Relation between axons and oligodendroglial cells during initial myelination. I. The glial unit

Río-Hortega's drawings revisited with fluorescent protein defines a cytoplasm-filled channel system of CNS myelin

A century ago this year, Pío del Río-Hortega (1921) coined the term 'oligodendroglia' for the 'interfascicular glia' with very few processes, launching an extensive discovery effort on his new cell type. One hundred years later, we review his original contributions to our understanding of the system of cytoplasmic channels within myelin in the context of what we observe today using light and electron microscopy of genetically encoded fluorescent reporters and immunostaining. We use the term myelinic channel system to describe the cytoplasm-delimited spaces associated with myelin; being the paranodal loops, inner and outer tongues, cytoplasm-filled spaces through compact myelin and further complex motifs associated to the sheath. Using a central nervous system myelinating cell culture model that contains all major neural cell types and produces compact myelin, we find that td-tomato fluorescent protein delineates the myelinic channel system in a manner reminiscent of the drawings of adult white matter by Río-Hortega, despite that he questioned whether some cytoplasmic figures he observed represented artefact. Together, these data lead us to propose a slightly revised model of the 'unrolled' sheath. Further, we show that the myelinic channel system, while relatively stable, can undergo subtle dynamic shape changes over days. Importantly, we capture an under-appreciated complexity of the myelinic channel system in mature myelin sheaths.

Human Muse cells-derived neural precursor cells as the novel seed cells for the repair of spinal cord injury

At present, stem cell transplantation has a significant therapeutic effect on spinal cord injury (SCI), however, it is still challenging for the seed cells selection. In this study, in order to explore cells with wide neural repair potentials, we selected the pluripotent stem cells multilineage-differentiating stress-enduring (Muse) cells, inducing the in vitro differentiation of human Muse cells into neural precursor cells (Muse-NPCs) by applying neural induction medium. Here, we found induced Muse-NPCs expressed neural stem cell markers Nestin and NCAM, capable of differentiating into three types of neural cells (neuron, astrocyte and oligodendrocyte), and have certain biological functions. When Muse-NPCs were transplanted into rats suffering from T10 SCI, motor function was improved. These results provide an insight for application of Muse-NPCs in cell therapy or tissue engineering for the repair of SCI in future.

The role of sleep and wakefulness in myelin plasticity

Myelin plasticity is gaining increasing recognition as an essential partner to synaptic plasticity, which mediates experience-dependent brain structure and function. However, how neural activity induces adaptive myelination and which mechanisms are involved remain open questions. More than two decades of transcriptomic studies in rodents have revealed that hundreds of brain transcripts change their expression in relation to the sleep-wake cycle. These studies consistently report upregulation of myelin-related genes during sleep, suggesting that sleep represents a window of opportunity during which myelination occurs. In this review, we summarize recent molecular and morphological studies detailing the dependence of myelin dynamics after sleep, wake, and chronic sleep loss, a condition that can affect myelin substantially. We present novel data about the effects of sleep loss on the node of Ranvier length and provide a hypothetical mechanism through which myelin changes in response to sleep loss. Finally, we discuss the current findings in humans, which appear to confirm the important role of sleep in promoting white matter integrity.

The adult oligodendrocyte can participate in remyelination

Endogenous remyelination of the CNS can be robust and restore function, yet in multiple sclerosis it becomes less complete with time. Promoting remyelination is a major therapeutic goal, both to restore function and to protect axons from degeneration. Remyelination is thought to depend on oligodendrocyte progenitor cells, giving rise to nascent remyelinating oligodendrocytes. Surviving, mature oligodendrocytes are largely regarded as being uninvolved. We have examined this question using two large animal models. In the first model, there is extensive demyelination and remyelination of the CNS, yet oligodendrocytes survive, and in recovered animals there is a mix of remyelinated axons interspersed between mature, thick myelin sheaths. Using 2D and 3D light and electron microscopy, we show that many oligodendrocytes are connected to mature and remyelinated myelin sheaths, which we conclude are cells that have reextended processes to contact demyelinated axons while maintaining mature myelin internodes. In the second model in vitamin B₁₂-deficient nonhuman primates, we demonstrate that surviving mature oligodendrocytes extend processes and ensheath demyelinated axons. These data indicate that mature oligodendrocytes can participate in remyelination.

Reduced white matter MRI transverse relaxation rate in cognitively normal H63D-HFE human carriers and H67D-HFE mice

Mutations within the HFE protein gene sequence have been associated with increased risk of developing a number of neurodegenerative disorders. To this effect, an animal model has been created which incorporates the mouse homologue to the human H63D-HFE mutation: the H67D-HFE knock-in mouse. These mice exhibit alterations in iron management proteins, have increased neuronal oxidative stress, and a disruption in cholesterol regulation. However, it remains undetermined how these differences translate to human H63D carriers in regards to white matter (WM) integrity. To this endeavor, MRI transverse relaxation rate (R₂) parametrics were employed to test the hypothesis that WM alterations are present in H63D human carriers and are recapitulated in the H67D mice. H63D carriers exhibit widespread reductions in brain R₂ compared to non-carriers within white matter association fibers in the brain. Similar R₂ decreases within white matter tracts were observed in the H67D mouse brain. Additionally, an exacerbation of age-related R₂ decrease is found in the H67D animal model in white matter regions of interest. The decrease in R₂ within white matter tracts of both species is speculated to be multifaceted. The R₂ changes are hypothesized to be due to alterations in axonal biochemical tissue composition. The R₂ changes observed in both the human-H63D and mouse-H67D data suggest that modified white matter myelination is occurring in subjects with HFE mutations, potentially increasing vulnerability to neurodegenerative disorders.

Imaging Myelination In Vivo Using Transparent Animal Models

Myelination by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system is essential for nervous system function and health. Despite its importance, we have a relatively poor understanding of the molecular and cellular mechanisms that regulate myelination in the living animal, particularly in the CNS. This is partly due to the fact that myelination commences around birth in mammals, by which time the CNS is complex and largely inaccessible, and thus very difficult to image live in its intact form. As a consequence, in recent years much effort has been invested in the use of smaller, simpler, transparent model organisms to investigate mechanisms of myelination in vivo. Although the majority of such studies have employed zebrafish, the Xenopus tadpole also represents an important complementary system with advantages for investigating myelin biology in vivo. Here we review how the natural features of zebrafish embryos and larvae and Xenopus tadpoles make them ideal systems for experimentally interrogating myelination by live imaging. We outline common transgenic technologies used to generate zebrafish and Xenopus that express fluorescent reporters, which can be used to image myelination. We also provide an extensive overview of the imaging modalities most commonly employed to date to image the nervous system in these transparent systems, and also emerging technologies that we anticipate will become widely used in studies of zebrafish and Xenopus myelination in the near future.

A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

Advances in brain connectomics set the need for detailed knowledge of functional properties of myelinated and non-myelinated (if present) axons in specific white matter pathways. The corpus callosum (CC), a major white matter structure interconnecting brain hemispheres, is extensively used for studying CNS axonal function. Unlike another widely used CNS white matter preparation, the optic nerve where all axons are myelinated, the CC contains also a large population of non-myelinated axons, making it particularly useful for studying both types of axons. Electrophysiological studies of optic nerve use suction electrodes on nerve ends to stimulate and record compound action potentials (CAPs) that adequately represent its axonal population, whereas CC studies use microelectrodes (MEs), recording from a limited area within the CC. Here we introduce a novel robust isolated "whole" CC preparation comparable to optic nerve. Unlike ME recordings where the CC CAP peaks representing myelinated and non-myelinated axons vary broadly in size, "whole" CC CAPs show stable reproducible ratios of these two main peaks, and also reveal a third peak, suggesting a distinct group of smaller caliber non-myelinated axons. We provide detailed characterization of "whole" CC CAPs and conduction velocities of myelinated and non-myelinated axons along the rostro-caudal axis of CC body and show advantages of this preparation for comparing axonal function in wild type and dysmyelinated shiverer mice, studying the effects of temperature dependence, bath-applied drugs and ischemia modeled by oxygen-glucose deprivation. Due to the isolation from gray matter, our approach allows for studying CC axonal function without possible "contamination" by reverberating signals from gray matter. Our analysis of "whole" CC CAPs revealed higher complexity of myelinated and non-myelinated axonal populations, not noticed earlier. This preparation may have a broad range of applications as a robust model for studying myelinated and non-myelinated axons of the CNS in various experimental models.

In vitro myelin formation using embryonic stem cells

Myelination in the central nervous system is the process by which oligodendrocytes form myelin sheaths around the axons of neurons. Myelination enables neurons to transmit information more quickly and more efficiently and allows for more complex brain functions; yet, remarkably, the underlying mechanism by which myelination occurs is still not fully understood. A reliable in vitro assay is essential to dissect oligodendrocyte and myelin biology. Hence, we developed a protocol to generate myelinating oligodendrocytes from mouse embryonic stem cells and established a myelin formation assay with embryonic stem cell-derived neurons in microfluidic devices. Myelin formation was quantified using a custom semi-automated method that is suitable for larger scale analysis. Finally, early myelination was followed in real time over several days and the results have led us to propose a new model for myelin formation.

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

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

Oligodendrocyte-protection and remyelination post-spinal cord injuries: a review

In the past four decades, the main focus of investigators in the field of spinal cord regeneration has been to devise therapeutic measures that enhance neural regeneration. More recently, emphasis has been placed on enhancing remyelination and providing oligodendrocyte-protection after a spinal cord injury (SCI). Demyelination post-SCI is part of the cascading secondary injury that takes place immediately after the primary insult; therefore, therapeutic measures are needed to reduce oligodendrocyte death and/or enhance remyelination during the acute stage, preserving neurological functions that would be lost otherwise. In this review a thorough investigation of the oligodendrocyte-protective and remyelinative molecular therapies available to date is provided. The advent of new biomaterials shown to promote remyelination post-SCI is discussed mainly in the context of a combinatorial approach where the biomaterial also provides drug delivery capabilities. The aim of these molecular and biomaterial-based therapies is twofold: (1) oligodendrocyte-protective therapy, which involves protecting already existing oligodendrocytes from undergoing apoptosis/necrosis; and (2) inductive remyelination, which involves harnessing the remyelinative capabilities of endogenous oligodendrocyte precursor cells (OPCs) at the lesion site by providing a suitable environment for their migration, survival, proliferation and differentiation. From the evidence reported in the literature, we conclude that the use of a combinatorial approach including biomaterials and molecular therapies would provide advantages such as: (1) sustained release of the therapeutic molecule, (2) local delivery at the lesion site, and (3) an environment at the site of injury that promotes OPC migration, differentiation and remyelination.

Individual axons regulate the myelinating potential of single oligodendrocytes in vivo

The majority of axons in the central nervous system (CNS) are eventually myelinated by oligodendrocytes, but whether the timing and extent of myelination in vivo reflect intrinsic properties of oligodendrocytes, or are regulated by axons, remains undetermined. Here, we use zebrafish to study CNS myelination at single-cell resolution in vivo. We show that the large caliber Mauthner axon is the first to be myelinated (shortly before axons of smaller caliber) and that the presence of supernumerary large caliber Mauthner axons can profoundly affect myelination by single oligodendrocytes. Oligodendrocytes that typically myelinate just one Mauthner axon in wild type can myelinate multiple supernumerary Mauthner axons. Furthermore, oligodendrocytes that exclusively myelinate numerous smaller caliber axons in wild type can readily myelinate small caliber axons in addition to the much larger caliber supernumerary Mauthner axons. These data indicate that single oligodendrocytes can myelinate diverse axons and that their myelinating potential is actively regulated by individual axons.

Conditional Tet-regulated over-expression of Hoxa2 in CG4 cells increases their proliferation and delays their differentiation into oligodendrocyte-like cells expressing myelin basic protein

Hoxa2 gene was reported to be expressed by oligodendrocytes (OLs) and down-regulated at the terminal differentiation stage during oligodendrogenesis in mice (Nicolay et al. 2004b). To further investigate the role of Hoxa2 in oligodendroglial development, a tetracycline regulated controllable expression system was utilized to establish a stable cell line (CG4-SHoxa2 [sense Hoxa2]), where the expression level of Hoxa2 gene could be up-regulated. The impact of Hoxa2 over-expression on the proliferation and differentiation of CG4-SHoxa2 cells was investigated. Up-regulation of Hoxa2 increased the proliferation of CG4-SHoxa2 cells. The mRNA levels of PDGFαR (platelet-derived growth factor [PDGF] alpha receptor), which is expressed by OL progenitor cells, were not different in CG4-SHoxa2 cells compared to wild-type CG4 cells. Semi-quantitative RT-PCR revealed that the mRNA levels of myelin basic protein (MBP) was lower in CG4-SHoxa2 cells than in wild-type CG4 cells indicating the differentiation of CG4-SHoxa2 cells was delayed when the Hoxa2 gene was up-regulated.

Disposition of axonal caspr with respect to glial cell membranes: Implications for the process of myelination

Neurofascin-155 (NF155) and caspr are transmembrane proteins found at discrete locations early during development of the nervous system. NF155 is present in the oligodendrocyte cell body and processes, whereas caspr is on the axonal surface. In mature nerves, these proteins are clustered at paranodes, flanking the node of Ranvier. To understand how NF155 and caspr become localized to the paranodal regions of myelinated nerves, we have studied their distribution over time in myelinating cultures. Our observations indicate that these two proteins are recruited to the cell surface at the contact zone between axons and oligodendrocytes, where they trans-interact. This association explains the early pattern of caspr distribution, a helical coil that winds around the axon, resembling the turns of the myelin sheath. Caspr, an axonal membrane protein, therefore seems to move in register with the overlying myelinating cell via its interactions with myelin proteins. We suggest that NF155 is the glial cell membrane protein responsible for caspr distribution. The pair act as interacting partners on either side of the axoglial contact area. Most likely, there are other proteins on the axonal surface whose distribution is equally influenced by interaction with the nascent myelin sheath. The fact that caspr follows the movement of the spiraling membrane has a direct affect on the interpretation of the way in which myelin is formed.

Human oligodendrocytes express Cx31.3: function and interactions with Cx32 mutants

Murine oligodendrocytes express the gap junction (GJ) proteins connexin32 (Cx32), Cx47, and Cx29. CNS phenotypes in patients with X-linked Charcot-Marie-Tooth disease may be caused by dominant effects of Cx32 mutations on other connexins. Here we examined the expression of Cx31.3 (the human ortholog of murine Cx29) in human brain and its relation to the other oligodendrocyte GJ proteins Cx32 and Cx47. Furthermore, we investigated in vitro whether Cx32 mutants with CNS manifestations affect the expression and function of Cx31.3. Cx31.3 was localized mostly in the gray matter along small myelinated fibers similar to Cx29 in rodent brain and was co-expressed with Cx32 in a subset of human oligodendrocytes. In HeLa cells Cx31.3 was localized at the cell membrane and appeared to form hemichannels but no GJs. Cx32 mutants with CNS manifestations were retained intracellularly, but did not alter the cellular localization or function of co-expressed Cx31.3. Thus, Cx31.3 shares many characteristics with its ortholog Cx29. Cx32 mutants with CNS phenotypes do not affect the trafficking or function of Cx31.3, and may have other toxic effects in oligodendrocytes.

Type III neuregulin-1 promotes oligodendrocyte myelination

The axonal signals that regulate oligodendrocyte myelination during development of the central nervous system (CNS) have not been established. In this study, we have examined the regulation of oligodendrocyte myelination by the type III isoform of neuregulin-1 (NRG1), a neuronal signal essential for Schwann cell differentiation and myelination. In contrast to Schwann cells, primary oligodendrocytes differentiate normally when cocultured with dorsal root ganglia (DRG) neurons deficient in type III NRG1. However, they myelinate type III NRG1-deficient neurites poorly in comparison to wild type cultures. Type III NRG1 is not sufficient to drive oligodendrocyte myelination as sympathetic neurons are not myelinated even with lentiviral-mediated expression of NRG1. Mice haploinsufficient for type III NRG1 are hypomyelinated in the brain, as evidenced by reduced amounts of myelin proteins and lipids and thinner myelin sheaths. In contrast, the optic nerve and spinal cord of heterozygotes are myelinated normally. Together, these results implicate type III NRG1 as a significant determinant of the extent of myelination in the brain and demonstrate important regional differences in the control of CNS myelination. They also indicate that oligodendrocyte myelination, but not differentiation, is promoted by axonal NRG1, underscoring important differences in the control of myelination in the CNS and peripheral nervous system (PNS).

Maternal infection and white matter toxicity

Studies examining maternal infection as a risk factor for neurological disorders in the offspring have suggested that altered maternal immune status during pregnancy can be considered as an adverse event in prenatal development. Infection occurring in the mother during the gestational period has been implicated in multiple neurological effects. The current manuscript will consider the issue of immune/inflammatory conditions during prenatal development where adverse outcomes have been linked to maternal systemic infection. The discussions will focus primary on white matter and oligodendrocytes as they have been identified as target processes. This white matter damage occurs in very early preterm infants and in various other human diseases currently being examined for a linkage to maternal or early developmental immune status. The intent is to draw attention to the impact of altered immune status during pregnancy on the offspring for the consideration of such contributing factors to the general assessment of developmental neurotoxicology.

Differential brain growth in the infant born preterm: current knowledge and future developments from brain imaging

Preterm birth is associated with a high prevalence of neuropsychiatric impairment in childhood and adolescence, but the neural correlates underlying these disorders are not fully understood. Quantitative magnetic resonance imaging techniques have been used to investigate subtle differences in cerebral growth and development among children and adolescents born preterm or with very low birth weight. Diffusion tensor imaging and computer-assisted morphometric techniques (including voxel-based morphometry and deformation-based morphometry) have identified abnormalities in tissue microstructure and cerebral morphology among survivors of preterm birth at different ages, and some of these alterations have specific functional correlates. This chapter reviews the literature reporting differential brain development following preterm birth, with emphasis on the morphological changes that correlate with neuropsychiatric impairment.

Unique distributions of the gap junction proteins connexin29, connexin32, and connexin47 in oligodendrocytes

Oligodendrocytes of adult rodents express three different connexins: connexin29 (Cx29), Cx32, and Cx47. In this study, we show that Cx29 is localized to the inner membrane of small myelin sheaths, whereas Cx32 is localized on the outer membrane of large myelin sheaths; Cx29 does not colocalize with Cx32 in gap junction plaques. All oligodendrocytes appear to express Cx47, which is largely restricted to their perikarya. Cx32 and Cx47 are colocalized in many gap junction plaques on oligodendrocyte somata, particularly in gray matter. Cx45 is detected in the cerebral vasculature, but not in oligodendrocytes or myelin sheaths. This diversity of connexins in oligodendrocytes (in different populations of cells and in different subcellular compartments) likely reflects functional differences between these connexins and perhaps the oligodendrocytes themselves.

Rho kinase regulates schwann cell myelination and formation of associated axonal domains

The myelin sheath forms by the spiral wrapping of a glial membrane around an axon. The mechanisms involved are poorly understood but are likely to involve coordinated changes in the glial cell cytoskeleton. Because of its key role as a regulator of the cytoskeleton, we investigated the role of Rho kinase (ROCK), a major downstream effector of Rho, in Schwann cell morphology, differentiation, and myelination. Pharmacologic inhibition of ROCK activity results in loss of microvilli and stress fibers in Schwann cell cultures and strikingly aberrant myelination in Schwann cell-neuron cocultures; there was no effect on Schwann cell proliferation or differentiation. Treated Schwann cells branch aberrantly and form multiple, small, independent myelin segments along the length of axons, each with associated nodes and paranodes. This organization partially resembles myelin formed by oligodendrocytes rather than the single long myelin sheath characteristic of Schwann cells. ROCK regulates myosin light chain phosphorylation, which is robustly, but transiently, activated at the onset of myelination. These results support a key role of Rho through its effector ROCK in coordinating the movement of the glial membrane around the axon at the onset of myelination via regulation of myosin phosphorylation and actomyosin assembly. They also indicate that the molecular machinery that promotes the wrapping of the glial membrane sheath around the axon is distributed along the entire length of the internode.

Three-dimensional electron microscopic studies of the transitional oligodendrocyte associated with the initial stage of myelination in developing rat hippocampal fimbria

We identified the transitional oligodendrocyte and their processes of rat hippocampal fimbria associated with the initial stage of myelination in both the morphological and functional classifications by means of three-dimensional ultrastructural analysis. Transitional oligodendrocytes appeared around P7, and their cell bodies were morphologically an intermediate form between the light and medium oligodendrocytes described by Mori and Leblond [J. Comp. Neurol. 139 (1970) 1]. Three phenotypes of the transitional oligodendrocytic processes were recognized. Spiral wrapping processes were ensheathing processes, club-like processes were nonensheathing processes, and sheet-like processes were possibly the transmuting form between the nonensheathing and ensheathing processes. Club-like processes were the major part of the nonensheathing processes, and most likely function as sensors to perceive axon maturation and find target axons. Multivesicular bodies that appeared to be associated with the initial ensheathment were observed in the transitional oligodendrocytic processes, suggesting that their roles are crucial in myelinogenesis.

Polarized domains of myelinated axons

The entire length of myelinated axons is organized into a series of polarized domains that center around nodes of Ranvier. These domains, which are crucial for normal saltatory conduction, consist of distinct multiprotein complexes of cell adhesion molecules, ion channels, and scaffolding molecules; they also differ in their diameter, organelle content, and rates of axonal transport. Juxtacrine signals from myelinating glia direct their sequential assembly. The composition, mechanisms of assembly, and function of these molecular domains will be reviewed. I also discuss similarities of this domain organization to that of polarized epithelia and present emerging evidence that disorders of domain organization and function contribute to the axonopathies of myelin and other neurologic disorders.

Regulation of oligodendrocyte development in the vertebrate CNS

The vertebrate central nervous system (CNS) contains two major classes of macroglial cells, oligodendrocytes and astrocytes. Oligodendrocytes are responsible for the formation of myelin in the central nervous system, while the functions of astrocytes are more diverse and less well established. Recent studies have provided new insights into when, where and how these different classes of cell arise during CNS development. The founder cells of the oligodendrocyte lineage initially arise in distinct regions of the ventricular zone during early development as the result of local signals including sonic hedgehog. In the spinal cord, oligodendrocyte precursors appear to share a developmental lineage with motor neurons, although they may also develop from restricted glial precursors. Immature oligodendrocyte precursors are highly migratory. They migrate from their site of origin to developing white matter tracts using a variety of guidance cues including diffusible chemorepellents. The majority of oligodendrocyte precursor proliferation occurs in developing white matter as a result of the local expression of mitogenic signals. Oligodendrocyte precursor cell proliferation is regulated by a number of distinct growth factors that act at distinct stages in the lineage and whose activity is modulated by synergy with other molecules including chemokines. The final matching of oligodendrocyte and axon number is accomplished through a combination of local regulation of cell proliferation, differentiation and cell death. Not all oligodendrocyte precursors differentiate during development, and the adult CNS contains a significant population of precursors. Understanding the regulation of oligodendrogenesis will facilitate the use of these endogenous precursors to enhance repair in a variety of pathological conditions.

Fibroblast growth factor-2 inhibits myelin production by oligodendrocytes in vivo

Fibroblast growth factor-2 (FGF-2) controls in part the timely differentiation of oligodendrocytes into the myelin-producing cells of the CNS. However, although differentiated oligodendrocytes express FGF receptors (R), the effect of FGF-2 on myelin-producing oligodendrocytes in vivo was unknown. In the present study, we show that delivery of FGF-2 into the cerebrospinal fluid of anaesthetized rat pups, aged postnatal day (P) 6 to 9, induced a severe loss of myelin in the caudal anterior medullary velum (AMV). Furthermore, we show that the caudal AMV was myelinated at the time of treatment, so the effects of FGF-2 represent a loss of myelin and not delayed differentiation. This was confirmed by injection of platelet-derived growth factor-AA (PDGF-AA), a factor known to affect the differentiation of PDGF-alphaR expressing oligodendrocyte progenitors, but which did not induce myelin loss in the caudal AMV and did not affect differentiated oligodendrocytes, which do not express PDGF-alphaR. Compared to controls treated with saline or PDGF-AA, FGF-2 induced an accumulation of PLP protein and MBP mRNA within the somata of myelin-producing oligodendrocytes. The results indicate that FGF receptor signalling disrupts myelin production in differentiated oligodendrocytes in vivo and interrupted the transport of myelin-related gene products from the oligodendrocyte cell body to their myelin sheaths.

Brevican in the developing hippocampal fimbria: differential expression in myelinating oligodendrocytes and adult astrocytes suggests a dual role for brevican in central nervous system fiber tract development

Brevican is one of the most abundant extracellular matrix proteoglycans in the mammalian brain. We have previously shown that brevican produced by gray matter astrocytes constitutes a major component of perineuronal extracellular matrix in the adult brain. In this paper, we investigate the expression of brevican in the postnatal hippocampal fimbria to explore the role of the proteoglycan in central nervous system fiber tract development. We demonstrate that brevican is expressed by both oligodendrocytes and white matter astrocytes in the fimbria, but the expression of brevican in these two glial cell types is differently regulated during development. At P14, brevican immunoreactivity was observed throughout the fimbria, with particularly strong immunoreactivity in the developing interfascicular glial rows. In situ hybridization showed that oligodendrocytes in the glial rows strongly express brevican during the second and third postnatal weeks. Expression in oligodendrocytes was then down-regulated after P21. In the adult fimbria, no brevican expression was observed in oligodendrocytes. The time window of brevican expression coincides with the phase in which immature oligodendrocytes actively extend membrane processes and enwrap axon fibers. In contrast, the expression in astrocytes started around P21 as oligodendrocytes began to down-regulate the expression. In the adult fimbria, brevican expression was restricted to astrocytes. In situ hybridization with isoform-specific probes and RNase protection assays showed that the authentic, secreted form of brevican, not the glycosylphosphatidylinositol-anchored variant, is the predominant species expressed in the developing fimbria. Our results suggest that brevican plays a dual role in developing and adult fiber tracts.

Intracellular distribution of myelin protein gene products is altered in oligodendrocytes of the taiep rat

Hypomyelination and subsequent demyelination of the taiep rat CNS are thought to result from the abnormal accumulation of microtubules (MTs) in oligodendrocytes that disrupts intracellular transport of components needed to form and maintain the myelin sheath. In this study, myelin gene expression was evaluated in mutant and age-matched controls to determine if MT abnormalities affect the distribution of myelin proteins and their mRNAs. Immunohistochemical analysis of taiep brains and spinal cords revealed a gradual decrease in levels of several myelin proteins including myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase. Accompanying early declines in MAG and PLP, accumulations of immunoreactive products were detected within oligodendrocytes, consistent with a defect in protein trafficking. Northern blot analysis indicated that diminishing protein levels could not be attributed to changes in transcriptional activity, except for MBP of which mRNA levels decreased with age. Cellular localization of MBP mRNA by in situ hybridization further revealed that transcripts were concentrated within oligodendrocyte cell bodies instead of uniformly distributed throughout processes. These results demonstrate that changes in expression and intracellular localization of myelin gene products are concurrent with increases in MT mass in taiep oligodendrocytes and support our hypothesis that cytoskeletal defects prevent the normal transport of elements required for the formation and maintenance of the myelin sheath.

Developing chicken oligodendrocytes express the type IV oligodendrocyte marker T4-O in situ, but not in vitro

Accumulating data suggest that the oligodendrocyte population includes morphological and biochemical subtypes. We recently reported that a polyclonal antiserum against an unknown antigen, the T4-O molecule, labels a subpopulation of chicken oligodendrocytes, obviously representing the type IV variety of Del Rio Hortega. The present study examines the developmental expression of the T4-O molecule in situ and in vitro. The results show that T4-O immunoreactive cells first appear at E15 in the ventral funiculus. But, oligodendrocytes cultured in vitro with or without neurones do not develop a T4-O immunoreactivity. We conclude that oligodendrocytes in the spinal cord of chicken embryos first express the T4-O molecule some time after onset of myelination, and that the T4-O immunoreactive phenotype does not develop in vitro.

Oligodendrocytes and the control of myelination in vivo: new insights from the rat anterior medullary velum

The rat anterior medullary velum (AMV) is representative of the brain and spinal cord, overall, and provides an almost two-dimensional preparation for investigating axon-glial interactions in vivo. Here, we review some of our findings on axon-oligodendrocyte unit relations in our adult, development, and injury paradigms: (1) adult oligodendrocytes are phenotypically heterogeneous, conforming to Del Rio Hortega's types I-IV, whereby differences in oligodendrocyte morphology, metabolism, myelin sheath radial and longitudinal dimensions, and biochemistry correlate with the diameters of axons in the unit; (2) oligodendrocytes derive from a common premyelinating oligodendrocyte phenotype, and divergence of types I-IV is related to the age they emerge and the presumptive diameter of axons in the unit; (3) during myelination, axon-oligodendrocyte units progress through a sequence of maturation phases, related to axon contact, ensheathment, establishment of internodal myelin sheaths, and finally the radial growth and compaction of the myelin sheath; (4) we provide direct in vivo evidence that platelet-derived growth factor-AA (PDGF-AA), fibroblast growth factor (FGF-2), and insulin-like growth factor-I (IGF-I) differentially regulate these events, by injecting the growth factors into the cerebrospinal fluid of neonatal rat pups; (5) in lesioned adult AMV, transected central nervous system (CNS) axons regenerate through the putatively inhibitory environment of the glial scar, but remyelination by oligodendrocytes is incomplete, indicating that axon-oligodendrocyte interactions are defective; and (6) in the adult AMV, cells expressing the NG2 chondroitin sulphate have a presumptive adult oligodendrocyte progenitor antigenic phenotype, but are highly complex cells and send processes to contact axolemma at nodes of Ranvier, suggesting they subserve a specific perinodal function. Thus, axons and oligodendrocyte lineage cells form interdependent functional units, but oligodendrocyte numbers, differentiation, phenotype divergence, and myelinogenesis are governed by axons in the units, mediated by growth factors and contact-dependent signals.

Regenerative potential of adult O1+ oligodendrocytes

Remyelination in the adult central nervous system (CNS) is preceded by the generation of new oligodendrocytes (ODCs) but the source of the new ODCs has not been resolved. Adult galactocerebroside positive (O1+)ODCs proliferate when cultured with purified sensory neurons (Wood and Bunge, Nature 320:756-758, 1999), implying that differentiated ODCs could be an important source of new myelinating ODCs. To test this possibility purified O1+ODCs (>96% purity) were plated at low density (20-50 cells/culture) into cultures of purified dorsal root ganglion neurons. Three days after plating, single O1+ODCs were located (209 ODCs/43 cultures) and sequentially observed for 4 weeks. The ODCs began to proliferate by the fifth day after plating and formed large colonies by the third week. Most cells in these colonies were 01- but positive for another ODC antigen, O4. A few O1+, myelin basic protein (MBP)+ODCs, and glial fibrillary acidic protein (GFAP)+cells with astrocytic morphology were observed in some colonies. In similar cultures plated with cell-sorted O1+ODCs (>99.5% purity), ciliary neurotrophic factor (CNTF, 1ng/ml) increased the number and size of colonies, the number of O1+MBP+ODCs (including ODCs producing myelin-like profiles in association with axons) and the number of GFAP+ astrocytes, relative to untreated controls. The results are evidence that CNTF exerts a trophic effect on adult O1+ODCs, and/or their progeny, and that cells generated by division of O1+ODCs can become either new myelin-producing ODC, or astrocytes. This plasticity in regenerative potential of adult O1+ODCs has not been previously demonstrated.

Contribution of O4+ oligodendrocyte precursors and astrocytes to the glial ensheathment of vessels in the rabbit myelinated streak

The barrier properties and glial ensheathment of blood vessels in the retinal myelinated streak of adult New Zealand White rabbits were characterized at the ultrastructural level by intravascular injection of horseradish peroxidase (HRP) and immuno-electron microscopy with monoclonal antibody O4 and antibodies to glial fibrillary acidic protein (GFAP). Vessels within the myelinated streak did not leak HRP, and they exhibited tight junctions between adjacent endothelial cells. However, unlike their adult counterparts, the retinal blood vessels at postnatal day 18 exhibited substantial endocytotic activity. Both GFAP+ astrocytes and O4+ cells were evident surrounding the preretinal blood vessels of the myelinated streak. Furthermore, O4+ cells exhibited features indicative of high synthetic activity, including a large proportion of extended chromatin and prominent nucleoli within the nucleus, as well as a well-developed Golgi apparatus and numerous mitochondria in the cytoplasm. O4+ cells also exhibited variable quantities of heterochromatin, indicative of early stages of cellular differentiation. These observations are consistent with previous data showing that O4+ cells in the myelinated streak include oligodendrocyte precursor cells, pre-oligodendrocytes, and immature oligodendrocytes (Morcos Y, Chan-Ling T. Glia 21:163-182, 1997). The present data indicate that the preretinal vessels of the myelinated streak possess barrier properties typical of microvasculature in the central nervous system, and that both O4+ cells and astrocytes contribute to the glial ensheathment of these vessels. These vessels thus differ markedly from the leaky preretinal vessels associated with pathological conditions such as retinopathy of prematurity.

Morphological heterogeneity of cultured spinal and cerebral rat oligodendrocytes

Oligodendrocytes in different central nervous system regions have dissimilar morphologies. In order to find out if immature oligodendrocytes develop different phenotypes spontaneously, neonatal spinal and cerebral rat oligodendrocytes were cultured in vitro in the absence of neurons. The results show that spinal oligodendrocytes tend to develop a bipolar structure with few processes, while cerebral cells tend to become multipolar with many processes. A similar spinal-cerebral difference was seen in oligodendrocyte-enriched cultures. Hence, spinal and cerebral oligodendrocytes develop partly different morphologies in vitro.

Central neuron-glial and glial-glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

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.

Characterization of a putative novel type of oligodendrocyte in cultures from rat spinal cord

Oligodendrocytes originate in different neural tube domains, within boundaries of expression of a series of patterning genes which condition the diverse morphogenetic programme of each area. Although neuronal and astrocyte heterogeneity are widely accepted, and despite accumulating evidence for oligodendrocyte heterogeneity in vivo, oligodendrocytes in vitro are currently considered as a homogeneous cell population. The present investigation demonstrates that oligodendrocyte diversity can be detected in vitro and characterizes a novel morphological class of O4-positive oligodendrocyte which is consistently identifiable in rat central nervous system cultures. These cells have a very characteristic epithelioid, unbranched and often lobulated morphology which enables their identification within 2 h of plating. Immunostaining shows that this morphological type is sometimes positive for GD3, A2B5 and vimentin, and most of the time positive for Ranscht antibody, O1 and Rip but negative for glial fibrillary acidic protein, OX-42, neuron-specific enolase, nestin and erbB2. The apparent levels and/or distributions of (i) microtubules, (ii) surface glycolipids recognized by O4, O1 and Ranscht antibody, and (iii) the less specific marker carbonic anhydrase II, typically differ from those of nearby classical, branched oligodendrocytes. Cells with this epithelioid morphology also express myelin basic protein and O10 (a proteolipid protein epitope), both of which are markers for mature oligodendrocytes. Conversely, O4+/O1- cells with this membranous appearance were also seen. Although these atypical oligodendrocytes were most abundant in spinal cord cultures (representing >10% of the O4+ population), they were not exclusive to this region and occurred at a low frequency in neonatal optic nerve cultures.

Oligodendroglial sheath lengths in developing rat ventral funiculus and corpus callosum

The lengths of uncompacted and partly compacted oligodendroglial sheaths in the developing rat spinal cord (SC) ventral funiculus (ages F19 and F21) and corpus callosum (CC; ages P12 and P17) were studied by serial section electron microscopy. The average newly formed uncompacted sheath is 21 and 33 microns long in the SC (F19) and CC (P12), respectively, many being less than 10 microns. In these early series, approximately 2/3 of the analysed axon length is unensheathed. The average partly compacted sheath is 102 and 69 microns long in the SC (F21) and CC (P17), respectively. Here, about 1/3 of the examined axon length is unensheathed. These results suggest that oligodendroglial sheaths initially are very short, and that they elongate actively before and during compaction. The limited unensheathed space along these axons indicate that some early sheaths must be eliminated.

Myelin/oligodendrocyte glycoprotein (MOG) expression is associated with myelin deposition

We investigated the onset of expression of the myelin/oligodendrocyte glycoprotein (MOG) mRNA and protein in the developing mouse central nervous system. In situ hybridization on brain sections at different stages of embryonic and postnatal development showed that MOG transcripts were first detected at birth in the medulla oblongata. During the first week after birth, cells expressing MOG mRNA were located in the ventral longitudinal funiculus. During the second postnatal week, the pattern of MOG mRNA expression extended rostrally to the mid-forebrain regions and reached completion by the beginning of the third week. MOG transcription was delayed by several days with respect to myelin basic protein (MBP), and it appeared that while the MBP probe labeled both non-myelinating and myelinating oligodendrocytes, only the latter were MOG-positive. In vitro, immunocytochemical analysis of MOG protein expression, performed on myelinating cultures derived from mouse brain embryos at 15 days of gestation, confirmed the strict restriction of MOG expression to myelinating oligodendrocytes. In particular, oligodendrocytes lining up their processes along axons, but not yet having started to deposit a myelin sheath, were still MOG negative. However, in the same cultures, pseudo-myelinating oligodendrocytes (i.e., cells not associated with neurites, but forming whorls of myelin-like figures) were MOG positive. Similarly, rat CG4 cells, an oligodendrocyte-like cell line, expressed MOG only after they had extended sheet-like processes, which suggested that the activation of MOG transcription depends more on an intrinsic oligodendroglial maturation program of myelination than on a neuronal signal.

Expression of myelin proteins in the opossum optic nerve: late appearance of inhibitors implicates an earlier non-myelin factor in preventing ganglion cell regeneration

The pattern of appearance of myelin-associated proteins in the visual system of the Brazilian opossum Monodelphis domestica is described. Whole mounts of optic nerve, chiasm, and optic tract were sectioned horizontally and incubated with antibodies to myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), "Rip," and the neurite inhibitory protein (IN-1), followed by visualization with diaminobenzidine and a peroxidase-conjugated secondary antibody. PLP is first detectable 24 days after birth (P24) at the centre of the optic chiasm. MBP, MAG, Rip, and IN-1 appear first in the same area at P26. By P28 the distribution of all proteins is similar, occupying the entire chiasm, optic tracts, and prechiasmatic portion of the optic nerves. Protein expression progresses along the optic nerve to reach the lamina cribrosa by P34, coincident with the time of eye opening. A critical period in which the retinofugal pathway has a regenerative capacity has recently been observed in Monodelphis. This period ends at P12, 2 weeks before the appearance of the myelin-associated inhibitory proteins MAG and IN-1. These results therefore suggest that regeneration in the developing retinofugal projection of the opossum is restricted by an earlier non-myelin factor, which is in contrast to current literature on the spinal cord.

Expression of type 1 inositol 1,4,5-trisphosphate receptor during axogenesis and synaptic contact in the central and peripheral nervous system of developing rat

Release of intracellular Ca2+ is triggered by the second messenger inositol 1,4,5-trisphosphate, which binds to the inositol 1,4,5-trisphosphate receptor and gates the opening of an intrinsic calcium channel in the endoplasmic reticulum. In order to understand the importance of this mechanism in development, we have examined the distribution of the type 1 inositol 1,4,5-trisphosphate receptor during development, in some areas of the rat brain and spinal cord and in peripheral neurons, using in situ hybridization and immunohistochemistry. In brain, we find that type 1 inositol 1,4,5-trisphosphate receptor is expressed in neurons from very early in development; low levels of expression are first detected after the neurons have migrated to their final positions, when they start to differentiate and begin axonal growth. Increasing levels of expression are observed later in development, during the time of synaptogenesis and dendritic contact. Glial cells do not express type 1 inositol 1,4,5-trisphosphate receptor, except for a transient period of expression, probably by oligodendrocytes, in developing fibre tracts during the onset of myelination. In contrast with the brain, both grey and white matter of the spinal cord express type 1 inositol 1,4,5-trisphosphate receptor throughout development, and it remains present in the adult spinal cord. We also show, for the first time, that type 1 inositol 1,4,5-trisphosphate receptor is expressed in the peripheral nervous system. Strong labelling was observed in the dorsal root ganglia and during development this expression seems to coincide with the onset of axogenesis. These results suggest that type 1 inositol 1,4,5-trisphosphate may be involved in the regulatory mechanism controlling Ca2+ levels in neurons during the periods of cell differentiation, axogenesis and synaptogenesis.

Rat brain oligodendrocytes do not interact selectively with axons expressing different calcium-binding proteins

A single oligodendrocyte may endow ten to twenty vicinal axons with internodal segments, but its radial domain is neither exclusive of processes from other like cells nor are all nerve fibres within this zone myelinated. Whether oligodendrocytes are able to discriminate between axons on the basis of chemical or electrophysiological differences, or whether the tactic response is random, has yet to be established. In order to shed some light on this process, we investigated the ensheathment, by single oligodendrocytes, of axons distinguished on the basis of their calcium-binding protein complexion. Rat brain oligodendrocytes were visualized either with the Rip-antibody or by intracellular injection of Lucifer Yellow; subclasses of axons were immunolabelled with antibodies against one of the two calcium-binding proteins parvalbumin or calretinin. Individual oligodendrocytes did not exhibit exclusivity with respect to their preferment for axons containing calcium-binding proteins, associations with both non-immunoreactive, as well as with parvalbumin- or calretinin-positive ones, being encountered.

Topographic variations in rat brain oligodendrocyte morphology elucidated by injection of Lucifer Yellow in fixed tissue slices

Visualisation of oligodendrocytes by fluorochrome labelling in fresh tissue is a relatively recent innovation, but its widespread applicability in comparative analyses between different regions of the brain has been hampered by the limited survival time of excised preparations. We here applied the technique of impaling and injecting these cells with Lucifer Yellow in fixed tissue slices. Using confocal laser scanning microscopy, we reconstructed the three-dimensional forms of oligodendrocytes derived from the optic nerve, corpus callosum, cerebellum and spinal cord of young adult rats. Differences in shape and size of the cell body, in the number of internodal segments supplied by a single cell, as well in their spatial orientation, and in the thickness of the myelinated fibre, were observed between the four white matter tracts analysed.

Axon-glial relationships in the anterior medullary velum of the adult rat

The anterior medullary velum is a thin sheet of CNS tissue which roofs the rostral part of the IVth ventricle and contains fascicles of myelinated fibres which, in part, arise from the nucleus of the IVth cranial nerve. This study used histochemical, immunohistochemical, and intracellular dye-injection techniques to describe cellular interrelationships in the velum in whole-mounts and in sections. Rip antibody-stained whole mounts provided a unique description of both oligodendrocyte units (defined as an oligodendrocyte and the complement of myelinated internodal segments it forms), and consecutive myelin sheaths along the same axon. A broad range of unit morphologies was categorised into four arbitrary groups, according to classical criteria, which comprised small cells supporting the short, thin myelin sheaths of 15-30 small diameter axons (Type I), through intermediate types (II & III), to the largest cells forming the long, thick myelin sheaths of 1-3 large diameter axons. Rip antibody and ferric ion-ferrocyanide staining, together with intracellular dye injection, revealed oligodendrocyte process branching patterns and their mode of engagement of myelin sheaths, nodes of Ranvier, and the spatial disposition of the outer cytoplasmic rims of myelin sheaths. The latter formed a conspicuous spiral ridge on the exterior surface of myelin sheaths which connected with the paranodal loops at each heminode. Large bundles of axons decussated through the velum, the bulk of which were IVth nerve fibres which constituted the IVth nerve rootlet. The PNS/CNS transitional zone of the IVth nerve was located 0.25-0.50 mm along the root, where astrocytic end-feet defined an abrupt margin, convex towards the periphery, where the heminodes of central and peripheral myelin were apposed, and where the basal lamina tubes of the Schwann cell units were discontinued. The basal processes of ependymal cells lining the ventricular wall of the velum, passed between axon bundles before abutting on the basal lamina of the pia. Many of these processes branched and ran along the axonal bundles. A monolayer of microglia occupied a subependymal stratum in which the non-overlapping dendritic territories of each cell formed a regular mosaic throughout the velum without any obvious interaction with either axons or other glial cells. Astrocytes were also uniformly distributed; their fine processes made up a dense lattice amongst axons, often running parallel and within the fibre bundles; stouter ones had terminal end-feet which undercoated the basal lamina of both the glia limitans externa and the blood vessels in the velum.

Myelin mutants: new models and new observations

The myelin mutants have been extensively used as tools to study the complex process of myelination in the central and peripheral nervous system. A multidisciplinary approach to the study of these models ultimately allows a correlation to be made between phenotype and genotype. This correlation may then lead to the formation of new hypotheses about the functions of the products of genes involved in myelination. This review presents a number of new myelin mutants which have recently been described. The species involved include mouse, rat, rabbit, hamster, and dog models. The genetic defect has not been elucidated in all of these animals, but most have been characterized clinically and pathologically, and, in some cases, biochemically. In addition, a better known myelin mutant, the trembler mouse, is discussed. Recent molecular findings have brought this fascinating mutant to the forefront of the field of peripheral nervous system research. The range of abnormalities in the mutants described in this review includes defects in specific myelin proteins, suspected abnormalities in membrane formation, and apparent defects of the oligodendrocyte cytoskeleton. These findings underscore the complexity of the myelination process and highlight the numerous ways in which it can be disrupted.

Calcium signaling in cultured rat oligodendrocytes

The syntax of neuronal-glial or axonal-glial interaction is frequently communicated through transient changes in internal calcium (Cai). We examined mechanisms for Cai signaling and intercellular propagation of Cai responses in cultured oligodendrocytes (OLGs) derived from adult spinal cord (SC), postnatal day 21 (P21) SC, and P21 brain. We found that (1) cultured OLGs exhibited a heterogeneous response to norepinephrine, carbachol, ATP, histamine, and glutamate; (2) receptor-mediated Cai increases were derived from both Ca2+ influx and intracellular Ca2+ release; (3) the percentage of responders to neuroligands varied as a function of cell origin; (4) cultured OLGs exhibited a thapsigargin-sensitive, but not a caffeine-sensitive, intracellular Ca2+ pool; and (5) gap junctional contacts between OLGs permitted limited intercellular propagation of mechanically stimulated Cai responses. Receptor-mediated Cai signaling appears to occur not only in cultured OLGs but also in acutely dissociated OLGs. The heterogeneity in Cai responses as a function of cell origin may reflect the existence of OLG subsets or differences in the maturation stage of OLGs.