Proliferation and differentiation of O4+ oligodendrocytes in postnatal rat cerebellum: analysis in unfixed tissue slices using anti-glycolipid antibodies

Small-molecule-induced epigenetic rejuvenation promotes SREBP condensation and overcomes barriers to CNS myelin regeneration

Remyelination failure in diseases like multiple sclerosis (MS) was thought to involve suppressed maturation of oligodendrocyte precursors; however, oligodendrocytes are present in MS lesions yet lack myelin production. We found that oligodendrocytes in the lesions are epigenetically silenced. Developing a transgenic reporter labeling differentiated oligodendrocytes for phenotypic screening, we identified a small-molecule epigenetic-silencing-inhibitor (ESI1) that enhances myelin production and ensheathment. ESI1 promotes remyelination in animal models of demyelination and enables de novo myelinogenesis on regenerated CNS axons. ESI1 treatment lengthened myelin sheaths in human iPSC-derived organoids and augmented (re)myelination in aged mice while reversing age-related cognitive decline. Multi-omics revealed that ESI1 induces an active chromatin landscape that activates myelinogenic pathways and reprograms metabolism. Notably, ESI1 triggered nuclear condensate formation of master lipid-metabolic regulators SREBP1/2, concentrating transcriptional co-activators to drive lipid/cholesterol biosynthesis. Our study highlights the potential of targeting epigenetic silencing to enable CNS myelin regeneration in demyelinating diseases and aging.

Myelinating Glia: Potential Therapeutic Targets in Polyglutamine Spinocerebellar Ataxias

Human studies, in combination with animal and cellular models, support glial cells as both major contributors to neurodegenerative diseases and promising therapeutic targets. Among glial cells, oligodendrocytes and Schwann cells are the myelinating glial cells of the central and peripheral nervous system, respectively. In this review, we discuss the contributions of these central and peripheral myelinating glia to the pathomechanisms of polyglutamine (polyQ) spinocerebellar ataxia (SCA) types 1, 2, 3, 6, 7, and 17. First, we highlight the function of oligodendrocytes in healthy conditions and how they are disrupted in polyQ SCA patients and diseased model systems. We then cover the role of Schwann cells in peripheral nerve function and repair as well as their possible role in peripheral neuropathy in polyQ SCAs. Finally, we discuss potential polyQ SCA therapeutic interventions in myelinating glial.

Live-cell metabolic analysis of oligodendroglia isolated from postnatal mouse brain and spinal cord

This protocol describes isolation and live-cell metabolic analysis of O4+ oligodendroglia from brain and spinal cord of postnatal mice. We have optimized existing protocols for O4+ isolation from neonatal brain and expanded the protocol to include isolation of highly viable oligodendroglia from spinal cords of postnatal mice up to 18 days of age. Isolated oligodendroglia can be used in multiple downstream analyses, and here we describe an optimized real-time metabolic assay using Agilent Seahorse Analyzer to measure mitochondrial respiration. For complete details on the use and execution of this protocol, please refer to Khandker et al. (2022).

Pathogenetic Mechanisms Underlying Spinocerebellar Ataxia Type 3 Are Altered in Primary Oligodendrocyte Culture

Emerging evidence has implicated non-neuronal cells, particularly oligodendrocytes, in the pathophysiology of many neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease and Spinocerebellar ataxia type 3 (SCA3). We recently demonstrated that cell-autonomous dysfunction of oligodendrocyte maturation is one of the of the earliest and most robust changes in vulnerable regions of the SCA3 mouse brain. However, the cell- and disease-specific mechanisms that underlie oligodendrocyte dysfunction remain poorly understood and are difficult to isolate in vivo. In this study, we used primary oligodendrocyte cultures to determine how known pathogenic SCA3 mechanisms affect this cell type. We isolated oligodendrocyte progenitor cells from 5- to 7-day-old mice that overexpress human mutant ATXN3 or lack mouse ATXN3 and differentiated them for up to 5 days in vitro. Utilizing immunocytochemistry, we characterized the contributions of ATXN3 toxic gain-of-function and loss-of-function in oligodendrocyte maturation, protein quality pathways, DNA damage signaling, and methylation status. We illustrate the utility of primary oligodendrocyte culture for elucidating cell-specific pathway dysregulation relevant to SCA3. Given recent work demonstrating disease-associated oligodendrocyte signatures in other neurodegenerative diseases, this novel model has broad applicability in revealing mechanistic insights of oligodendrocyte contribution to pathogenesis.

Cholesterol biosynthesis defines oligodendrocyte precursor heterogeneity between brain and spinal cord

Brain and spinal cord oligodendroglia have distinct functional characteristics, and cell-autonomous loss of individual genes can result in different regional phenotypes. However, a molecular basis for these distinctions is unknown. Using single-cell analysis of oligodendroglia during developmental myelination, we demonstrate that brain and spinal cord precursors are transcriptionally distinct, defined predominantly by cholesterol biosynthesis. We further identify the mechanistic target of rapamycin (mTOR) as a major regulator promoting cholesterol biosynthesis in oligodendroglia. Oligodendroglia-specific loss of mTOR decreases cholesterol biosynthesis in both the brain and the spinal cord, but mTOR loss in spinal cord oligodendroglia has a greater impact on cholesterol biosynthesis, consistent with more pronounced deficits in developmental myelination. In the brain, mTOR loss results in a later adult myelin deficit, including oligodendrocyte death, spontaneous demyelination, and impaired axonal function, demonstrating that mTOR is required for myelin maintenance in the adult brain.

Using single-cell RNA sequencing, Khandker et al. reveal that oligodendroglia in the brain and spinal cord are distinct. These differences arise from mechanisms regulating cholesterol acquisition, necessary for maintenance of the lipid-rich myelin sheath, and involve mTOR in the regulation of cholesterol biosynthesis in oligodendroglia.

Premyelinating Oligodendrocytes: Mechanisms Underlying Cell Survival and Integration

In the central nervous system, oligodendrocytes produce myelin sheaths that enwrap neuronal axons to provide trophic support and increase conduction velocity. New oligodendrocytes are produced throughout life through a process referred to as oligodendrogenesis. Oligodendrogenesis consists of three canonical stages: the oligodendrocyte precursor cell (OPC), the premyelinating oligodendrocyte (preOL), and the mature oligodendrocyte (OL). However, the generation of oligodendrocytes is inherently an inefficient process. Following precursor differentiation, a majority of premyelinating oligodendrocytes are lost, likely due to apoptosis. If premyelinating oligodendrocytes progress through this survival checkpoint, they generate new myelinating oligodendrocytes in a process we have termed integration. In this review, we will explore the intrinsic and extrinsic signaling pathways that influence preOL survival and integration by examining the intrinsic apoptotic pathways, metabolic demands, and the interactions between neurons, astrocytes, microglia, and premyelinating oligodendrocytes. Additionally, we will discuss similarities between the maturation of newly generated neurons and premyelinating oligodendrocytes. Finally, we will consider how increasing survival and integration of preOLs has the potential to increase remyelination in multiple sclerosis. Deepening our understanding of premyelinating oligodendrocyte biology may open the door for new treatments for demyelinating disease and will help paint a clearer picture of how new oligodendrocytes are produced throughout life to facilitate brain function.

Oligodendrocytes in intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a cerebrovascular disorder with high mortality and disability rates. Although a lot of effort has been put in ICH, there is still no effective treatment for this devastating disease. Recent studies suggest that oligodendrocytes play an important role in brain repair after ICH and thus may be targeted for the therapies of ICH. Here in this review, we first introduce the origin, migration, proliferation, differentiation, and myelination of oligodendrocytes under physiological condition. Second, recent findings on how ICH affects oligodendrocyte biology and function are reviewed. Third, potential crosstalk between oligodendrocytes and other cells in the brain is also summarized. Last, we discuss the therapeutic potential of oligodendrocyte‐based treatments in ICH. Our goal is to provide a comprehensive review on the biology and function of oligodendrocytes under both physiological and ICH conditions.

Systemic Interleukin-4 Administration after Spinal Cord Injury Modulates Inflammation and Promotes Neuroprotection

Traumatic spinal cord injury (SCI) causes dramatic disability and dysfunction in the motor, sensory and autonomic systems. The severe inflammatory reaction that occurs after SCI is strongly associated with further tissue damage. As such, immunomodulatory strategies have been developed, aimed at reducing inflammation, but also at shaping the immune response in order to protect, repair and promote regeneration of spared neural tissue. One of those promising strategies is the intraspinal administration of the cytokine interleukin-4 (IL-4) that was shown to promote a phenotype on specific immune cells associated with neuroprotection and repair. In this work, we evaluated if a systemic delivery of IL-4 for a 7-days period was also capable of promoting neuroprotection after SCI by analyzing different neural cells populations and motor recovery. IL-4 treatment promoted an elevation of the anti-inflammatory cytokine IL-10 in the serum both at 24 h and 7 days after injury. Locally, treatment with IL-4 led to a reduction on cells expressing markers associated with inflammation, CD11b/c and iNOS. Importantly, IL-4 treatment increased the neuronal markers βIII-tubulin and NeuN, and the oligodendrocyte marker O4, suggesting a neuroprotective effect. Moreover, 100% of the animals treated with IL-4 were able to recover weight support against only 33% of saline treated animals. Overall, these results show that systemic administration of IL-4 positively impacts different aspects of spinal cord injury, creating a more favorable environment for recovery to take place.

Embracing oligodendrocyte diversity in the context of perinatal injury

Emerging evidence is fueling a new appreciation of oligodendrocyte diversity that is overturning the traditional view that oligodendrocytes are a homogenous cell population. Oligodendrocytes of distinct origins, maturational stages, and regional locations may differ in their functional capacity or susceptibility to injury. One of the most unique qualities of the oligodendrocyte is its ability to produce myelin. Myelin abnormalities have been ascribed to a remarkable array of perinatal brain injuries, with concomitant oligodendrocyte dysregulation. Within this review, we discuss new insights into the diversity of the oligodendrocyte lineage and highlight their relevance in paradigms of perinatal brain injury. Future therapeutic development will be informed by comprehensive knowledge of oligodendrocyte pathophysiology that considers the particular facets of heterogeneity that this lineage exhibits.

MiR-125a-3p timely inhibits oligodendroglial maturation and is pathologically up-regulated in human multiple sclerosis

In the mature central nervous system (CNS), oligodendrocytes provide support and insulation to axons thanks to the production of a myelin sheath. During their maturation to myelinating cells, oligodendroglial precursors (OPCs) follow a very precise differentiation program, which is finely orchestrated by transcription factors, epigenetic factors and microRNAs (miRNAs), a class of small non-coding RNAs involved in post-transcriptional regulation. Any alterations in this program can potentially contribute to dysregulated myelination, impaired remyelination and neurodegenerative conditions, as it happens in multiple sclerosis (MS). Here, we identify miR-125a-3p, a developmentally regulated miRNA, as a new actor of oligodendroglial maturation, that, in the mammalian CNS regulates the expression of myelin genes by simultaneously acting on several of its already validated targets. In cultured OPCs, over-expression of miR-125a-3p by mimic treatment impairs while its inhibition with an antago-miR stimulates oligodendroglial maturation. Moreover, we show that miR-125a-3p levels are abnormally high in the cerebrospinal fluid of MS patients bearing active demyelinating lesions, suggesting that its pathological upregulation may contribute to MS development, at least in part by blockade of OPC differentiation leading to impaired repair of demyelinated lesions.

CXCR7 Is Involved in Human Oligodendroglial Precursor Cell Maturation

Differentiation of oligodendroglial precursor cells (OPCs), a crucial prerequisite for central nervous system (CNS) remyelination in diseases such as Multiple Sclerosis (MS), is modulated by a multitude of extrinsic and intrinsic factors. In a previous study we revealed that the chemokine CXCL12 stimulates rodent OPC differentiation via activation of its receptor CXCR7. We could now demonstrate that CXCR7 is also expressed on NogoA- and Nkx2.2-positive oligodendroglial cells in human MS brains and that stimulation of cultured primary fetal human OPCs with CXCL12 promotes their differentiation as measured by surface marker expression and morphologic complexity. Pharmacological inhibition of CXCR7 effectively blocks these CXCL12-dependent effects. Our findings therefore suggest that a specific activation of CXCR7 could provide a means to promote oligodendroglial differentiation facilitating endogenous remyelination activities.

Role of adenosine in oligodendrocyte precursor maturation

Differentiation and maturation of oligodendroglial cells are postnatal processes that involve specific morphological changes correlated with the expression of stage-specific surface antigens and functional voltage-gated ion channels. A small fraction of oligodendrocyte progenitor cells (OPCs) generated during development are maintained in an immature and slowly proliferative or quiescent state in the adult central nervous system (CNS) representing an endogenous reservoir of immature cells. Adenosine receptors are expressed by OPCs and a key role of adenosine in oligodendrocyte maturation has been recently recognized. As evaluated on OPC cultures, adenosine, by stimulating A₁ receptors, promotes oligodendrocyte maturation and inhibits their proliferation; on the contrary, by stimulating A_2A receptors, it inhibits oligodendrocyte maturation. A₁ and A_2A receptor-mediated effects are related to opposite modifications of outward delayed rectifying membrane K⁺ currents (I_K) that are involved in the regulation of oligodendrocyte differentiation. Brain A₁ and A_2A receptors might represent new molecular targets for drugs useful in demyelinating pathologies, such as multiple sclerosis (MS), stroke and brain trauma.

Naturally occurring antibodies as therapeutics for neurologic disease: can human monoclonal IgMs replace the limited resource IVIG?

Naturally occurring autoantibodies (NAbs) are common in normal humans. The majority of NAbs are IgMs, but a small proportion are IgGs. Therefore a certain portion of pooled whole human IgG (IVIG) can be considered NAbs. While the applications of IVIG to modulate human disease have increased dramatically, the use of IgMs as drugs has lagged. In fact, much of the contaminating IgM component of IVIG is disposed of as waste. However, a number of model studies, including those targeting Alzheimer and multiple sclerosis (MS) suggest that IgMs may better modulate disease at much lower doses than IVIG. Our own studies in a model of MS show that polyclonal human IgM promotes better remyelination than IVIG and that monoclonal IgMs promote greater remyelination than monoclonal IgGs containing identical variable region sequences. We propose that this difference is due to the ability of IgM to cross link cell surface antigens better than IgGs and induce signals in nervous system cells. Monoclonal antibodies (mAbs) that promote remyelination induce a transient Ca(2+) influx in myelin forming cells, whereas IgGs with identical variable sequences do not. MAbs that promote remyelination were identified in human serum and in EBV-immortalized human B-cell lines obtained from normal adults, fetal cord blood, and rheumatoid arthritis and MS patients. Therefore therapeutic mAbs are present and common in normal circulation. All therapeutic mAbs were IgMs and bound to nervous system cells, however, the tissue binding patterns suggest that binding any one of multiple antigens induces repair. An expression vector was constructed that can manufacture gram quantities of recombinant monoclonal human IgM. Therefore the technology exists to determine whether human monoclonal NAbs can modulate human disease. IVIG can modulate neurologic disease, but using IVIG to treat these chronic diseases is unsustainable. A long-term solution is to identify the functional component of IVIG and test whether a recombinant human monoclonal can replicate its efficacy.

Phosphorylation and lipid raft association of fibroblast growth factor receptor-2 in oligodendrocytes

Fibroblast growth factors (FGFs) and their receptors (FGFRs) initiate diverse cellular responses that contribute to the regulation of oligodendrocyte (OL) function. To understand the mechanisms by which FGFRs elicit these cellular responses, we investigated the phosphorylation of signal transduction proteins and the role of cholesterol-glycosphingolipid-enriched "lipid raft" microdomains in differentiated OLs. Surprisingly, we found that the most abundant tyrosine-phosphorylated protein in OLs was the 120-kd isoform of FGFR2 and that it was phosphorylated even in the absence of FGF2, suggesting a potential ligand-independent function for this receptor. Furthermore, FGFR2, but not FGFR1, was associated with lipid raft microdomains in OLs and myelin (but not in astrocytes). This provides the first evidence for the association of FGFR with TX-100-insoluble lipid raft fractions. FGFR2 phosphorylated the key downstream target, FRS2 in OLs. Raft disruption resulted in loss of phosphorylated FRS2 from lipid rafts, coupled with the loss of Akt but not of Mek or Erk phosphorylation. This suggests that FGFR2-FRS2 signaling in lipid rafts operates via the PI3-Kinase/Akt pathway rather than the Ras/Mek/Erk pathway, emphasizing the importance of microenvironments within the cell membrane. Also present in lipid rafts in OLs and myelin, but not in astrocytes, was a novel 52-kd isoform of FGFR2 that lacked the extracellular ligand-binding region. These results demonstrate that FGFR2 in OLs and myelin possess unique characteristics that are specific both to receptor type and to OLs and provide a novel mechanism to elicit distinct cellular responses that mediate both FGF-dependent and -independent functions.

Oligodendrocyte development and the onset of myelination in the human fetal brain

Oligodendrocytes are cells that myelinate axons, providing saltatory conduction of action potentials and proper function of the central nervous system. Myelination begins prenatally in the human, and the sequence of oligodendrocyte development and the onset of myelination are not thoroughly investigated. This knowledge is important to better understand human diseases, such as periventricular leukomalacia, one of the leading causes of motor deficit in premature babies, and demyelinating disorders such as multiple sclerosis (MS). In this review we discuss the spatial and temporal progression of oligodendrocyte lineage characterized by the expression of specific markers and transcription factors in the human fetal brain from the early embryonic period (5 gestational weeks, gw) until midgestation (24 gw). Our in vitro evidence indicated that a subpopulation of human oligodendrocytes may have dorsal origin, from cortical radial glia cells, in addition to their ventral telencephalic origin. Furthermore, we demonstrated that the regulation of myelination in the human fetal brain includes positive and negative regulators. Chemokines, such as CXCL1, abundant in proliferative zones during brain development and in regions of remyelination in adult, are discussed in the view of their potential roles in stimulating oligodendrocyte development. Other signals are inhibitory and may include, but are not limited to, polysialic acid modification of the neural cell adhesion molecule on axons. Overall, important differences in temporal and spatial distribution and regulatory signals for oligodendrocyte differentiation exist between human and rodent brains. Those differences may underlie the unique susceptibility of humans to demyelinating diseases, such as MS.

Biochemical characterization of tau protein and its associated syndapin 1 and protein kinase Cepsilon for their functional regulation in rat brain

BACKGROUND

We recently reported that both sulfatide and cholesterol-3-sulfate (SCS) function as potent stimulators for the GSK-3beta-mediated phosphorylation of tau protein (TP) in vitro [J. Biochem. 143 (2008) 359-367].

METHODS

By means of successive gel filtration on a Superdex 200 pg column and three distinct ion-exchange column chromatographies, TP and its associated proteins were highly purified from the extract of rat brain.

RESULTS

We found that (i) syndapin 1 and novel protein kinase Cepsilon (nPKCepsilon) were identified as the TP-associated proteins; (ii) SCS highly stimulated the phosphorylation of TP and syndapin 1 by nPKCepsilon as well as CK1; (iii) the full phosphorylation of TP and syndapin 1 by nPKCepsilon in the presence of sulfatide resulted in their dissociation; (iv) TP primed by CK1 functioned as an effective phosphate acceptor for GSK-3beta; (v) syndapin 1 highly stimulated the GSK-3beta-mediated phosphorylation of TP; and (vi) TP isoforms were highly expressed in aged brain, whereas syndapin 1 was consistently detected in adult brain, but not in newborn brain.

GENERAL SIGNIFICANCE

These results provided here suggest that (i) TP-associated nPKCepsilon suppresses the GSK-3beta-mediated phosphorylation of TP through the phosphorylation of GSK-3beta by the kinase in vitro; and (ii) SCS act as effective sole mediators to induce the GSK-3beta-mediated high phosphorylation of both TP and its associated syndapin 1 involved in the biochemical processes of neuronal diseases, including Alzheimer's disease.

Immunostaining for oligodendrocyte-specific galactosphingolipids in fixed brain sections using the cholesterol-selective detergent digitonin

Galactocerebroside (GalC) and its sulfated derivative sulfatide (SUL) are galactosphingolipids abundantly expressed in oligodendrocytes (OLs). Despite their biological importance in OL development and function, attempts to visualize GalC/SUL in tissue sections have met with limited success. This is at least in part because permeabilization of tissue sections with detergents such as Triton X-100 results in significant degradation of GalC/SUL immunoreactivity. Here we establish a novel method that enables visualization of endogenous GalC/SUL in OLs and myelin throughout the entire depth of brain sections. We show that treating brain sections with the cholesterol-specific detergent digitonin instead of Triton X-100 or methanol leads to efficient antibody penetration into tissue sections without disrupting GalC/SUL immunoreactivity. We also determine the optimal concentrations of digitonin using confocal microscopy. With our method, the morphology and the number of GalC/SUL-expressing OLs can be visualized three-dimensionally. Furthermore, our method is applicable to double immunostaining with anti-GalC/SUL antibody and other antibodies which recognize intracellular antigens. Our simple method using digitonin should prove to be useful in enabling detailed examination of GalC/SUL expression in the brain in both physiological and pathological conditions.

Remyelination-promoting human IgMs: developing a therapeutic reagent for demyelinating disease

Promoting remyelination following injury to the central nervous system (CNS) promises to be an effective neuroprotective strategy to limit the loss of surviving axons and prevent disability. Studies confirm that multiple sclerosis (MS) and spinal cord injury lesions contain myelinating cells and their progenitors. Recruiting these endogenous cells to remyelinate may be of therapeutic value. This review addresses the use of antibodies reactive to CNS antigens to promote remyelination. Antibody-induced remyelination in a virus-mediated model of chronic spinal cord injury was initially observed in response to treatment with CNS reactive antisera. Monoclonal mouse and human IgMs, which bind to the surface of oligodendrocytes and myelin, were later identified that were functionally equivalent to antisera. A recombinant form of a human remyelination-promoting IgM (rHIgM22) targets areas of CNS injury and promotes maximal remyelination within 5 weeks after a single low dose (25 microg/kg). The IgM isoform of this reparative antibody is required for in vivo function. We hypothesize that the IgM clusters membrane domains and associated signaling molecules on the surface of target cells. Current therapies for MS are designed to modulate inflammation. In contrast, remyelination promoting IgMs are the first potential therapeutic molecules designed to induce tissue repair by acting within the CNS at sites of damage on the cells responsible for myelin synthesis.

White matter axon vulnerability to AMPA/kainate receptor-mediated ischemic injury is developmentally regulated

Periventricular white matter injury (PWMI) is the leading cause of neurodevelopmental morbidity in survivors of premature birth. Cerebral ischemia is considered a major etiologic factor in the generation of PWMI. In adult white matter (WM), ischemic axonal damage is mediated by AMPA/kainate receptors. Mechanisms of ischemic axonal injury during development are not well defined. We used a murine brain slice model to characterize mechanisms of ischemic axonal injury in developing WM. Acute coronal brain slices were prepared from thy1-yellow fluorescent protein (YFP) mice at postnatal day 3 (P3), P7, P10, and P21. Ischemia was simulated by oxygen-glucose deprivation (OGD). YFP-positive axon morphology in the corpus callosum was preserved for at least 15 h under normoxic conditions. OGD resulted in delayed degeneration of YFP-positive axons, characterized by axonal beading, fragmentation, and loss of YFP. AMPA and cyclothiazide damaged WM axons at P7, P10, and P21 but not at P3. The AMPA/kainate receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) decreased OGD-induced axonal degeneration and oligodendrocyte loss at P10 and P21. At P3 and P7, NBQX protected oligodendrocytes but did not prevent axonal degeneration after OGD. The NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] provided no protection at any age. These results indicate that developing WM axons are susceptible to ischemic injury. However, mechanisms of axonal degeneration are developmentally regulated. At P3 and P7, corresponding developmentally to the window of peak vulnerability to PWMI in humans, ischemic axonal injury is not mediated by AMPA/kainate receptors. Strategies to protect WM during this period may be substantially different from those used at later developmental stages.

Endogenous alpha2-adrenergic receptor-mediated neuroprotection after severe hypoxia in preterm fetal sheep

Central alpha-adrenergic receptor activity is important for fetal adaptation to hypoxia before birth. It is unclear whether it is also important during recovery. We therefore tested the hypothesis that an infusion of the specific alpha(2)-adrenergic receptor antagonist idazoxan (1 mg/kg/h i.v.) from 15 min to 4 h after profound hypoxia induced by 25 min umbilical cord occlusion in fetal sheep at 70% of gestation (equivalent to the 28-32 weeks in humans) would increase neural injury. After 3 days' recovery, idazoxan infusion was associated with a significant increase in neuronal loss in the hippocampus (P<0.05), expression of cleaved caspase-3 (P<0.05), and numbers of activated microglia (P<0.05). There was no significant effect on other neuronal regions or on loss of O4-positive premyelinating oligodendrocytes in the subcortical white matter. Idazoxan was associated with an increase in evolving epileptiform electroencephalographic (EEG) transient activity after occlusion (difference at peak 2.5+/-1.0 vs. 11.7+/-4.7 counts/min, P<0.05) and significantly reduced average spectral edge frequency, but not EEG intensity, from 54 until 72 h after occlusion (P<0.05). Hippocampal neuronal loss was correlated with total numbers of epileptiform transients during idazoxan infusion (P<0.01; r(2)=0.7). In conclusion, endogenous inhibitory alpha(2)-adrenergic receptor activation after severe hypoxia appears to significantly limit evolving hippocampal damage in the immature brain.

Sulfatide with short fatty acid dominates in astrocytes and neurons

Glycosphingolipids are located in cell membranes and the brain is especially enriched. We speculated that the subcellular location of glycosphingolipids depends on their fatty acid chain length because their sugar residues are constant, whereas fatty acid chain length can vary within the same molecule. To test this hypothesis we analysed the glycosphingolipid sulfatide, which is highly abundant in myelin and has mostly long fatty acids. We used a negative ion electrospray tandem mass spectrometry precursor ion scan to analyse the molecular species of sulfatide in cultured astrocytes and a mouse model of the human disease metachromatic leukodystrophy. In these arylsulfatase A (ASA)-deficient mice sulfatide accumulates intracellularly in neurons and astrocytes. Immunocytochemistry was also performed on cultured astrocytes and analysed using confocal laser scanning microscopy. Analyses of the molecular species showed that cultured astrocytes contained sulfatide with a predominance of stearic acid (C18), which was located in large intracellular vesicles throughout the cell body and along the processes. The same was seen in ASA-deficient mice, which accumulated a higher proportion (15 mol% compared with 8 mol% in control mice) of sulfatide with stearic acid. We conclude that the major fatty acid composition of sulfatide differs between white and grey matter, with neurons and astrocytes containing mostly short-chain fatty acids with an emphasis on stearic acid. Based on our results, we speculate that the fatty acid chain length of sulfatide might determine its intracellular (short chain) or extracellular (long chain) location and thereby its functions.

Induction of oligodendrocyte progenitors in dorsal forebrain by intraventricular microinjection of FGF-2

During embryonic development, oligodendrocyte progenitors (OLPs) originate from the ventral forebrain under the regulation of Sonic hedgehog (Shh). Shh controls the expression of transcription factor Olig2, which is strongly implicated in OLP generation. Studies of mice deficient in Shh expression suggest, however, that an alternative pathway for OLP generation may exist. The generation of OLPs in dorsal forebrain has been suggested since treatment of dorsal-neural progenitor cells in culture with fibroblast growth factor (FGF-2) results in OLP induction. To ask if dorsal induction of OLPs in embryonic forebrain can occur in vivo and if FGF-2 could initiate an alternative pathway of regulation, we used in utero microinjection of FGF-2 into the lateral ventricles of mouse fetal forebrain. A single injection of FGF-2 at E13.5 resulted in the expression of the OLP markers Olig2 and PDGFRalpha mRNA in dorsal forebrain ventricular and intermediate zones. However, FGF-2 did not induce dorsal expression of Shh, Patched1 or Nkx2.1, and co-injection of FGF-2 and a Shh inhibitor did not attenuate the induction of Olig2 and PDGFRalpha, suggesting that Shh signaling was not involved in this FGF-2-mediated dorsal induction. These results demonstrate that the dorsal embryonic forebrain in vivo has the potential to generate OLPs in the presence of normal positional cues and that this can be driven by FGF-2 independent of Shh signaling.

Growth conelike sensorimotor structures are characteristic features of postmigratory, premyelinating oligodendrocytes

During development, postmigratory, premyelinating oligodendrocytes extend processes that navigate through the central nervous system (CNS) environment, where they recognize a number of extracellular cues, including axonal segments to be myelinated. Ultimately this recognition event leads to the formation of the CNS myelin sheath. However, the morphological structures and molecular mechanisms that control such oligodendroglial pathfinding are poorly understood. Here we show that postmigratory, premyelinating oligodendrocyte processes possess at their distal tips expansions that ultrastructurally resemble growth cones of postmigratory neurons and that we will refer to as OLG-growth cones. OLG-growth cones are highly motile and capable of mediating process outgrowth, retraction, and branching. In addition, they express regulators of cytoskeletal organization, GAP43 and cofilin, that are known to mediate neuronal growth cone navigation. In a choice situation, processes of postmigratory, premyelinating oligodendrocytes and their OLG-growth cones have the ability to selectively avoid a nonpermissive substrate, that is, collagen IV. Thus, our findings provide, for the first time, a detailed characterization of sensorimotor structures present at the tips of postmigratory, premyelinating oligodendrocyte processes. Furthermore, the data presented here suggest that, although the cellular mechanisms involved in growth cone steering may be similar for postmigratory neuronal and oligodendroglial cells, extracellular cues may be interpreted in a cell-type-specific fashion.

Increased myelinating capacity of embryonic stem cell derived oligodendrocyte precursors after treatment by interleukin-6/soluble interleukin-6 receptor fusion protein

Neurosphere cells (NSc) derived from embryonic stem cells have characteristics of neural stem cells and can differentiate into oligodendrocyte precursors. Culture of NSc with IL6RIL6 chimera (soluble interleukin-6 receptor fused to interleukin-6) enhances their differentiation into oligodendrocytes with longer and more numerous branches and with peripheral accumulation of myelin basic protein (MBP) in myelin membranes indicating maturation. Gene expression profiling reveals that one of the proteins strongly induced by IL6RIL6 is a regulator of microtubule dynamics, stathmin-like 2 (SCG10/Stmn2), and gene silencing shows that Stmn2 plays an important role in the development of the mature oligodendrocyte morphology. IL6RIL6 acts as an effective stimulator of the myelinating function of ES cell-derived oligodendrocyte precursors, as observed upon transplantation of the IL6RIL6- pretreated cells into brain slices of MBP-deficient shiverer mice.

A new ENU-induced allele of mouse quaking causes severe CNS dysmyelination

The mutant allelic series of the mouse quaking gene consists of the spontaneous quaking(viable) (qk(v)) allele, which is homozygous viable with a dysmyelination phenotype, and four ENU-induced alleles (qk(kt 1), qk(k2), qk(kt3/4), and qk(l-1)), which are homozygous embryonic lethal. Here we report the isolation of qk(e5), the first ENU-induced viable allele of quaking. Unlike qk(v)/qk(v), qk(e5)/qk(e5) animals have early-onset seizures, severe ataxia, and a dramatically reduced lifespan. Ultrastructural analysis of qk(e5)/qk(e5) brains reveals severe dysmyelination when compared with both wild-type and qk(v)/qk(v) brains. In addition, Calbindin detection in young adult qk(e5)/qk(e5) mice reveals Purkinje cell axonal swellings indicative of neurodegeneration , which is not seen in young adult qk(v)/qk(v) mice. Although the molecular defect in the qk(e5) allele is not evident by sequencing, protein expression studies show that qk(e5)/qk(e5) postnatal oligodendrocytes lack the QKI-6 and QKI-7 isoforms and have reduced QKI-5 levels. The oligodendrocyte developmental markers PDGF alpha R, NG 2, O4, CNP, and MBP are also present in the qk(e5)/qk(e5) postnatal brain although CNP and MBP levels are considerably reduced. Because the qk(v) allele is a large deletion that affects the expression of three genes, the new neurologic qk(e5) allele is an important addition to this allelic series.

S-phase entry of oligodendrocyte lineage cells is associated with increased levels of p21Cip1

The mechanisms regulating the number of myelinating cells in the central nervous system are crucial for both normal development and repair in pathological conditions. Among relevant growth factors involved in this process, fibroblast growth factor-2 (FGF2) induces oligodendrocyte progenitors (OLPs) to proliferate and stimulates mature oligodendrocytes (OLs) to reenter the S-phase of the cell cycle. S-phase entry is modulated by the formation of complexes between cyclins and cyclin-dependent kinases (CDKs), on one hand, and by their interactions with cell cycle inhibitors (e.g., p18INK, p27Kip1, p21Cip1), on the other. Although the roles of cyclin E/CDK2 complexes and the inhibitor p27Kip1 have been extensively investigated relative to proliferation and differentiation in the OL lineage, less is known about the regulation of the formation of cyclin D1/CDK4 complexes and the role of p21Cip1 in these events. In this study, we show that the FGF2-mediated increase in bromodeoxyuridine (BrdU) incorporation into OL progenitors and mature OLs occurs concomitantly with increase in the levels of p21Cip1 and the formation of p21Cip1/cyclin D1/CDK4 ternary complexes. These complexes are functionally active is indicated by the ensuing FGF2-dependent hyperphosphorylation of the downstream target Rb. In untreated mature OLs that do not incorporate BrdU, the levels of p21Cip1 are low, and the level of the inhibitor p18INK is high. Furthermore, p18INK sequesters CDK2 into binary complexes, precluding the formation of p21Cip1/cyclin D1/CDK4 ternary complexes in these cells. Therefore, we propose that p21Cip1 is acting as a positive regulator, rather than an inhibitor, of cell cycle entry by favoring the assembly of active cyclin D1/CDK4 complexes.

Sequence of oligodendrocyte development in the human fetal telencephalon

Oligodendrocytes (OL), cells that myelinate axons in the CNS, differentiate from early to late oligodendrocyte progenitor cells (OPC) to become mature OL. Unlike the case in the rodent brain, myelin formation starts prenatally in the human brain, but the sequence of OL development and the onset of myelination are not well understood. We studied the human fetal forebrain at midgestation (17-23 gestational weeks, g.w.) using OL lineage-specific antibodies and mRNA probes. Early OPC were present in a gradient from the subventricular zone to the cortical plate. Their close apposition to radial glia fibers suggests a possible role of these fibers in OPC migration. Late OPC reached peak density in the subplate layer, whereas multipolar cells with the morphology of mature OL were restricted to the emerging white matter. At 20 g.w., myelinated axons were observed in the diencephalon, but not in the telencephalon, consistent with caudal-to-rostral progression of myelination. Interestingly, in organotypic slice cultures of the same gestational ages, the subventricular zone contained a considerably greater number of the mature OL cells, suggesting the presence of inhibitory signals in vivo. Overall, in addition to considerable similarities with rodents, important differences in temporal and spatial distribution and regulatory signals for OL differentiation exist in the human brain.

Spatial and temporal expression of S100B in cells of oligodendrocyte lineage

The analysis of oligodendrocyte (OL) lineage development has been facilitated by the immunocytochemical characterization of OL-specific antigens and definition of the phenotypes sequentially acquired by differentiating OLs. The purpose of the present study was to address an enduring discrepancy between several reported cases of S100B immunodetection in CNS myelin and myelinating OLs on the one hand, and the systematic use of the S100B protein as an alleged astrocytic marker in studies of the mammalian CNS on the other. To resolve this discrepancy, we have compared the developmental distribution of EGFP+ cells in the CNS of s100b-enhanced green fluorescent protein (EGFP) (Vives et al., 2003) and cnp-EGFP (Yuan et al., 2002) mice, and examined the degree of overlap between EGFP expression and that of stage-specific markers of OL differentiation during the embryonic and postnatal phases of development. We demonstrate that the S100B protein is expressed in postnatal and adult populations of NG2+ progenitors of mouse brain, as well as in immature and mature myelinating OLs present in the brain and spinal cord of embryonic and adult mice, respectively. Comparison between EGFP and endogenous S100B expression in the s100b-EGFP and cnp-EGFP mice indicates that S100B protein expression is upregulated in immature and mature OLs. These results argue against the current view that S100B expression is restricted to the astrocytic lineage in the CNS, and indicate that the use of S100B in combination with other molecular markers will help discriminate oligodendrocytes from astrocytes.

Generation of purified oligodendrocyte progenitors from embryonic stem cells

Demyelination is a key component in the pathogenesis of many neurological disorders. Transplantation of myelinating cells may offer a therapeutic approach to restore neurological function in these diseases. Recent findings suggest that pluripotent embryonic stem (ES) cells can serve as an unlimited donor source for neural transplantation. The clinical application of ES cells for myelin repair will depend critically on the ability to enrich oligodendroglial progenitors in high purity. Combining controlled differentiation in the presence of growth factors and genetic lineage selection, we devised a cell culture protocol yielding highly purified oligodendrocyte progenitors. Murine ES cell clones stably transfected with a construct encoding the beta-galactosidase-neomycine phosphotransferase fusion protein (beta(geo)) under control of the 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter were differentiated into bipotential glial precursors. Subsequent induction of a CNP-positive stage and selection in neomycine resulted in a homogenous cell population with a pre-oligodendrocyte phenotype. The selected cells continued to proliferate in the presence of FGF-2 and PDGF and, upon growth factor withdrawal, differentiated into mature galactocerebroside (GalC)-positive oligodendrocytes. Transplantation studies in myelin-deficient (md) rats indicate that ES cell-derived oligodendrocyte progenitors generated with this method may serve as an attractive donor source for myelin repair.

Neuron-binding human monoclonal antibodies support central nervous system neurite extension

Two human IgMs (sHIgM12 and sHIgM42) were identified that supported in vitro central nervous system (CNS) neurite extension equal to the potent neurite stimulatory molecule laminin. Both IgMs bound to multiple cell types in unfixed CNS tissue and to the surface of neurons in culture. Both monoclonal antibodies (mAbs) overrode the inhibitory effect of CNS mouse myelin on granule cell neurite extension. Neither mAb bound to the surface of mature oligodendrocytes or strictly colocalized with myelin proteins. Sialidase treatment eliminated the neuronal surface binding of both mAbs, whereas blocking sphingolipid synthesis with Fumonisin B1 or removing GPI-linked proteins with PIPLC did not. When used as substrates for mixed neuron/glia aggregates, sHIgM12 and sHIgM42 supported robust neurite extension while astrocytes remained in the aggregates. In contrast, laminin supported astrocyte migration and spreading. Human mAbs that support neurite extension are novel factors that may be of use in encouraging axon repair following injury while minimizing glial cell infiltration. Both human mAbs were isolated from individuals with monoclonal gammopathy. Each individual has carried high mAb titers in circulation for years without detriment. sHIgM12 and sHIgM42 are therefore unlikely to be systemically pathogenic.

Antiapoptotic signaling by a remyelination-promoting human antimyelin antibody

Stabilizing the survival of oligodendrocytes and oligodendrocyte precursors within and near lesions in patients suffering from multiple sclerosis (MS) and other demyelinating diseases is an important therapeutic goal. Previous studies have identified a human-derived monoclonal IgM antibody designated rHIgM22 that induces remyelination in a mouse model of MS. We provide evidence that this antibody, directed against myelin, induces antiapoptotic signaling in premyelinating oligodendrocytes and reduces caspase-3 activation and caspase gene expression in mice undergoing antibody-induced remyelination. This effect was dependent on calcium entry via CNQX-sensitive channels and on lipid raft integrity, and was correlated with suppression of JNK signaling. We conclude that rHIgM22 may induce remyelination via rescue of oligodendrocytes, and suggest that such autoantibody-mediated signaling may have important therapeutic implications for a variety of neurological diseases, including stroke and Alzheimer's disease.

Sulfatide is a negative regulator of oligodendrocyte differentiation: Development in sulfatide-null mice

Galactosylceramide (GalC) and its sulfated analogue, sulfatide, are major galactosphingolipid components of myelin and oligodendrocyte plasma membranes in the nervous system. We previously hypothesized that these galactolipids play functional roles in the regulation of oligodendrocyte terminal differentiation by acting as sensors/transmitters of environmental information. Evidence strongly supports this idea. First, these molecules are initially expressed on the cell surface at the interface at which oligodendrocyte progenitors first enter terminal differentiation. Second, exposure of oligodendrocyte progenitors to anti-GalC/-sulfatide (RmAb) or antisulfatide (O4), but not anti-GalC (O1), antibodies leads to the reversible arrest of oligodendrocyte lineage progression at this interface. Third, in cerebroside galactosyl transferase-null mice (Cgt(-/-)) that are unable to synthesize either GalC or sulfatide, terminal differentiation and morphological maturation of oligodendrocytes are enhanced. In the present study, we examined oligodendrocytes differentiation in cerebroside sulfotransferase-null mice (Cst(-/-)) that lack sulfatide but express GalC. We show that cerebroside sulfotransferase mRNA expression begins already in the embryonic spinal cord and progressively increases with age, that the late progenitor marker POA is not synthesized in the absence of this enzyme, and that, most notably, there is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo, similar to that in mice lacking both GalC and sulfatide. We conclude that primarily sulfatide, rather than GalC, is a key molecule for the negative regulation of oligodendrocyte terminal differentiation.

Specific inhibitor of FGF receptor signaling: FGF-2-mediated effects on proliferation, differentiation, and MAPK activation are inhibited by PD173074 in oligodendrocyte-lineage cells

Multiple studies have shown that migration, proliferation, and differentiation of oligodendrocyte (OL) lineage cells are influenced by fibroblast growth factor-2 (FGF-2) signaling through its receptors (FGFR) FGFR-1, FGFR-2, and FGFR-3. We report the effectiveness and specificity of a unique inhibitor, PD173074, for inhibiting FGF receptor signaling in OL-lineage cells. Three FGF-mediated responses of OL progenitors and two of differentiated OLs were examined by immunofluorescence microscopy and immunoblotting. PD173074 effectively antagonized the effect of FGF-2 on proliferation and differentiation of OL progenitors in culture. One dose of PD173074 at nanomolar concentrations was sufficient to inhibit ongoing FGF-2 mediated proliferation for prolonged periods, in a non-toxic, dose-dependent manner. In contrast, platelet-derived growth factor (PDGF)-induced proliferation was unaffected by PD173074. Similarly, mitogen-activated protein kinase (MAPK) activation, a downstream event after activation of either FGFR or PDGFR, was also blocked by PD173074 in OL progenitors stimulated with FGF-2 but not PDGF. A general tyrosine kinase inhibitor (PD166285), however, antagonized both FGF-2- and PDGF-mediated responses. PD173074 also completely antagonized two phenotypic alterations of differentiated OLs, specifically downregulation of myelin proteins, and their re-entry into the cell cycle. We conclude that PD173704 is an effective and specific inhibitor for multiple FGF-2-mediated responses of both OL progenitors and differentiated OLs. This inhibitor provides a direct approach for identifying the importance of FGF signaling, comparable in effect to a knockout of all FGF receptors and all FGF ligands, while leaving other pathways unaffected. Thus, PD173704 is an excellent tool for investigating the role of FGF signaling in vivo in the context of combinatorial interactions of other signals.

Effect of a traditional Chinese herbal medicine, Ren-Shen-Yang-Rong-Tang (Japanese name: Ninjin-Youei-To), on oligodendrocyte precursor cells from aged-rat brain

Understanding of oligodendrocyte precursor cells and their role in the generation of oligodendrocytes in developing and adult rodents has been considered, particularly much less is known about aged-rodent oligodendrocyte precursor cells and their cell lineage. In this present study, we have developed oligodendrocyte cultures from the 30-month-old rat brain and examined whether oligodendrocyte precursor cells can proliferate in vitro. Adult oligodendrocyte precursor cells (O1(-), O4(+)) and oligodendrocytes (O1(+), O4(+)) are present in the cultures of the 30-month-old rat brain. They are also capable of proliferating and differentiating in the cultures. These capabilities increased four- to fivefold, when the aged rats are treated with Ninjin-Youei-To for 3 months in comparison with those of control aged rats. These results suggest that Ninjin-Youei-To has a potential mitotic effect on oligodendrocyte precursor cells in aged-rat brains and may be expected to have a therapeutic effect on brain aging.

Quantitative analysis of perinatal rodent oligodendrocyte lineage progression and its correlation with human

The development of a rodent model in the perinatal rat or mouse that reproduces the principal features of human perinatal white matter injury (periventricular leukomalacia) has been hampered by uncertainty about the developmental window in the rodent that coincides temporally with cerebral white matter development in the premature infant. We recently determined oligodendrocyte (OL) lineage progression in human cerebral white matter and found that the late OL progenitor (preOL) predominates throughout the high-risk period for periventricular leukomalacia [J. Neurosci. 21(2001), 1302-1312]. Here, we determined in the perinatal rat and mouse when each species displays a distribution of OL stages that is similar to the premature human cerebral white matter. PreOLs are abundant in the rat and mouse at P2. By P7, extensive OL maturation occurs in both species and coincides with the onset of early myelination. PreOLs and immature OLs mature in the P2 white matter along a medial to lateral gradient. This may provide an explanation for regional variation in the susceptibility of perinatal white matter to injury. We propose that the sequence of OL lineage progression is a useful means to estimate developmental windows of white matter maturation in perinatal rodents that coincide with those of developing human cerebral white matter. These studies support that the vulnerable period for white matter injury in the rodent is centered around P2 and should decline thereafter, coincident with the onset of myelination.

Fibroblast growth factor receptor 3 signaling regulates the onset of oligodendrocyte terminal differentiation

Fibroblast growth factor receptor (FGFR) signaling is essential for nervous system development. We have shown that, in the normal postnatal brain, the spatial and temporal expression pattern of FGFR3 parallels the appearance of differentiated oligodendrocytes and that in culture FGFR3 is expressed maximally at the critical stage in the lineage at which oligodendrocyte late progenitors (Pro-OLs) enter terminal differentiation. Therefore, FGFR3 expression is positioned ideally to have an impact on oligodendrocyte differentiation. In support of this we show that, during the onset and active phase of myelination in FGFR3-deficient mice, there are reduced numbers of differentiated oligodendrocytes in the forebrain, cerebellum, hindbrain, and spinal cord. Furthermore, myelination is delayed in parallel. Delay of oligodendrocyte differentiation also is observed in primary cell culture from this mutant. On the other hand, no differences are observed in the survival or proliferation of oligodendrocyte progenitors. This suggests that the decrease in the number of differentiated oligodendrocytes is attributable to a delay in the timing of their differentiation process. Astrocytes also express FGFR3, and in mice lacking FGFR3 there is an enhancement of the astrocytic marker glial fibrillary acidic protein expression in a region-specific manner. Thus our findings suggest that there are cell type- and region-specific functions for FGFR3 signaling and in particular emphasize a prominent role for FGFR3 as part of a system regulating the onset of oligodendrocyte terminal differentiation.

Progenitors in the postnatal cerebellar white matter are antigenically heterogeneous

Progenitors that migrate through the white matter of the postnatal cerebellum give rise to cortical interneurons, astroglia, and oligodendroglia. To determine whether this progenitor population is heterogeneous with respect to specific lineage markers, we infected progenitors in vivo with a retrovirus encoding the green fluorescent protein on postnatal day 4/5 and labeled them in situ with various antibodies 2 days postviral injection: the neuronal marker was the transcription factor SOX1; early oligodendroglial markers were chondroitin sulfate proteoglycan antigen and platelet-derived growth factor receptor-alpha. Markers for astroglial progenitors were vimentin, nestin, zebrin II, and the astroglial-specific glutamate transporter subtype GLAST. None of the progenitors was doubly labeled with any combination of markers characteristic for different cell lineages. Most progenitors were not labeled with any of the various combinations of antibodies used. Progenitors did not express markers characteristic for mature astroglia (GFAP), oligodendroglia (CNPase), or neurons (MAP2). Thus, although these progenitors are morphologically indistinguishable, a minority expresses markers of early neuronal or glial lineages, suggesting that they begin to differentiate during migration.

Syndecan-3 and perlecan are differentially expressed by progenitors and mature oligodendrocytes and accumulate in the extracellular matrix

Oligodendrocyte progenitors originate in the subventricular zone, proliferate, migrate to their final destinations, differentiate, and interact with axons to produce multilamellar myelin sheaths. These processes are regulated by a variety of environmental signals, including growth factors, the extracellular matrix, and adhesion molecules. Heparan sulfate proteoglycans are premier candidates as participants in this regulation by virtue of their structural diversity and their capacity to function as coreceptors for both growth factors and extracellular matrix molecules. Consistently with this, we have previously shown that oligodendrocyte progenitors are unable to proliferate in response to fibroblast growth factor-2 (FGF-2) in the absence of sulfated heparan sulfate proteoglycan. Here we show that members of three families of heparan sulfate proteoglycans, syndecan, perlecan, and glypican, are developmentally and posttranscriptionally regulated during oligodendrocyte-lineage progression: Syndecan-3 is synthesized by oligodendrocyte progenitors (but not terminally differentiated oligodendrocytes) and is up-regulated by FGF-2; perlecan synthesis increases as oligodendrocytes undergo terminal differentiation; glypican-1 is expressed by both progenitors and differentiated oligodendrocytes. Astrocytes express glypican-1 and perlecan but not syndecan-3. All three of these heparan sulfate proteoglycans are shed from the cell surface and bind to specific substrates. The developmentally regulated expression of these heparan sulfate proteoglycans is indicative of their participation in events involving growth factor receptors and the extracellular matrix that may regulate oligodendrocyte progenitor proliferation, migration, and adhesion phenomena.

Direct evidence that a human antibody derived from patient serum can promote myelin repair in a mouse model of chronic-progressive demyelinating disease

Certain human sera from patients with monoclonal gammopathies contain factors that induce myelin repair in animals with demyelinating disease. We hypothesize that antibodies functionally distinguish the serum of one patient from another. However, pooled normal polyclonal human IgM antibodies also induce remyelination. Definitive proof that specific antibodies are the biologically active components of serum is missing because unquestionably pure preparations of antibody molecules cannot be generated by fractionation. To demonstrate definitively that antibody is the biologically active component of patient serum, recombinant antibody was generated for evaluation in bioassays. The induction of remyelination in vivo requires milligram quantities of antibody. Consequently, an expression system was engineered to express high-titer, recombinant human IgM antibodies in vitro. A resulting recombinant antibody (rHIgM22) was evaluated for its ability to induce remyelination in the Theiler's virus mouse model of chronic-progressive demyelinating disease. We demonstrate that a single recombinant monoclonal antibody recapitulates the key characteristics of patient serum, including binding specificity, the induction of calcium signals in oligodendrocytes in vitro, and the induction of myelin repair within demyelinated plaques in vivo. The rHIgM22 antibody provides a new venue for the analysis of mechanisms governing remyelination and may prove useful in the treatment of demyelinating diseases.

AMPA glutamate receptor-mediated calcium signaling is transiently enhanced during development of oligodendrocytes

Cells of the oligodendroglial lineage express Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-preferring glutamate receptors (AMPA-GluR) during development. Prolonged activation of their AMPA-GluR causes Ca2+ overload, resulting in excitotoxic death. Prior studies have shown that oligodendroglial progenitors and immature oligodendrocytes are susceptible to excitotoxicity, whereas mature oligodendrocytes are resistant. An unresolved issue has been why Ca2+-permeability of AMPA-GluR varies so markedly with oligodendroglial development, although the level of expression of edited GluR2, an AMPA-GluR subunit which blocks Ca2+ entry, is relatively constant. To address this question, we performed Ca2+ imaging, molecular and electrophysiological analyses using purified cultures of the rat oligodendroglial lineage. We demonstrate that transient up-regulation of expression of GluR3 and GluR4 subunits in oligodendroglial progenitors and immature oligodendrocytes results in the assembly by these cells, but not by oligodendroglial pre-progenitors or mature oligodendrocytes, of a population of AMPA-GluR which lack GluR2. This stage-specific up-regulation of edited GluR2-free, and hence Ca2+-permeable, AMPA-GluR explains the selective susceptibility to excitotoxicity of cells at these stages of oligodendroglial differentiation, and is likely to be important to these cells in the trans-synaptic Ca2+-signaling from glutamatergic neurons, which occurs in hippocampus

Human antibodies accelerate the rate of remyelination following lysolecithin-induced demyelination in mice

Immunoglobulin-based therapies are becoming increasingly common for the treatment of neurologic and autoimmune diseases in humans. In this study, we demonstrate that systemic administration of either polyclonal human immunoglobulins or specific human monoclonal antibodies can accelerate the rate of CNS remyelination following toxin-induced demyelination. Injection of lysolecithin directly into the spinal cord results in focal demyelinated lesions. In contrast to other murine models of demyelinating disease, the mechanism of demyelination following lysolecithin injection is independent of immune system activation, and chronic inflammation at the site of the lesion is minimal. Administration of polyclonal human IgM (pHIgM) or a serum-derived human monoclonal antibody (sHIgM22) resulted in approximately a twofold increase in remyelinating axons when compared to animals treated with saline or with antibodies that do not promote repair. Both pHIgM and sHIgM22 show strong binding to CNS white matter and oligodendrocytes, while antibodies that did not accelerate remyelination do not. This differential staining pattern suggests that enhanced remyelination may result from direct stimulation of oligodendrocyte remyelination by binding to surface receptors on oligodendrocytes or glial progenitor cells. We propose the use of human polyclonal IgM or specific human monoclonal IgM antibodies as potential therapies to enhance myelin repair following CNS injury and disease.

Proteolipid promoter activity distinguishes two populations of NG2-positive cells throughout neonatal cortical development

Transgenic mice expressing enhanced green fluorescent protein (EGFP) driven by the mouse myelin proteolipid protein (PLP) gene promoter have been developed to investigate cells in the oligodendrocyte lineage. Transgene expression is consistent with the developmental expression of PLP, with cells at all stages of oligodendrocyte differentiation clearly visualized. These animals were analyzed to establish the time course of oligodendrocyte progenitor migration, proliferation, and differentiation in neonatal cortex. In these animals, two populations of NG2 proteoglycan-positive oligodendrocyte progenitor cells were identified that exist in postnatal subventricular zone and cortex. These two populations are distinguished by the presence or absence of PLP gene expression. Thus, PLP gene expression defines a subpopulation of NG2-positive cells from very early postnatal ages, which migrates toward the pial surface and proliferates in situ. EGFP(+)/NG2(+) cells are present in the cortex from postnatal day 1, and they remain in the cortex as undifferentiated oligodendrocyte progenitors for up to 3 weeks before myelination begins. These data could be explained by the presence of an important inhibitor of oligodendrocyte differentiation in the cortex during this period, which is downregulated in a region-specific manner to allow myelination. On the other hand, it is possible that oligodendrocyte progenitor cells remain undifferentiated in cortex until an essential signal is produced in situ to induce differentiation.

The tripotential glial-restricted precursor (GRP) cell and glial development in the spinal cord: generation of bipotential oligodendrocyte-type-2 astrocyte progenitor cells and dorsal-ventral differences in GRP cell function

We have found that the tripotential glial-restricted precursor (GRP) cell of the embryonic rat spinal cord can give rise in vitro to bipotential cells that express defining characteristics of oligodendrocyte-type-2 astrocyte progenitor cells (O2A/OPCs). Generation of O2A/OPCs is regulated by environmental signals and is promoted by platelet-derived growth factor (PDGF), thyroid hormone (TH) and astrocyte-conditioned medium. In contrast to multiple observations indicating that oligodendrocyte precursor cells in the embryonic day 14 (E14) spinal cord are ventrally restricted, GRP cells are already present in both the dorsal and ventral spinal cord at E13.5. Ventral-derived GRP cells, however, were more likely to generate O2A/OPCs and/or oligodendrocytes than were their dorsal counterparts when exposed to TH, PDGF, or even bone morphogenetic protein-4. The simplest explanation of our results is that oligodendrocyte generation occurs as a result of generation of GRP cells from totipotent neuroepithelial stem cells, of O2A/OPCs from GRP cells and, finally, of oligodendrocytes from O2A/OPCs. In this respect, the responsiveness of GRP cells to modulators of this process may represent a central control point in the initiation of this critical developmental sequence. Our findings provide an integration between the earliest known glial precursors and the well-studied O2A/OPCs while opening up new questions concerning the intricate spatial and temporal regulation of precursor cell differentiation in the CNS.

Arrested oligodendrocyte lineage progression during human cerebral white matter development: dissociation between the timing of progenitor differentiation and myelinogenesis

Immature oligodendrocytes (OLs) derive from a large pool of late OL progenitors that populate human cerebral white matter throughout the latter half of gestation. We recently reported that a minor population of immature OLs are present in human cerebral white matter for at least 3 months before these cells commit to myelinogenesis around 30 wk postconceptional age. Since this finding supports dissociation between the events that regulate human immature OL maturation and their commitment to myelinogenesis, we characterized here the cellular sequence of events that characterize immature OLs during the transition from a premyelinating to a myelinating state. Commitment of immature OLs to myelinogenesis in human cerebral white matter correlated with the longitudinal extension of specialized processes, designated "pioneer processes," that made multiple types of apparent contacts with axons. This event coincided with the appearance of 3 distinct populations of sheaths that varied in their labeling for myelin basic protein (MBP). Since few axons initially labeled for MBP, this supported the occurrence in vivo of O4-negative, O1-positive premyelin sheaths that precede MBP-positive compacted myelin. These observations identify 3 sequential stages of early myelinogenesis: 1) the initial ensheathment of axons by premyelin sheaths generated by immature OLs; 2) the initial insertion of MBP into transitional sheaths; and 3) the generation of MBP-rich mature myelin.

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.

Multipotent stem cells from the mouse basal forebrain contribute GABAergic neurons and oligodendrocytes to the cerebral cortex during embryogenesis

During CNS development, cell migrations play an important role, adding to the cellular complexity of different regions. Earlier studies have shown a robust migration of cells from basal forebrain into the overlying dorsal forebrain during the embryonic period. These immigrant cells include GABAergic neurons that populate the cerebral cortex and hippocampus. In this study we have examined the fate of other basal forebrain cells that migrate into the dorsal forebrain, identifying basal cells using an antibody that recognizes both early (dlx1/2) and late (dlx 5/6) members of the dlx homeobox gene family. We found that a subpopulation of cortical and hippocampal oligodendrocytes are also ventral-derived. We traced the origin of these cells to basal multipotent stem cells capable of generating both GABAergic neurons and oligodendrocytes. A clonal analysis showed that basal forebrain stem cells produce significantly more GABAergic neurons than dorsal forebrain stem cells from the same embryonic age. Moreover, stem cell clones from basal forebrain are significantly more likely to contain both GABAergic neurons and oligodendrocytes than those from dorsal. This indicates that forebrain stem cells are regionally specified. Whereas dlx expression was not detected within basal stem cells growing in culture, these cells produced dlx-positive products that are capable of migration. These data indicate that the developing cerebral cortex incorporates both neuronal and glial products of basal forebrain and suggest that these immigrant cells arise from a common progenitor, a dlx-negative basal forebrain stem cell.

Proliferation of NG2-positive cells and altered oligodendrocyte numbers in the contused rat spinal cord

Given the numerous reparative roles glia may play after spinal cord injury (SCI), glial proliferation and cell number were examined in a model of traumatic SCI. Emphasis was placed on analysis of oligodendrocytes and NG2-positive (NG2+) cells, an endogenous cell population that may be involved in oligodendrocyte replacement. Overall, proliferation (assessed by bromodeoxyuridine incorporation) was markedly elevated during the first 2 weeks after injury and declined thereafter; a large portion of these dividing cells likely consisted of microglia-macrophages. Although the total number of NG2+ cells in the epicenter was reduced by half, we noted protracted proliferation in surviving NG2+ cells, with values sevenfold greater than in uninjured controls. Elevated proliferation of NG2+ cells persisted throughout the first 4 weeks after injury. However, the absolute number of NG2+ cells was not increased over controls, suggesting that the daughter cells either did not survive or they differentiated into other cell types. As expected, oligodendrocyte numbers were drastically altered after SCI. By 7 d after injury, the number of oligodendrocytes at the impact site was reduced by 93%. Despite ongoing tissue loss, the number of oligodendrocytes in spared tissue rose threefold at 14 d after injury. Although the function of NG2+ cells within the spinal cord is not completely understood, several studies suggest that they may differentiate into oligodendrocytes. Thus, proliferating NG2+ cells may contribute to the increased oligodendrocyte number observed at 2 weeks after injury. Future studies are required, however, to definitively determine the role NG2+ cells play in oligodendrocyte genesis, remyelination, and other post-injury events.

Late oligodendrocyte progenitors coincide with the developmental window of vulnerability for human perinatal white matter injury

Hypoxic-ischemic injury to the periventricular cerebral white matter [periventricular leukomalacia (PVL)] results in cerebral palsy and is the leading cause of brain injury in premature infants. The principal feature of PVL is a chronic disturbance of myelination and suggests that oligodendrocyte (OL) lineage progression is disrupted by ischemic injury. We determined the OL lineage stages at risk for injury during the developmental window of vulnerability for PVL (23-32 weeks, postconceptional age). In 26 normal control autopsy human brains, OL lineage progression was defined in parietal white matter, a region of predilection for PVL. Three successive OL stages, the late OL progenitor, the immature OL, and the mature OL, were characterized between 18 and 41 weeks with anti-NG2 proteoglycan, O4, O1, and anti-myelin basic protein (anti-MBP) antibodies. NG2+O4+ late OL progenitors were the predominant stage throughout the latter half of gestation. Between 18 and 27 weeks, O4+O1+ immature OLs were a minor population (9.9 +/- 2.1% of total OLs; n = 9). Between 28 and 41 weeks, an increase in immature OLs to 30.9 +/- 2.1% of total OLs (n = 9) was accompanied by a progressive increase in MBP+ myelin sheaths that were restricted to the periventricular white matter. The developmental window of high risk for PVL thus precedes the onset of myelination and identifies the late OL progenitor as the major potential target. Moreover, the decline in incidence of PVL at approximately 32 weeks coincides with the onset of myelination in the periventricular white matter and suggests that the risk for PVL is related to the presence of late OL progenitors in the periventricular white matter.