Rat optic nerve oligodendrocytes develop in the absence of viable retinal ganglion cell axons

Remyelination in multiple sclerosis, along with its immunology and association with gut dysbiosis, lifestyle, and environmental factors

Multiple sclerosis (MS) is a chronic inflammatory disease that damages the myelin sheath around the axons of the central nervous system. While there are periods of inflammation and remyelination in MS, the latter can sometimes be insufficient and lead to the formation of lesions in the brain and spinal cord. Environmental factors such as vitamin D deficiency, viral or bacterial infections, tobacco smoking, and anxiety have been shown to play a role in the development of MS. Dysbiosis, where the composition of the microbiome changes, may also be involved in the pathogenesis of MS by affecting the gut's microbial population and negatively impacting the integrity of the epithelia. While the cause of MS remains unknown, genetic susceptibility, and immunological dysregulation are believed to play a key role in the development of the disease. Further research is needed to fully understand the complex interplay between genetic, environmental, and microbial factors in the pathogenesis of MS.

Dark Rearing in the Visual Critical Period Causes Structural Changes in Myelinated Axons in the Adult Mouse Visual Pathway

An appropriate sensory experience during the early developmental period is important for brain maturation. Dark rearing during the visual critical period delays the maturation of neuronal circuits in the visual cortex. Although the formation and structural plasticity of the myelin sheaths on retinal ganglion cell axons modulate the visual function, the effects of dark rearing during the visual critical period on the structure of the retinal ganglion cell axons and their myelin sheaths are still unclear. To address this question, mice were reared in a dark box during the visual critical period and then normally reared to adulthood. We found that myelin sheaths on the retinal ganglion cell axons of dark-reared mice were thicker than those of normally reared mice in both the optic chiasm and optic nerve. Furthermore, whole-mount immunostaining with fluorescent axonal labeling and tissue clearing revealed that the myelin internodal length in dark-reared mice was shorter than that in normally reared mice in both the optic chiasm and optic nerve. These findings demonstrate that dark rearing during the visual critical period affects the morphology of myelin sheaths, shortens and thickens myelin sheaths in the visual pathway, despite the mice being reared in normal light/dark conditions after the dark rearing.

GDE2-Dependent Activation of Canonical Wnt Signaling in Neurons Regulates Oligodendrocyte Maturation

Neurons and oligodendrocytes communicate to regulate oligodendrocyte development and ensure appropriate axonal myelination. Here, we show that Glycerophosphodiester phosphodiesterase 2 (GDE2) signaling underlies a neuronal pathway that promotes oligodendrocyte maturation through the release of soluble neuronally derived factors. Mice lacking global or neuronal GDE2 expression have reduced mature oligodendrocytes and myelin proteins but retain normal numbers of oligodendrocyte precursor cells (OPCs). Wild-type (WT) OPCs cultured in conditioned medium (CM) from Gde2-null (Gde2KO) neurons exhibit delayed maturation, recapitulating in vivo phenotypes. Gde2KO neurons show robust reduction in canonical Wnt signaling, and genetic activation of Wnt signaling in Gde2KO neurons rescues in vivo and in vitro oligodendrocyte maturation. Phosphacan, a known stimulant of oligodendrocyte maturation, is reduced in CM from Gde2KO neurons but is restored when Wnt signaling is activated. These studies identify GDE2 control of Wnt signaling as a neuronal pathway that signals to oligodendroglia to promote oligodendrocyte maturation.

Promoting remyelination in multiple sclerosis

The greatest unmet need in multiple sclerosis (MS) are treatments that delay, prevent or reverse progression. One of the most tractable strategies to achieve this is to therapeutically enhance endogenous remyelination; doing so restores nerve conduction and prevents neurodegeneration. The biology of remyelination-centred on the activation, migration, proliferation and differentiation of oligodendrocyte progenitors-has been increasingly clearly defined and druggable targets have now been identified in preclinical work leading to early phase clinical trials. With some phase 2 studies reporting efficacy, the prospect of licensed remyelinating treatments in MS looks increasingly likely. However, there remain many unanswered questions and recent research has revealed a further dimension of complexity to this process that has refined our view of the barriers to remyelination in humans. In this review, we describe the process of remyelination, why this fails in MS, and the latest research that has given new insights into this process. We also discuss the translation of this research into clinical trials, highlighting the treatments that have been tested to date, and the different methods of detecting remyelination in people.

Individual neuronal subtypes control initial myelin sheath growth and stabilization

Background

In the developing central nervous system, pre-myelinating oligodendrocytes sample candidate nerve axons by extending and retracting process extensions. Some contacts stabilize, leading to the initiation of axon wrapping, nascent myelin sheath formation, concentric wrapping and sheath elongation, and sheath stabilization or pruning by oligodendrocytes. Although axonal signals influence the overall process of myelination, the precise oligodendrocyte behaviors that require signaling from axons are not completely understood. In this study, we investigated whether oligodendrocyte behaviors during the early events of myelination are mediated by an oligodendrocyte-intrinsic myelination program or are over-ridden by axonal factors.

Methods

To address this, we utilized in vivo time-lapse imaging in embryonic and larval zebrafish spinal cord during the initial hours and days of axon wrapping and myelination. Transgenic reporter lines marked individual axon subtypes or oligodendrocyte membranes.

Results

In the larval zebrafish spinal cord, individual axon subtypes supported distinct nascent sheath growth rates and stabilization frequencies. Oligodendrocytes ensheathed individual axon subtypes at different rates during a two-day period after initial axon wrapping. When descending reticulospinal axons were ablated, local spinal axons supported a constant ensheathment rate despite the increased ratio of oligodendrocytes to target axons.

Conclusion

We conclude that properties of individual axon subtypes instruct oligodendrocyte behaviors during initial stages of myelination by differentially controlling nascent sheath growth and stabilization.

Role of glia in optic nerve

Glial cells are critically important for maintenance of neuronal activity in the central nervous system (CNS), including the optic nerve (ON). However, the ON has several unique characteristics, such as an extremely high myelination level of retinal ganglion cell (RGC) axons throughout the length of the nerve (with virtually all fibers myelinated by 7 months of age in humans), lack of synapses and very narrow geometry. Moreover, the optic nerve head (ONH) - a region where the RGC axons exit the eye - represents an interesting area that is morphologically distinct in different species. In many cases of multiple sclerosis (demyelinating disease of the CNS) vision problems are the first manifestation of the disease, suggesting that RGCs and/or glia in the ON are more sensitive to pathological conditions than cells in other parts of the CNS. Here, we summarize current knowledge on glial organization and function in the ON, focusing on glial support of RGCs. We cover both well-established concepts on the important role of glial cells in ON health and new findings, including novel insights into mechanisms of remyelination, microglia/NG2 cell-cell interaction, astrocyte reactivity and the regulation of reactive astrogliosis by mitochondrial fragmentation in microglia.

Initiation of CNS Myelination in the Optic Nerve Is Dependent on Axon Caliber

Emerging evidence suggests that neuronal signaling is important for oligodendrocyte myelination; however, the necessity of this signaling during development is unclear. By eliminating dynamic neuronal signaling along the developing optic nerve, we find that oligodendrocyte differentiation is not dependent on neuronal signaling and that the initiation of myelination is dependent on a permissive substrate, namely supra-threshold axon caliber. Furthermore, we show that loss of dynamic neuronal signaling results in hypermyelination of axons. We propose that oligodendrocyte differentiation is regulated by non-neuronal factors during optic nerve development, whereas myelination is sensitive to the biophysical properties of axonal diameter.

Mayoral et al. show that elimination of neuronal signaling via enucleation of the developing optic nerve of Wlds mice results in normal oligodendrocyte differentiation but disrupted myelination. Myelination is rescued when axons are enlarged prior to enucleation, showing that supra-threshold axon caliber, but not neuronal signaling, is necessary for myelination.

Modulation of Neural Activity for Myelination in the Central Nervous System

Electrical stimulation has been playing a significant role in revealing various functions and mechanisms of the nervous system. It is no different for myelination, a process in which oligodendrocytes in the central nervous system (CNS) or Schwann Cells in the peripheral nerve system (PNS) wrap around axons to provide an insulating layer in vitro and in vivo. It has been widely recognized that the myelin sheath accelerates axon signal conduction and provides neuroprotection. Recent studies have begun to reveal its role in plasticity. The major mechanism that enables this process is activity-dependent myelination - the phenomenon where neuronal activity supports oligodendrocyte maturation and myelin sheath formation. In light of recent discoveries, a better understanding of this phenomenon has a potential to provide therapeutic targets for not only demyelinating diseases, but also psychiatric disorders. There is a growing need for experimental platforms capable of dissecting the effect of neural activity on myelination in health and disease. The effect of neural activity is commonly studied by comparing the myelination levels in cultures with neurons of low and high activity. Electrical stimulation is particularly well suited as a method of inducing neural activity in these systems. In this review, we describe in vitro platforms for studying activity-dependent myelination, which utilize neuron stimulation via electrical field. We also discuss stimulation profiles, as well as the alternatives to electrical stimulation in the context of regular, compartmentalized, and organotypic co-cultures.

An augmentation in histone dimethylation at lysine nine residues elicits vision impairment following traumatic brain injury

Traumatic Brain Injury (TBI) affects more than 1.7 million Americans each year and about 30% of TBI-patients having visual impairments. The loss of retinal ganglion cells (RGC) in the retina and axonal degeneration in the optic nerve have been attributed to vision impairment following TBI; however, the molecular mechanism has not been elucidated. Here we have shown that an increase in histone di-methylation at lysine 9 residue (H3K9Me2), synthesized by the catalytic activity of a histone methyltransferase, G9a is responsible for RGC loss and axonal degeneration in the optic nerve following TBI. To elucidate the molecular mechanism, we found that an increase in H3K9Me2 results in the induction of oxidative stress both in the RGC and optic nerve by decreasing the mRNA level of antioxidants such as Superoxide dismutase (sod) and catalase through impairing the transcriptional activity of Nuclear factor E2-related factor 2 (Nrf2) via direct interaction. The induction of oxidative stress is associated with death in RGC and oligodendrocyte precursor cells (OPCs). The death in OPCs is correlated with a reduction in myelination, and the expression of myelin binding protein (MBP) in association with degeneration of neurofilaments in the optic nerve. This event allied to an impairment of the retrograde transport of axons and loss of nerve fiber layer in the optic nerve following TBI. An administration of G9a inhibitor, UNC0638 attenuates the induction of H3K9Me2 both in RGC and optic nerve and subsequently activates Nrf2 to reduce oxidative stress. This event was concomitant with the rescue in the loss of retinal thickness, attenuation in optic nerve degeneration and improvement in the retrograde transport of axons following TBI.

Targeted Oligodendrocyte Apoptosis in Optic Nerve Leads to Persistent Demyelination

The optic nerve represents one of the simplest regions of the CNS and has been useful in developing an understanding of glial development and myelination. While the visual system is frequently affected in demyelinating conditions, utilizing the optic nerve to model demyelination/remyelination studies has been difficult due to its accessibility, relatively small size, and dense nature that makes direct injections challenging. Taking advantage of the lack of oligodendrocytes and myelination in the mouse retina, we have developed a model in which the induction of apoptosis in mature oligodendrocytes allows for the selective, non-invasive generation of demyelinating lesions in optic nerve. Delivery of an inducer of oligodendrocyte apoptosis by intravitreous injection minimizes trauma to the optic nerve and allows for the assessment of oligodendrocyte death in the absence of injury related factors. Here we show that following induction of apoptosis, oligodendrocytes are lost within 3 days. The loss of oligodendrocytes is associated with limited microglial and astrocyte response, is patchy along the nerve, and results in localized myelin loss. Unlike in other regions of the murine CNS, where local demyelination stimulates activation of local oligodendrocyte precursors and remyelination, optic nerve demyelination induced by oligodendrocyte apoptosis fails to recover and results in persistent areas of myelin loss. Over time these chronic lesions change cellular composition and ultimately become devoid of GFAP+ astrocytes and OPCs. Why the optic nerve lesions fail to repair may reflect the lack of early immune responsiveness and provide a novel model of chronic demyelination.

Activity-dependent central nervous system myelination throughout life

Myelin, the multilayered membrane surrounding many axons in the nervous system, increases the speed by which electrical signals travel along axons and facilitates neuronal communication between distant regions of the nervous system. However, how neuronal signals influence the myelinating process in the CNS is still largely unclear. Recent studies have significantly advanced this understanding, identifying important roles for neuronal activity in controlling oligodendrocyte development and their capacity of producing myelin in both developing and mature CNS. Here, we review these recent advances, and discuss potential mechanisms underpinning activity-dependent myelination and how remyelination may be stimulated via manipulating axonal activity, raising new questions for future research.

The Absolute Number of Oligodendrocytes in the Adult Mouse Brain

The central nervous system is a highly complex network composed of various cell types, each one with different subpopulations. Each cell type has distinct roles for the functional operation of circuits, and ultimately, for brain physiology in general. Since the absolute number of each cell type is considered a proxy of its functional complexity, one approach to better understand how the brain works is to unravel its absolute cellularity and the quantitative relations between cell populations; in other words, how one population of cells is quantitatively structured, in relation to another. Oligodendrocytes are one of these cell types - mainly, they provide electric insulation to axons, optimizing action potential conduction. Their function has recently been revisited and their role extended, one example being their capability of providing trophic support to long axons. To determine the absolute cellularity of oligodendroglia, we have developed a protocol of oligodendrocyte quantification using the isotropic fractionator with a pan-marker for this cell type. We report a detailed assessment of specificity and universality of the oligodendrocyte transcription factor 2 (Olig2), through systematic confocal analyses of the C57BL/6 mouse brain. In addition, we have determined the absolute number (17.4 million) and proportion (about 20%) of this cell type in the brain (and in different brain regions), and tested if this population, at the intraspecific level, scales with the number of neurons in an allometric-based approach. Considering these numbers, oligodendrocytes proved to be the most numerous of glial cells in the mouse brain.

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.

Axonal Regulation of Central Nervous System Myelination: Structure and Function

Approximately half of the human brain consists of myelinated axons. Central nervous system (CNS) myelin is made by oligodendrocytes and is essential for nervous system formation, health, and function. Once thought simply as a static insulator that facilitated rapid impulse conduction, myelin is now known to be made and remodeled in to adult life. Oligodendrocytes have a remarkable capacity to differentiate by default, but many aspects of their development can be influenced by axons. However, how axons and oligodendrocytes interact and cooperate to regulate myelination in the CNS remains unclear. Here, we review recent advances in our understanding of how such interactions generate the complexity of myelination known to exist in vivo. We highlight intriguing results that indicate that the cross-sectional size of an axon alone may regulate myelination to a surprising degree. We also review new studies, which have highlighted diversity in the myelination of axons of different neuronal subtypes and circuits, and structure-function relationships, which suggest that myelinated axons can be exquisitely fine-tuned to mediate precise conduction needs. We also discuss recent advances in our understanding of how neuronal activity regulates CNS myelination, and aim to provide an integrated overview of how axon-oligodendrocyte interactions sculpt neuronal circuit structure 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.

Modulation of oligodendrocyte generation during a critical temporal window after NG2 cell division

Oligodendrocytes in the mammalian brain are continuously generated from NG2 cells throughout postnatal life. However it has remained unclear when the decision of NG2 cells to self-renew or differentiate into oligodendrocytes occurs after cell division. Using a combination of in vivo and ex vivo imaging and fate analysis of proliferated NG2 cells in fixed tissue, we demonstrate that in the postnatal developing mouse brain, the majority of divided NG2 cells differentiate into oligodendrocytes during a critical age-specific temporal window of 3–8 days. Importantly, within this time period, myelin and oligodendrocyte damage accelerated oligodendrocyte differentiation from divided cells, while whisker removal decreased the survival of divided cells in the deprived somatosensory cortex. These findings indicate that during the critical temporal window of plasticity, the fate of divided NG2 cells is sensitive to modulation by external signals.

Regulation of oligodendrocyte precursor migration during development, in adulthood and in pathology

Oligodendrocytes are the myelin-forming cells in the central nervous system (CNS). These cells originate from oligodendrocyte precursor cells (OPCs) during development, and they migrate extensively from oligodendrogliogenic niches along the neural tube to colonise the entire CNS. Like many other such events, this migratory process is precisely regulated by a battery of positional and signalling cues that act via their corresponding receptors and that are expressed dynamically by OPCs. Here, we will review the cellular and molecular basis of this important event during embryonic and postnatal development, and we will discuss the relevance of the substantial number of OPCs existing in the adult CNS. Similarly, we will consider the behaviour of OPCs in normal and pathological conditions, especially in animal models of demyelination and of the demyelinating disease, multiple sclerosis. The spontaneous remyelination observed after damage in demyelinating pathologies has a limited effect. Understanding the cellular and molecular mechanisms underlying the biology of OPCs, particularly adult OPCs, should help in the design of neuroregenerative strategies to combat multiple sclerosis and other demyelinating diseases.

Central axons preparing to myelinate are highly sensitive [corrected] to ischemic injury

OBJECTIVE

Developing central white matter is subject to ischemic-type injury during the period that precedes myelination. At this stage in maturation, central axons initiate a program of radial expansion and ion channel redistribution. Here we test the hypothesis that during radial expansion axons display heightened ischemic sensitivity, when clusters of Ca(2+) channels decorate future node of Ranvier sites.

METHODS

Functionality and morphology of central axons and glia were examined during and after a period of modeled ischemia. Pathological changes in axons undergoing radial expansion were probed using electrophysiological, quantitative ultrastructural, and morphometric analysis in neonatal rodent optic nerve and periventricular white matter axons studied under modeled ischemia in vitro or after hypoxia-ischemia in vivo.

RESULTS

Acute ischemic injury of central axons undergoing initial radial expansion was mediated by Ca(2+) influx through Ca(2+) channels expressed in axolemma clusters. This form of injury operated only in this axon population, which was more sensitive to injury than neighboring myelinated axons, smaller axons yet to initiate radial expansion, astrocytes, or oligodendroglia. A pharmacological strategy designed to protect both small and large diameter premyelinated axons proved 100% protective against acute ischemia studied under modeled ischemia in vitro or after hypoxia-ischemia in vivo.

INTERPRETATION

Recent clinical data highlight the importance of axon pathology in developing white matter injury. The elevated susceptibility of early maturing axons to ischemic injury described here may significantly contribute to selective white matter pathology and places these axons alongside preoligodendrocytes as a potential primary target of both injury and therapeutics.

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.

Effects of axon degeneration on oligodendrocyte lineage cells: dorsal rhizotomy evokes a repair response while axon degeneration rostral to spinal contusion induces both repair and apoptosis

Wallerian degeneration in the dorsal columns (DC) after spinal cord injury (SCI) is associated with microglial activation and prolonged oligodendrocyte (OL) apoptosis that may contribute to demyelination and dysfunction after SCI. But, there is an increase in OL lineage cells after SCI that may represent a reparative response, and there is evidence for remyelination after SCI. To assess the role of axonal degeneration per se in OL apoptosis and proliferation, we cut the L2-S2 dorsal roots producing massive axonal degeneration and microglial activation in the DC, and found no evidence of OL loss or apoptosis. Rather, the numbers of OL-lineage cells positive for NG2 and APC (CC1) increased, and BrdU studies suggested new OL formation. We then tested contusion SCI (cSCI) that results in comparable degeneration in the DC rostral to the injury, microglial activation, and apoptosis of DC OLs by eight days. NG2+ cell proliferation and oligodendrogenesis was seen as after rhizotomy. The net result of this combination of proliferation and apoptosis was a reduction in DC OLs, confirming earlier studies. Using an antibody to oxidized nucleic acids, we found rapid and prolonged RNA oxidation in OLs rostral to cSCI, but no evidence of oxidative stress in DC OLs after rhizotomy. These results suggest that signals associated with axonal degeneration are sufficient to induce OL proliferation, and that secondary injury processes associated with the central SCI, including oxidative stress, rather than axonal degeneration per se, are responsible for OL apoptosis.

The lysosomal sialic acid transporter sialin is required for normal CNS myelination

Salla disease and infantile sialic acid storage disease are autosomal recessive lysosomal storage disorders caused by mutations in the gene encoding sialin, a membrane protein that transports free sialic acid out of the lysosome after it is cleaved from sialoglycoconjugates undergoing degradation. Accumulation of sialic acid in lysosomes defines these disorders, and the clinical phenotype is characterized by neurodevelopmental defects, including severe CNS hypomyelination. In this study, we used a sialin-deficient mouse to address how loss of sialin leads to the defect in myelination. Behavioral analysis of the sialin(-/-) mouse demonstrates poor coordination, seizures, and premature death. Analysis by histology, electron microscopy, and Western blotting reveals a decrease in myelination of the CNS but normal neuronal cytoarchitecture and normal myelination of the PNS. To investigate potential mechanisms underlying CNS hypomyelination, we studied myelination and oligodendrocyte development in optic nerves. We found reduced numbers of myelinated axons in optic nerves from sialin(-/-) mice, but the myelin that was present appeared grossly normal. Migration and density of oligodendrocyte precursor cells were normal; however, a marked decrease in the number of postmitotic oligodendrocytes and an associated increase in the number of apoptotic cells during the later stages of myelinogenesis were observed. These findings suggest that a defect in maturation of cells in the oligodendrocyte lineage leads to increased apoptosis and underlies the myelination defect associated with sialin loss.

Experimental autoimmune encephalomyelitis-induced upregulation of tumor necrosis factor-alpha in the dorsal root ganglia

Background: Multiple sclerosis (MS) is a chronic, neurological disease characterized by targeted destruction of central nervous system (CNS) myelin. The autoimmune theory is the most widely accepted explanation of disease pathology. Circulating Th1 cells become activated by exposure to CNS-specific antigens such as myelin basic protein. The activated Th1 cells secrete inflammatory cytokines, which are pivotal for inflammatory responses. We hypothesize that enhanced production of inflammatory cytokines triggers cellular events within the dorsal root ganglia (DRG) and/or spinal cord, facilitating the development of neuropathic pain (NPP) in MS. NPP, the second worst disease-induced symptom suffered by patients with MS, is normally regulated by DRG and/or spinal cord.

Objective: To determine gene and protein expression levels of tumor necrosis factor-alpha (TNFα) within DRG and/or spinal cord in an animal model of MS.

Methods: Experimental autoimmune encephalomyelitis (EAE) was induced in adolescent female Lewis rats. Animals were sacrificed every 3 days post-disease induction. DRG and spinal cords were harvested for protein and gene expression analysis.

Results: We show significant increases in TNFα expression in the DRG and of EAE animals at peak disease stage, as assessed by clinical symptoms.

Conclusion: Antigen-induced production of inflammatory cytokines such as TNFα within the DRG identifies a potential novel mechanism for MS-induced NPP.

Antisense suppression of tau in cultured rat oligodendrocytes inhibits process formation

The microtubule-associated protein tau is integral to neuronal process development and has a role in the pathogenesis of several neurodegenerative conditions. We examined possible roles for tau in cultured oligodendrocyte process formation by using antisense oligonucleotide treatment. Inhibition of tau synthesis with single oligonucleotides resulted in decreased tau protein levels and significantly shorter cellular processes. Simultaneous use of two nonoverlapping oligonucleotides caused a major reduction in tau levels and severely inhibited process outgrowth. The timing of oligonucleotide addition to oligodendrocyte cultures was important, with addition of antisense at the time of plating into culture having the most significant effect on morphology through reduction of tau expression.

Axon-glial signaling and the glial support of axon function

Oligodendrocytes and Schwann cells are highly specialized glial cells that wrap axons with a multilayered myelin membrane for rapid impulse conduction. Investigators have recently identified axonal signals that recruit myelin-forming Schwann cells from an alternate fate of simple axonal engulfment. This is the evolutionary oldest form of axon-glia interaction, and its function is unknown. Recent observations suggest that oligodendrocytes and Schwann cells not only myelinate axons but also maintain their long-term functional integrity. Mutations in the mouse reveal that axonal support by oligodendrocytes is independent of myelin assembly. The underlying mechanisms are still poorly understood; we do know that to maintain axonal integrity, mammalian myelin-forming cells require the expression of some glia-specific proteins, including CNP, PLP, and MAG, as well as intact peroxisomes, none of which is necessary for myelin assembly. Loss of glial support causes progressive axon degeneration and possibly local inflammation, both of which are likely to contribute to a variety of neuronal diseases in the central and peripheral nervous systems.

Nogo-A and myelin-associated glycoprotein differently regulate oligodendrocyte maturation and myelin formation

Nogo-A is one of the most potent oligodendrocyte-derived inhibitors for axonal regrowth in the injured adult CNS. However, the physiological function of Nogo-A in development and in healthy oligodendrocytes is still unknown. In the present study, we investigated the role of Nogo-A for myelin formation in the developing optic nerve. By quantitative real-time PCR, we found that the expression of Nogo-A increased faster in differentiating oligodendrocytes than that of the major myelin proteins MBP (myelin basic protein), PLP (proteolipid protein)/DM20, and CNP (2',3'-cyclic nucleotide 3'-phosphodiesterase). The analysis of optic nerves and cerebella of mice deficient for Nogo-A (Nogo-A(-/-)) revealed a marked delay of oligodendrocyte differentiation, myelin sheath formation, and axonal caliber growth within the first postnatal month. The combined deletion of Nogo-A and MAG caused a more severe transient hypomyelination. In contrast to MAG(-/-) mice, Nogo-A(-/-) mutants did not present abnormalities in the structure of myelin sheaths and Ranvier nodes. The common binding protein for Nogo-A and MAG, NgR1, was exclusively upregulated in MAG(-/-) animals, whereas the level of Lingo-1, a coreceptor, remained unchanged. Together, our results demonstrate that Nogo-A and MAG are differently involved in oligodendrocyte maturation in vivo, and suggest that Nogo-A may influence also remyelination in pathological conditions such as multiple sclerosis.

Control of glial precursor cell development in the mouse optic nerve by sonic hedgehog from retinal ganglion cells

The development of glial precursor cells in the mammalian optic nerve depends on retinal ganglion cell (RGC) axons, but the signals that mediate this neuron-to-glia interaction have not been fully characterized. Sonic hedgehog (Shh) is expressed by RGCs, and we showed previously that it is required for the specification of astrocyte lineage cells at the optic disc. To study the role of RGC-derived Shh on astrocyte development at later developmental stages, we generated mice with a conditional ablation of Shh in the peripheral retina and analyzed gene expression and glial cell development in the optic nerve. Astrocyte development was initiated in the optic nerves of these mutant mice; however, the expression of Hedgehog (Hh) target genes, Gli1 and Ptch1 and cell cycle genes, Ccnd1 and Cdc25b in the optic nerves were downregulated. Astrocyte proliferation was markedly reduced. Oligodendrocyte precursor cells were fewer in the optic nerves of mutant mice, possibly as a consequence of reduced secretion of growth factors by astrocytes. At a later developmental stage, optic nerve axons displayed signs of Wallerian degeneration, including reduction of astrocyte processes, degenerating glial cells and formation of distended axons. We also demonstrate that the Hh pathway can be activated in optic nerve-derived astrocytes in vitro, but fails to induce cell cycle gene expression and proliferation. RGC-derived Shh signalling isthus necessary in vivo for maintenance of astrocyte proliferation, affecting both axo-glial and normal glial cell development in the optic nerve.

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).

NG2-positive cells generate A2B5-positive oligodendrocyte precursor cells

Cellular specification of the oligodendrocyte lineage occurs through a series of stages identified by expression of distinct biochemical characteristics. The best characterized oligodendrocyte progenitor cell (OPC) in vitro is the bipotential O2-A progenitor, identified by labeling with monoclonal antibody A2B5, which proliferates predominantly in response to platelet derived growth factor (PDGF). The cellular ancestors of O2-A progenitor cells are currently unclear. In vivo OPCs can be identified by expression of the cell surface markers NG2 (a sulfated proteoglycan) and platelet derived growth factor receptor alphaR). Substantial evidence supports the generation of oligodendrocytes from NG2(+), PDGFalphaR(+) cells both in vivo and in vitro. The developmental relationship between NG2(+) cells and A2B5(-) positive cells is unknown and it is unclear whether they represent identical, partially overlapping or nonoverlapping populations of cells. Here we show that in cultures of developing brain NG2(+) and A2B5(+) cells arise from overlapping cell populations. NG2(+) cells appear prior to the expression of A2B5(+) cells and generate A2B5(+) cells. We propose that during development NG2(+)/A2B5(-) cells (pre-OPCs) represent the direct ancestor to A2B5(+) O2A progenitor cells (OPCs).

Anosmin-1 modulates the FGF-2-dependent migration of oligodendrocyte precursors in the developing optic nerve

Oligodendrocyte precursors (OPCs) originate at specific domains within the neural tube before migrating to colonize the entire CNS. Once in their target areas, these cells differentiate into oligodendrocytes, the myelin-forming cells in the CNS. Using the embryonic mouse optic nerve as an experimental model, we have analyzed the influence of FGF-2 on OPC development. FGF-2 exerts a dose-dependent motogenic effect on the migration of plp-dm20+ and it also acts as a chemoattractant on these cells. These effects produced by FGF-2 are principally mediated by the FGFR1 receptor, which is expressed by OPCs. Anosmin-1 is the protein that is defective in the X-linked form of human Kallmann syndrome. This protein is expressed by retinal axons and it also interacts with FGFR1, thereby impairing the migration of OPCs. Because both Anosmin-1 and FGF-2 are present in the optic nerve in vivo, we propose a model whereby the relative concentration of these two proteins modulates the migration of OPCs during development through their interaction with FGFR1. This FGF-2/FGFR1/Anosmin-1 system may be relevant in the context of demyelinating diseases.

Progesterone increases oligodendroglial cell proliferation in rat cerebellar slice cultures

We have previously demonstrated that progesterone significantly increases the rate of myelination in organotypic slice cultures of 7-day-old rat and mouse cerebellum. Here, we show that progesterone (20microM) stimulates the proliferation of oligodendrocyte precursors in cultured cerebellar slices of 7-day-old rats. The steroid increased the number of pre-oligodendrocytes (NG2(+), O4(+)) and to some extent of oligodendrocyte precursors, corresponding to an earlier developmental stage (nestin(+), PDGFalphaR(+), NG2(+), O4(-)). Progesterone stimulated the proliferation of both NG2(+) and O4(+) cells as shown by increased double-immunolabeling with the cell proliferation marker Ki67. The mitogenic effect of progesterone was inhibited by the progesterone receptor antagonist mifepristone (10microM) and could not be mimicked by its GABA-active metabolite 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone), even at the high concentration of 50microM. Results indicate that progesterone first strongly and transiently stimulates the proliferation of oligodendrocyte precursors, and that it may thereafter accelerate their maturation into myelinating oligodendrocytes. Although oligodendrocyte precursors may be a direct target for the actions of progesterone, their number may also be indirectly influenced by the effects of the steroid on neurons and microglial cells, since treatment of the cerebellar slices with progesterone enhanced staining of the neuronal cytoskeleton marker microtubule-associated protein-2 and increased the number of OX-42(+) microglia. A small percentage (about 0.1%) of the NG2(+) cells transiently became OX-42(+) in response to progesterone. These results point to novel mechanisms by which progesterone may promote myelination in the CNS, specifically by stimulating the proliferation and maturation of oligodendrocyte precursors into myelinating oligodendrocytes.

The molecular orchestra of the migration of oligodendrocyte precursors during development

During development of the central nervous system (CNS), postmitotic cells (including neurons and myelin-generating cells, the oligodendrocytes) migrate from the germinal areas of the neural tube where they originate to their final destination sites. The migration of neurons during development has been extensively studied and has been the topic of detailed reviews. The migration of oligodendrocyte precursor cells (OPCs) is also an extremely complex and precise event, with a widespread migration of OPCs across many regions to colonize the entire CNS. Different mechanisms have been shown to direct the migration of OPCs, among them contact-mediated mechanisms (adhesion molecules) and long-range cues (chemotropic molecules). This review provides a detailed overview and discussion of the cellular and molecular basis of OPCs migration during development. Because it has been shown that neuronal and oligodendroglial lineages share some of these mechanisms, we briefly summarize similarities and differences between these two types of neural cells. We also summarize the changes in the normal migration of OPCs during development that would be relevant for different neurological diseases (including demyelinating diseases, such as multiple sclerosis, and glial cancers). We also examine the relevance of these migratory properties of the oligondendrocytic cell lineage for the repair of neural damage.

Plp gene regulation in the developing murine optic nerve: correlation with oligodendroglial process alignment along the axons

The factors regulating the expression and splicing of the major myelin gene, proteolipid protein (Plp), are unclear. The gene encodes two splice variants, Plp and Dm20. During active myelination, transcription of the Plp gene is markedly upregulated and the splice variant ratio becomes Plp-mRNA dominant. We hypothesised that these aspects of Plp gene regulation are linked to overt axonal contact. Using the developing optic nerve of mice, we demonstrate that alignment of oligodendroglial processes with the axon correlates with both the expression of Plp-mRNA and the transcriptional upregulation of the gene. We test the above hypothesis more extensively in a subsequent study.

Beta IV tubulin is selectively expressed by oligodendrocytes in the central nervous system

Oligodendrocyte differentiation and myelination involve dramatic changes in cell signaling pathways, gene expression patterns, cell shape, and cytoskeletal organization. In a pilot study investigating CNS angiogenesis, oligodendrocytes were intensely labeled by antisera directed against the C-terminal of Tie-2, a 140-kDa transmembrane receptor for angiopoietin. Immunoprecipitation of rat brain proteins with Tie-2 C-terminal antisera, however, produced a single spot of approximately 55-kDa pI approximately 5 by two-dimensional (2D) electrophoresis, which was identified as beta-tubulin by mass spectrometry. Isotype-specific antibodies for beta(IV) tubulin selectively labeled oligodendrocytes. First detected in premyelinating oligodendrocytes, beta(IV) tubulin was abundant in myelinating oligodendrocyte perinuclear cytoplasm and processes extending to and along developing myelin internodes. Beta(IV) tubulin-positive MTs were diffusely distributed in oligodendrocyte perinuclear cytoplasm and not organized around the centrosome. Beta(IV) tubulin may play a role in establishing the oligodendrocyte MT network, which is essential for the transport of myelin proteins, lipids, and RNA during myelination.

Oligodendrocyte precursors on the move: mechanisms directing migration

Oligodendrocyte precursor cells traverse long distances to reach their axonal targets. The molecules that influence their migration include a combination of short-range attractants and repellents and long-range chemoattractants and chemorepellents. Here, the authors review mechanisms that direct oligodendrocyte precursor cells as they migrate throughout the developing CNS.

Phosphodiesterase-Ialpha/autotaxin: a counteradhesive protein expressed by oligodendrocytes during onset of myelination

The initial stages of central nervous system (CNS) myelination require complex interactions of oligodendrocytes with their surrounding extracellular environment. In the present study, we demonstrate that commencing with active myelination oligodendrocytes express phosphodiesterase-Ialpha/autotaxin [PD-Ialpha/ATX (NPP-2)] as a non-membrane-associated extracellular factor. As such a component of the extracellular environment, PD-Ialpha/ATX has the ability to antagonize the adhesive interactions between oligodendroglial cells and known extracellular matrix (ECM) molecules present in the developing CNS. This counteradhesion requires intracellular signaling through heterotrimeric G proteins on fibronectin substrates and thus represents an active cellular response. Similar counteradhesive effects in other systems have been attributed to the activity of matricellular proteins, which support intermediate stages of cell adhesion thought to facilitate cellular locomotion and remodeling. Thus, the release of PD-Ialpha/ATX may be critically involved in the regulation of the initial stages of myelination, i.e., oligodendrocyte remodeling, via modulation of oligodendrocyte-ECM interactions in a matricellular fashion.

Oligodendrogenesis is differentially regulated in gray and white matter of jimpy mice

The factors that regulate oligodendrogenesis have been studied extensively in optic nerve, where oligodendrocyte production and myelination quickly follow colonization of the nerve by progenitor cells. In contrast, oligodendrocyte production in the cerebral cortex begins approximately 1 week after progenitor cell colonization and continues for 3-4 weeks. This and other observations raise the possibility that oligodendrogenesis is regulated by different mechanisms in white and gray matter. The present study examined oligodendrocyte production in the developing cerebral cortex of jimpy (jp) and jimpy(msd) (msd) mice, which exhibit hypomyelination and oligodendrocyte death due to mutations in and toxic accumulations of proteolipid protein, the major structural protein of CNS myelin. Proliferation of oligodendrocyte progenitors and production of myelinating oligodendrocytes was reduced in jp cerebral cortex when compared to wild-type (wt) and msd mice. The incidence of oligodendrocyte cell death was similar in jp and msd cortex, but total dying oligodendrocytes were greater in msd. We confirm previous reports of increased oligodendrocyte production in white matter of both jp and msd mice. The jp mutation, therefore, reduces oligodendrocyte production in cerebral cortex but not in white matter. These data provide additional evidence that oligodendrogenesis is differentially regulated in white matter and gray matter and implicate PLP/DM20 as a modulator of these differences.

Differentiation of proliferated NG2-positive glial progenitor cells in a remyelinating lesion

Cells that express the NG2 proteoglycan (NG2+ cells) constitute a large cell population in the adult mammalian central nervous system (CNS). They give rise to mature oligodendrocytes in culture and are thus considered to be oligodendrocyte progenitor cells (OPCs). They proliferate in response to a variety of insults to the CNS, but their ability to differentiate into oligodendrocytes in vivo has not been established. We used bromodeoxyuridine (BrdU) to trace the fate of NG2+ cells that proliferated in response to a chemically induced demyelinating lesion in the adult rat spinal cord. Cells that were proliferating 24 hr after lesioning were labeled by a single injection of BrdU, and their antigenic phenotype was examined at various times up to 28 days post-lesioning (28 dpl). Initially, at 2 dpl, NG2+/BrdU+ cells were found almost exclusively at the periphery of the lesion. At 7 dpl, the number of NG2+/BrdU+ cells increased in the lesion center and decreased from the surrounding areas. The number of NG2+/BrdU+ cells inside the lesion further decreased with time, concomitant with progression of remyelination and appearance of BrdU+ mature oligodendrocytes. Double labeling with (3)H-thymidine and BrdU combined with NG2 immunohistochemistry showed that some NG2+ cells in the lesion had undergone at least two rounds of cell division. These observations strongly suggest that NG2+/BrdU+ cells that appeared in response to the demyelinating insult gave rise to mature remyelinating oligodendrocytes, providing an in vivo evidence for the differentiation of NG2+ cells into oligodendrocytes.

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.

Premyelinating oligodendrocytes in chronic lesions of multiple sclerosis

BACKGROUND

Multiple sclerosis is an inflammatory disease of the central nervous system that destroys myelin, oligodendrocytes, and axons. Since most of the lesions of multiple sclerosis are not remyelinated, enhancement of remyelination is a possible therapeutic strategy that could perhaps be achieved with the transplantation of oligodendrocyte-producing cells into the lesions. We investigated the frequency distribution and configuration of oligodendrocytes in chronic lesions of multiple sclerosis to determine whether these factors limit remyelination.

METHODS

Forty-eight chronic lesions obtained at autopsy from 10 patients with multiple sclerosis were examined immunocytochemically for oligodendrocytes and oligodendrocyte progenitor cells. Using confocal microscopy, we examined the three-dimensional relations between axons and the processes of premyelinating oligodendrocytes.

RESULTS

Thirty-four of the 48 chronic lesions of multiple sclerosis contained oligodendrocytes with multiple extended processes that associated with demyelinated axons but failed to myelinate them. These axons were dystrophic and contained multiple swellings. In some regions, the densities of premyelinating oligodendrocytes (25 per square millimeter of tissue) were similar to those in the developing rodent brain (23 per square millimeter). In the patients with disease of long duration (more than 20 years), there were fewer lesions with premyelinating oligodendrocytes (P<0.001).

CONCLUSIONS

Premyelinating oligodendrocytes are present in chronic lesions of multiple sclerosis, so remyelination is not limited by an absence of oligodendrocyte progenitors or their failure to generate oligodendrocytes. Our findings suggest that in the chronic lesions of multiple sclerosis, the axons are not receptive for remyelination. Understanding the cellular interactions between premyelinating oligodendrocytes, axons, and the microenvironment of lesions of multiple sclerosis may lead to effective strategies for enhancing remyelination.

N-acetylaspartate is an axon-specific marker of mature white matter in vivo: a biochemical and immunohistochemical study on the rat optic nerve

Axonal pathology is a major cause of neurological disability in multiple sclerosis. Axonal transection begins at disease onset but remains clinically silent because of compensatory brain mechanisms. Noninvasive surrogate markers for axonal injury are therefore essential to monitor cumulative disease burden in vivo. The neuronal compound N-acetylaspartate, as measured by magnetic resonance spectroscopy, is currently the best and most specific noninvasive marker of axonal pathology in multiple sclerosis. The possibility has been raised, however, that N-acetylaspartate is expressed also by oligodendroglial lineage cells. In order to investigate N-acetylaspartate specificity for white matter axons, transected rat optic nerves were analyzed by high-performance liquid chromatography and immunohistochemistry. In transected adult nerves, N-acetylaspartate and N-acetyl aspartylglutamate decreased in concordance with axonal degeneration and were undetectable 24 days posttransection. Nonproliferating oligodendrocyte progenitor cells, oligodendrocytes, and myelin were abundant in these axon-free nerves. At 24 days posttransection, N-acetylaspartate was increased (42%; p = 0.02) in nontransected contralateral nerves. After transection at postnatal day 4, total N-acetylaspartate decreased by 80% (P14; p = 0.002) and 94% (P20; p = 0.003). In these developing axon-free nerves, 25 to 33% of oligodendrocyte progenitor cells were proliferating. These data validate magnetic resonance spectroscopy measurements of N-acetylaspartate as an axon-specific monitor of central nervous system white matter in vivo. In addition, the results indicate that neuronal adaptation can increase N-acetylaspartate levels, and that 5 to 20% of the N-acetylaspartate in developing white matter is synthesized by proliferating oligodendrocyte progenitor cells.

The erbB2 gene is required for the development of terminally differentiated spinal cord oligodendrocytes

Development of oligodendrocytes and the generation of myelin internodes within the spinal cord depends on regional signals derived from the notochord and axonally derived signals. Neuregulin 1 (NRG)-1, localized in the floor plate as well as in motor and sensory neurons, is necessary for normal oligodendrocyte development. Oligodendrocytes respond to NRGs by activating members of the erbB receptor tyrosine kinase family. Here, we show that erbB2 is not necessary for the early stages of oligodendrocyte precursor development, but is essential for proligodendroblasts to differentiate into galactosylcerebroside-positive (GalC+) oligodendrocytes. In the presence of erbB2, oligodendrocyte development is normal. In the absence of erbB2 (erbB2-/-), however, oligodendrocyte development is halted at the proligodendroblast stage with a >10-fold reduction in the number of GalC+ oligodendrocytes. ErbB2 appears to function in the transition of proligodendroblast to oligodendrocyte by transducing a terminal differentiation signal, since there is no evidence of increased oligodendrocyte death in the absence of erbB2. Furthermore, known survival signals for oligodendrocytes increase oligodendrocyte numbers in the presence of erbB2, but fail to do so in the absence of erbB2. Of the erbB2-/- oligodendrocytes that do differentiate, all fail to ensheath neurites. These data suggest that erbB2 is required for the terminal differentiation of oligodendrocytes and for development of myelin.

N-cadherin is involved in axon-oligodendrocyte contact and myelination

We have analyzed the influence of the calcium-dependent cell adhesion molecule, N-cadherin, on events leading to CNS myelination. Interactions between axons and oligodendrocyte progenitor (OP) cells and the CG4 OP cell line were examined by video-microscopy. OPs cocultured with dorsal root ganglia explants migrated around the culture and formed numerous contacts with axons. The duration of these contacts depended on the morphology of the OP, with OPs containing four or more processes forming long-lasting contacts and OPs with three or fewer processes forming short-termed contacts. Treatment with N-cadherin function blocking peptides approximately halved the duration of contacts made by cells with four or more processes but contact times for cells with three or less processes were unaffected. The L7 cadherin-blocking antibody and calcium withdrawal had similar effects. Contacts with axons regenerating from explants of adult retina, which do not have N-cadherin on their surface was examined. The contact duration of OPs to adult retina axons was short and similar in length to those formed between OPs and dorsal root ganglion axons in the presence of cadherin blocking reagents. Oligodendrocyte myelination was examined in organotypic rat cerebellar slice cultures, taken before myelination at postnatal day 10 and then allowed to myelinate in vitro over 7 days. When incubated with an N-cadherin function-blocking peptide, myelination of Purkinje cell axons was reduced to about half of control levels, while control peptides were without effect. Cadherin-blockade did not prevent maturation of OPs, since oligodendrocytes showing myelin basic protein immunostaining were still found in these cultures. However, many of the cell processes did not colocalize with calbindin-positive axons. From these data we conclude that N-cadherin is important for the initial contact between a myelinating oligodendrocyte and axons and significantly contributes to the success of myelination.

Extensive proliferation of oligodendrocyte precursors in the parenchyma of the embryonic chick central nervous system

The proliferation of oligodendrocyte lineage cells in the chick embryo central nervous system (CNS) was examined by double-immunolabeling with a lineage marker monoclonal antibody (mAb) O4 or mAb O1 and 5-bromo-3'-deoxyuridine (BrdU). In all regions examined, the first O4-positive (O4+) cells appeared in restricted regions of the ventricular zone (VZ), regarded as a site of oligodendrocyte origin. Within the O4+ focus, less than 20% of the O4+ cells incorporated BrdU. In contrast, O4+ cells in the parenchyma were mitotically active; for example, 40-50% of early O4+ cells were labeled with BrdU. Some of these were unipolar in shape, indicative of migratory precursor cells. The frequency of O4+/BrdU+ cell appearance decreased to less than 20% with further development. O1+ oligodendrocytes were largely mitotically inactive, with only approximately 5% of O1+ cells incorporating BrdU. These results clearly demonstrated that the VZ generates relatively few precursor cells and that these oligodendrocyte precursors actively generate their cohort in the parenchyma of the CNS.