactivity; s-laminin appears to be sequestered intracellularly. However, both molecules are found in the astrocyte conditioned medium. Thus, under these growth conditions, astrocytes produce and release laminin and s-laminin, but only incorporate the former into an ECM. In contrast, neither molecule is present in comparable cultures of oligodendrocytes. Although no established ECM is seen in cultures of Schwann cells or schwannomas, laminin and s-laminin immunoreactivity are present within cells and in the conditioned media. These results indicate that certain populations of non-neuronal support cells and cell lines can produce and release both synaptic and extrasynaptic components of the ECM. The assembly of these different molecules into an organized basal lamina may require the presence of additional factors or interaction with neurons.

Fetal alcohol delays the developmental expression of myelin basic protein and transferrin in rat primary oligodendrocyte cultures

This study has examined the development of immunoreactive myelin basic protein and transferrin in primary glial cell cultures. Cultures were initiated from control and experimental Sprague-Dawley rats 1-2 days postnatally. Experimental treatment involved exposure to 5% (w/v) ethanol in a liquid diet during the last two weeks of gestation. Prenatal alcohol administration delayed the expression of myelin basic protein and transferrin during the first three weeks postnatally. Other oligodendroglial and astroglial markers were little affected, if at all, by fetal alcohol exposure.

Glass micro-fibers: a model system for study of early events in myelination

A system was developed to analyze early events in the process of myelination. Primary cultures of rat oligodendrocytes were maintained in the presence of glass micro-fibers which served as artificial axons. A culture chamber was constructed which allowed the close apposition of fibers and cells in a three-dimensional arrangement designed to resemble an in vivo environment. Cells cultured in the presence of glass micro-fibers coated with a glial cell matrix extract were induced to organize into clusters around the fibers. Examination of oligodendrocyte-fiber sandwiches by SEM revealed the presence of a number of cell contacts with the fibers. TEM images showed that, in most cases, fibers were surrounded by the cells and not multiply wrapped. Only occasionally was a loose wrapping of cell membrane observed around the fibers. Cells cultured in the presence of matrix-coated glass micro-fibers showed an increased production of sulfolipids that was at least partially dependent on the presence of the matrix coating. Coating of these "artificial axons" may aid in the identification of signal molecules produced by neurons which enable them to be myelinated.

P0 mRNA expression in cultures of Schwann cells and neurons that lack basal lamina and myelin

Schwann cells of the peripheral nervous system depend on the presence of both axons and basal lamina to achieve a myelinating phenotype. Furthermore, removal of axonal influence results in the cessation of myelination and down-regulation of myelin protein expression by Schwann cells. Here we examine whether both axons and basal lamina are required by Schwann cells for the expression of mRNA encoding the major myelin glycoprotein, P0. Cultures of Schwann cells and neurons obtained from dorsal root ganglia of 15 day embryonic rat pups were grown for up to 20 days in vitro under conditions that either allowed or prohibited basal lamina and myelin formation. These cultures were assayed for the expression of P0 mRNA by using an S1 nuclease-protection assay. After 20 days in vitro, the cultures that did not assemble basal lamina and that were incapable of myelin formation expressed P0 mRNA at a level which was comparable to that seen in identically aged, myelinating cultures. Both the myelinating and nonmyelinating cultures demonstrated an appreciable increase in P0 mRNA when compared to the starting embryonic dorsal root ganglia Schwann cells. The latter had a low, but detectable, level of mRNa for this myelin glycoprotein. The cultures that were devoid of basal lamina and myelin showed a clear increase in P0 mRNA by 11-15 days in culture. This increase in expression depended on the presence of neurons/neurites, since Schwann cells which were grown in neuron-depleted cultures expressed little, if any, P0 mRNA. In contrast to the levels of P0 mRNA, the nonmyelinating cultures had a significantly lower amount of P0 glycoprotein than did the cultures which assemble myelin. This suggests that the nonmyelinating Schwann cells regulate the level of this glycoprotein at the translational and/or the posttranslational level. The data presented here suggest that myelin protein mRNA expression and myelin assembly by Schwann cells are separable events, with the former depending on one or more neuronal/axonal factors.

Neuronal modulation of calcium channel activity in cultured rat astrocytes

The patch-clamp technique was used to study whether cocultivation of neurons and astrocytes modulates the expression of calcium channel activity in astrocytes. Whole-cell patch-clamp recordings from rat brain astrocytes cocultured with rat embryonic neurons revealed two types of voltage-dependent inward currents carried by Ca2+ and blocked by either Cd2+ or Co2+ that otherwise were not detected in purified astrocytes. This expression of calcium channel activity in astrocytes was neuron dependent and was not observed when astrocytes were cocultured with purified oligodendrocytes.

Expression of beta-nerve growth factor in cultured cells derived from the hypothalamus and cerebral cortex

Although the synthesis of nerve growth factor (NGF) in brain regions innervated by magnocellular cholinergic neurons of the basal forebrain is well documented, the cell type(s) able to produce NGF in the central nervous system (CNS) remain only partially characterized. Moreover, little is known regarding the ability of brain areas not innervated by magnocellular cholinergic neurons to express NGF protein. The hypothalamus, which controls the endocrine system, is one of such regions. Primary culture of mixed populations of cells from the fetal hypothalamus were used to identify the presence of NGF in this brain area. Immunocytochemistry revealed that hypothalamic oligodendrocytes and a subpopulation of neurons expressed the NGF protein. In contrast, astrocytes were either immunonegative or equivocally stained. To define whether synthesis of NGF is restricted to a particular cell type, cultures of purified astrocytes, oligodendrocyte progenitor (oligoP) cells and neurons were utilized. They were obtained from the neonatal cerebral cortex to ensure an adequate yield of glial cells. Virtually the entire population of cerebral oligoP cells were found to express NGF protein. In contrast, and similar to hypothalamic astrocytes, cerebral type I astrocytes isolated at the same time as oligoP cells exhibited little or no NGF staining. When type I astrocytes were induced to differentiate in the presence of a serumless, chemically defined medium, a subpopulation of the culture became more robustly positive for the NGF protein. Contrasting with these differences in NGF immunoreactivity, Northern analysis of RNA isolated from purified cerebral type I astrocytes, oligoP cells and neurons demonstrated that NGF mRNA was expressed in each of these cell types at approximately the same levels. The results indicate that: (a) when placed in culture, each of the major cell types within the CNS has the capability of transcribing the NGF gene, and (b) despite similar NGF mRNA levels the cellular content of NGF protein is greater in a subpopulation of neurons and in oligodendrocytes than in astrocytes, suggesting differences in NGF post-transcriptional regulation between these cell types. In addition, the presence of NGF in hypothalamic cells suggests that NGF may be involved in the regulation of specific hypothalamic neuronal systems.

Regulation of cerebroside sulfotransferase activity in cultured oligodendrocytes: effect of growth factors and insulin

Cerebroside sulfotransferase (EC 2.8.2.11, CST) specific activity has been determined in oligodendrocyte (OL)-enriched glial cell cultures from newborn rat brain grown in serum supplemented medium. This activity is detectable at 5 days in vitro (DIV) and reaches its maximum value at 12 DIV. This period corresponds to that of oligodendrocyte precursor proliferation in these cultures. The activity decreases thereafter and remains nearly constant after 24 DIV. The developmental curve of CST activity is parallel in pure oligodendrocyte subcultures but twice higher than in primary cultures. These data confirm that CST is highly enriched in OL. Basic fibroblast growth factor (bFGF) (15 ng/ml) and platelet derived growth factor (PDGF) (0.75 U/ml) both enhance CST activity by 90% and 72%, respectively. This increase is in the same range than that of DNA content in treated cultures, whereas protein increase is smaller (50% and 22%, respectively). In contrast, transforming growth factor beta 1 (TGF beta 1, 0.5 and 5 ng/ml) does not significantly enhance CST activity nor DNA content of OL cultures. Insulin at high concentrations (5 micrograms/ml) also enhances CST activity but has no effect at physiological concentrations (20 ng/ml). These results show that CST activity can be controlled by growth factors. They suggest that CST activity is more closely related to OL and OL precursor proliferation than to myelination itself since its maximal activity preceeds myelination in vitro.

Myelination of axons within cytosine arabinoside treated mouse cerebellar explants by cultured rat oligodendrocytes

Cell suspensions of cultured purified rat oligodendrocytes prepared by the differential substrate adhesion method were applied to neonatal mouse cerebellar explant cultures in which myelination and oligodendrocyte maturation had been irreversibly inhibited by exposure to cytosine arabinoside. Myelination of Purkinje cell axons within 92% of the host explants was observed 2-5 days after oligodendrocyte application. Ultrastructurally, mature oligodendrocytes and axons surrounded by compact myelin, as well as spherules of compact myelin membranes without axons, were present within the cerebellar explants. It is evident that cultured dissociated purified oligodendrocytes retain the ability to myelinate appropriate axons. Such oligodendrocytes may be hyperreactive with regard to myelin membrane formation, as suggested by the presence of spheres of compact myelin without axons.

Myelin-oligodendrocyte glycoprotein (MOG) is a surface marker of oligodendrocyte maturation

The myelin-oligodendrocyte glycoprotein (MOG) is a minor component of central nervous system myelin. Using neonatal rat optic nerve oligodendrocyte cultures we have compared the development in vitro of MOG with galactocerebroside, myelin basic protein and 2' ,3'-cyclic-nucleotide 3'-phosphodiesterase. MOG appears on the surface of oligodendrocytes 1-2 days later than these other oligodendrocyte markers, suggesting that MOG may be a useful indicator of oligodendrocyte maturation. The relevance of these findings for investigating mechanisms of myelin injury in vitro and the role of oligodendrocyte damage in demyelinating disease is discussed.

Region-specific appearance of myelin constituents in the developing rat spinal cord

The appearance of myelin-specific glycolipids and of myelin basic protein was studied with regard to the detailed anatomy of the rat cervical spinal cord. The expression of these constituents in particular fibre tracts and regions occurs at specific times of development between postnatal days 1 and 14. This mosaic-like appearance started in the ventral funiculus (day 1) followed by fasciculus cuneatus and ventro-lateral funiculus (day 2), and fasciculus gracilis and dorso-lateral funiculus (days 3 to 4). Cortico-spinal tract (day 11), Lissauer tract (day 14) and the commissures started to acquire myelin very late. In the grey matter, myelin constituents appeared around days 11 to 14 in a patchy pattern. These results support a concept of highly local interactions regulating oligodendrocyte differentiation. In addition, a general rostro-caudal gradient of myelin development exists in the spinal cord, which is independent of the ascending or descending nature of the fibre tracts. Appearance of myelin constitutents in the caudal spinal cord was not prevented by a neonatal transection at mid-thoracic levels.

Immunohistochemical localization of myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphohydrolase in flattened membrane expansions produced by cultured oligodendrocytes

Oligodendrocytes, the cells responsible for myelin sheath formation in the central nervous system, were isolated from primary dissociated mixed glial cultures prepared from newborn mouse forebrain, and further cultured in a serum-free defined culture medium. Single and double indirect immunofluorescence using antibodies against the myelin glycolipids, galactocerebroside and sulfatide, and the myelin proteins, myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphohydrolase, was used to investigate the composition of the flat membrane extensions produced by some oligodendrocytes in culture. Galactocerebroside and sulfatide were both expressed on the external surface of the plasma membrane of oligodendrocyte cell bodies and processes and also the membrane expansions. Neither myelin basic protein nor 2',3'-cyclic nucleotide 3'-phosphohydrolase were expressed on the external surface of oligodendrocytes. Myelin basic protein could be localized to the cell body and the membrane expansions but not the major and fine processes. The localization of these myelin components suggests that the expansions have characteristics of the mature myelin membrane. 2',3'-Cyclic nucleotide 3'-phosphohydrolase was found to be localized in the cell body, and in total contrast to myelin basic protein, in the major processes and the fine interconnecting processes, but not the membrane expansions. In some of the cells 2',3'-cyclic nucleotide 3'-phosphohydrolase was present at the outer extremities of the flat membrane sheets, giving the appearance of an extending growth region. Our results thus clearly show that 2',3'-cyclic nucleotide 3'-phosphohydrolase is localized within oligodendrocytes in discrete regions of plasma membranes and suggest that this protein has a possible role in the early stages of myelin formation.

Differential expression of galactocerebroside, myelin basic protein, and 2',3'-cyclic nucleotide 3'-phosphohydrolase during development of oligodendrocytes in vitro

This paper describes the differential expression and localization of myelin components within membrane sheets produced by oligodendrocytes in vitro. In double-labeling experiments using antibodies to the myelin antigens 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and galactocerebroside (GC), the two antigens were coexpressed in at least 95% of oligodendrocytes at all ages examined. A small population of relatively undifferentiated cells expressed one antigen before the other. Within the membrane sheets produced by the cultured cells, CNP and GC are distributed differently. CNP is highly concentrated in cell bodies, in a network of processes extending from the cell body into the sheets, and around the perimeter of the sheets. CNP staining cannot be detected in some areas within the body of the sheet. When present, it is of low intensity. Under our labeling conditions, GC staining is found throughout the membrane sheets, except in the network of veins which are CNP+. GC and myelin basic protein (MBP) staining are seen in similar membrane domains even though GC is a surface component while MBP resides on the cytoplasmic face. Both the timing and localization of CNP immunostaining show that CNP is as early a marker for oligodendrocytes as GC, and support the idea that CNP may play a structural role in the myelin membrane. Double-labeling studies with GC and CNP antibodies also show that the true shape of a cell and the extent of its development are not always revealed by a single antigen. The differential distribution of antigens within membrane sheets illustrates that they contain areas of structural specialization that may reflect the situation in intact myelin.

Myelin basic protein and transferrin characterize different subpopulations of oligodendrocytes in rat primary glial cultures

The iron transport glycoprotein, transferrin (Tf), localizes exclusively in oligodendrocytes in brain tissue sections. Previously, we showed that Tf is also expressed in oligodendrocytes in primary cultures established from newborn rat brains. Its developmental appearance precedes that of galactocerebroside (GC). In this study, Tf expression in primary brain cell cultures was investigated over a 4-week period in relation to GC and myelin basic protein (MBP), respectively, early and late markers of oligodendrocyte development. From 9 days in vitro and thereafter, all Tf+ cells were also found to be GC+. With increasing age the number of Tf+ cells decreased while the number of MBP+ cells increased. However, less than 10% of oligodendrocytes co-expressed Tf and MBP at any age. MBP+ cells were largely found in cell clusters which increased in size and number with age in culture. Interestingly, Tf+ cells were located around the clusters of MBP+ cells which displayed elaborate branched processes. The transient expression of Tf in oligodendrocytes which become MBP+, suggests a role for Tf in the early stages of myelinogenesis. The results also demonstrate the existence of three phenotypically distinct populations of oligodendrocytes. A new model of developmental and functional subpopulations of oligodendrocytes is proposed.

Evidence that an RGD-dependent receptor mediates the binding of oligodendrocytes to a novel ligand in a glial-derived matrix

A simple adhesion assay was used to measure the interaction between rat oligodendrocytes and various substrata, including a matrix secreted by glial cells. Oligodendrocytes bound to surfaces coated with fibronectin, vitronectin and a protein component of the glial matrix. The binding of cells to all of these substrates was inhibited by a synthetic peptide (GRGDSP) modeled after the cell-binding domain of fibronectin. The component of the glial matrix responsible for the oligodendrocyte interaction is a protein which is either secreted by the glial cells or removed from serum by products of these cultures; serum alone does not promote adhesion to the same extent as the glial-derived matrix. The interaction of cells with this glial-derived matrix requires divalent cations and is not mediated by several known RGD-containing extracellular proteins, including fibronectin, vitronectin, thrombospondin, type I and type IV collagen, and tenascin.

RGD-containing peptides inhibit the synthesis of myelin-like membrane by cultured oligodendrocytes

A synthetic peptide derived from the fibronectin cell-binding domain, GRGDSP, inhibits the adhesion of rat oligodendrocytes to a number of substrates. However, while GRGDSP inhibited the adhesion of cells in a short term adhesion assay, the presence of the peptide did not prevent cells from adhering and thriving in longer term culture. The morphological characteristics of individual cells cultured with 0.1 mg/ml GRGDSP were similar to untreated cultures; small rounded cell bodies radiating numerous fine processes. Peptide-treated cultures were inhibited in their ability to produce myelin specific components. The characteristic developmental peak in sulfolipid synthesis which occurs both in vivo and in vitro was completely inhibited when cells were cultured with GRGDSP. In addition, the synthesis of myelin basic protein was inhibited. Ultrastructurally, cells treated with GRGDSP showed a greatly reduced number of multilamellar myelin-like membrane figures than cells grown without peptide or those grown with GRADSP. Cultured oligodendrocytes did not become sensitive to inhibition of sulfolipid synthesis by GRGDSP until a period immediately preceding the peak in sulfolipid biosynthesis. The effects of pretreatment with peptide for 5 d before this time were completely reversible. Pretreatment which extended into the time of peak myelin synthesis resulted in permanent impairment in the cell's ability to synthesize sulfolipid. The oligodendrocyte's ability to synthesize a myelin-like membrane in culture is, in part, inherent since it occurs in the absence of neurons. The present results indicate that myelin membrane production is also subject to external control since it appears that occupancy of an RGD-dependent cell surface receptor during a critical period of in vitro development is required for the oligodendrocyte to produce myelin-like membrane.

Serum contains inducers and repressors of oligodendrocyte differentiation

An important stage in oligodendrocyte development is the expression of galactocerebroside (GC), the major glycolipid in myelin. Although oligodendrocyte cell lineage and differentiation in vitro have been the object of many studies, to date there is sparse information on the regulation of GC expression in oligodendrocytes already committed to be positive for GC. We report here that GC expression in these cells is controlled by three serum factors. Two of these, possibly a lipoprotein and a mucoprotein, increase GC levels, whereas the third, probably a glycoprotein, exerts an inhibitory effect. The developmental increase of GC in postnatal rat brain cerebral cultures and its induction by serum factors are reversible phenomena. The isolation of the GC-regulatory factors would allow experimental manipulation of impaired GC expression by differentiated oligodendrocytes.

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.