Differentiation of the O-2A progenitor cell line CG-4 into oligodendrocytes and astrocytes following transplantation into glia-deficient areas of CNS white matter

Cell replacement therapy with stem cells in multiple sclerosis, a systematic review

Multiple sclerosis (MS) is a chronic inflammatory, autoimmune, and neurodegenerative disease of the central nervous system (CNS), characterized by demyelination and axonal loss. It is induced by attack of autoreactive lymphocytes on the myelin sheath and endogenous remyelination failure, eventually leading to accumulation of neurological disability. Disease-modifying agents can successfully address inflammatory relapses, but have low efficacy in progressive forms of MS, and cannot stop the progressive neurodegenerative process. Thus, the stem cell replacement therapy approach, which aims to overcome CNS cell loss and remyelination failure, is considered a promising alternative treatment. Although the mechanisms behind the beneficial effects of stem cell transplantation are not yet fully understood, neurotrophic support, immunomodulation, and cell replacement appear to play an important role, leading to a multifaceted fight against the pathology of the disease. The present systematic review is focusing on the efficacy of stem cells to migrate at the lesion sites of the CNS and develop functional oligodendrocytes remyelinating axons. While most studies confirm the improvement of neurological deficits after the administration of different stem cell types, many critical issues need to be clarified before they can be efficiently introduced into clinical practice.

Therapeutic Potential of Mesenchymal Stromal Cells and Extracellular Vesicles in the Treatment of Radiation Lesions-A Review

Ionising radiation-induced normal tissue damage is a major concern in clinic and public health. It is the most limiting factor in radiotherapy treatment of malignant diseases. It can also cause a serious harm to populations exposed to accidental radiation exposure or nuclear warfare. With regard to the clinical use of radiation, there has been a number of modalities used in the field of radiotherapy. These includes physical modalities such modified collimators or fractionation schedules in radiotherapy. In addition, there are a number of pharmacological agents such as essential fatty acids, vasoactive drugs, enzyme inhibitors, antioxidants, and growth factors for the prevention or treatment of radiation lesions in general. However, at present, there is no standard procedure for the treatment of radiation-induced normal tissue lesions. Stem cells and their role in tissue regeneration have been known to biologists, in particular to radiobiologists, for many years. It was only recently that the potential of stem cells was studied in the treatment of radiation lesions. Stem cells, immediately after their successful isolation from a variety of animal and human tissues, demonstrated their likely application in the treatment of various diseases. This paper describes the types and origin of stem cells, their characteristics, current research, and reviews their potential in the treatment and regeneration of radiation induced normal tissue lesions. Adult stem cells, among those mesenchymal stem cells (MSCs), are the most extensively studied of stem cells. This review focuses on the effects of MSCs in the treatment of radiation lesions.

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.

Radiation-induced brain injury: low-hanging fruit for neuroregeneration

Brain radiation is a fundamental tool in neurooncology to improve local tumor control, but it leads to profound and progressive impairments in cognitive function. Increased attention to quality of life in neurooncology has accelerated efforts to understand and ameliorate radiation-induced cognitive sequelae. Such progress has coincided with a new understanding of the role of CNS progenitor cell populations in normal cognition and in their potential utility for the treatment of neurological diseases. The irradiated brain exhibits a host of biochemical and cellular derangements, including loss of endogenous neurogenesis, demyelination, and ablation of endogenous oligodendrocyte progenitor cells. These changes, in combination with a state of chronic neuroinflammation, underlie impairments in memory, attention, executive function, and acquisition of motor and language skills. Animal models of radiation-induced brain injury have demonstrated a robust capacity of both neural stem cells and oligodendrocyte progenitor cells to restore cognitive function after brain irradiation, likely through a combination of cell replacement and trophic effects. Oligodendrocyte progenitor cells exhibit a remarkable capacity to migrate, integrate, and functionally remyelinate damaged white matter tracts in a variety of preclinical models. The authors here critically address the opportunities and challenges in translating regenerative cell therapies from rodents to humans. Although valiant attempts to translate neuroprotective therapies in recent decades have almost uniformly failed, the authors make the case that harnessing human radiation-induced brain injury as a scientific tool represents a unique opportunity to both successfully translate a neuroregenerative therapy and to acquire tools to facilitate future restorative therapies for human traumatic and degenerative diseases of the central nervous system.

Mechanisms of cellular plasticity in cerebral perivascular region

Brain vasculature acts in synergism with neurons to maintain brain function. This neurovascular coupling, or trophic coupling between cerebral endothelium and neuron, is now well accepted as a marker for mapping brain activity. Neurovascular coupling is most active in the perivascular region, in which there are ample opportunities for cell-cell interactions within the neurovascular unit. This trophic coupling between cells maintains neurovascular function and cellular plasticity. Recent studies have revealed that even adult brains contain multiple stem cells of various lineages, which may provide cellular plasticity through the process of differentiation among these stem cell populations. In this chapter, we provide an overview of the process by which neurovascular components contribute to cellular plasticity in the cerebral perivascular regions, focusing on mechanisms of cell-cell interaction in adult brain.

Phenotype overlap in glial cell populations: astroglia, oligodendroglia and NG-2(+) cells

The extent to which NG-2(+) cells form a distinct population separate from astrocytes is central to understanding whether this important cell class is wholly an oligodendrocyte precursor cell (OPC) or has additional functions akin to those classically ascribed to astrocytes. Early immuno-staining studies indicate that NG-2(+) cells do not express the astrocyte marker GFAP, but orthogonal reconstructions of double-labeled confocal image stacks here reveal a significant degree of co-expression in individual cells within post-natal day 10 (P10) and adult rat optic nerve (RON) and rat cortex. Extensive scanning of various antibody/fixation/embedding approaches identified a protocol for selective post-embedded immuno-gold labeling. This first ultrastructural characterization of identified NG-2(+) cells revealed populations of both OPCs and astrocytes in P10 RON. NG-2(+) astrocytes had classic features including the presence of glial filaments but low levels of glial filament expression were also found in OPCs and myelinating oligodendrocytes. P0 RONs contained few OPCs but positively identified astrocytes were observed to ensheath pre-myelinated axons in a fashion previously described as a definitive marker of the oligodendrocyte lineage. Astrocyte ensheathment was also apparent in P10 RONs, was absent from developing nodes of Ranvier and was never associated with compact myelin. Astrocyte processes were also shown to encapsulate some oligodendrocyte somata. The data indicate that common criteria for delineating astrocytes and oligodendroglia are insufficiently robust and that astrocyte features ascribed to OPCs may arise from misidentification.

Demyelination causes adult CNS progenitors to revert to an immature state and express immune cues that support their migration

The declining efficiency of myelin regeneration in individuals with multiple sclerosis has stimulated a search for ways by which it might be therapeutically enhanced. Here we have used gene expression profiling on purified murine oligodendrocyte progenitor cells (OPCs), the remyelinating cells of the adult CNS, to obtain a comprehensive picture of how they become activated after demyelination and how this enables them to contribute to remyelination. We find that adult OPCs have a transcriptome more similar to that of oligodendrocytes than to neonatal OPCs, but revert to a neonatal-like transcriptome when activated. Part of the activation response involves increased expression of two genes of the innate immune system, IL1β and CCL2, which enhance the mobilization of OPCs. Our results add a new dimension to the role of the innate immune system in CNS regeneration, revealing how OPCs themselves contribute to the postinjury inflammatory milieu by producing cytokines that directly enhance their repopulation of areas of demyelination and hence their ability to contribute to remyelination.

Allosteric inhibition of a stem cell RNA-binding protein by an intermediary metabolite

Gene expression and metabolism are coupled at numerous levels. Cells must sense and respond to nutrients in their environment, and specialized cells must synthesize metabolic products required for their function. Pluripotent stem cells have the ability to differentiate into a wide variety of specialized cells. How metabolic state contributes to stem cell differentiation is not understood. In this study, we show that RNA-binding by the stem cell translation regulator Musashi-1 (MSI1) is allosterically inhibited by 18-22 carbon ω-9 monounsaturated fatty acids. The fatty acid binds to the N-terminal RNA Recognition Motif (RRM) and induces a conformational change that prevents RNA association. Musashi proteins are critical for development of the brain, blood, and epithelium. We identify stearoyl-CoA desaturase-1 as a MSI1 target, revealing a feedback loop between ω-9 fatty acid biosynthesis and MSI1 activity. We propose that other RRM proteins could act as metabolite sensors to couple gene expression changes to physiological state.

Germline disruption of Pten localization causes enhanced sex-dependent social motivation and increased glial production

PTEN Hamartoma Tumor Syndrome (PHTS) is an autosomal-dominant genetic condition underlying a subset of autism spectrum disorder (ASD) with macrocephaly. Caused by germline mutations in PTEN, PHTS also causes increased risks of multiple cancers via dysregulation of the PI3K and MAPK signaling pathways. Conditional knockout models have shown that neural Pten regulates social behavior, proliferation and cell size. Although much is known about how the intracellular localization of PTEN regulates signaling in cancer cell lines, we know little of how PTEN localization influences normal brain physiology and behavior. To address this, we generated a germline knock-in mouse model of cytoplasm-predominant Pten and characterized its behavioral and cellular phenotypes. The homozygous Pten(m3m4) mice have decreased total Pten levels including a specific drop in nuclear Pten and exhibit region-specific increases in brain weight. The Pten(m3m4) model displays sex-specific increases in social motivation, poor balance and normal recognition memory-a profile reminiscent of some individuals with high functioning ASD. The cytoplasm-predominant protein caused cellular hypertrophy limited to the soma and led to increased NG2 cell proliferation and accumulation of glia. The animals also exhibit significant astrogliosis and microglial activation, indicating a neuroinflammatory phenotype. At the signaling level, Pten(m3m4) mice show brain region-specific differences in Akt activation. These results demonstrate that differing alterations to the same autism-linked gene can cause distinct behavioral profiles. The Pten(m3m4) model is the first murine model of inappropriately elevated social motivation in the context of normal cognition and may expand the range of autism-related behaviors replicated in animal models.

High yields of oligodendrocyte lineage cells from human embryonic stem cells at physiological oxygen tensions for evaluation of translational biology

We have established and efficient system to specify NG2/PDGF-Rα/OLIG2+ oligodendrocyte precursor cells (OPCs) from human embryonic stem cells (hESCs) at low, physiological (3%) oxygen levels. This was achieved via both forebrain and spinal cord origins, with up to 98% of cells expressing NG2. Developmental insights reveal a critical role for fibroblast growth factor 2 (FGF-2) in OLIG2 induction via ventral forebrain pathways. The OPCs mature in vitro to express O4 (46%) and subsequently become galactocerebroside (GALC), O1, and myelin basic protein-positive (MBP+) multibranching oligodendrocytes. These were cultured alongside hESC-derived neurons. The electrophysiological properties of human OPCs are similar to those of rat OPCs, with large voltage-gated sodium currents and the ability to fire action potentials. Exposure to a selective retinoid X receptor agonist increased the proportion of O4+ oligodendrocytes that express MBP from 5% to 30%. Thus, we have established a developmentally engineered system to investigate the biological properties of human OPCs and test the effects of putative remyelinating agents prior to clinical application.

Highly malignant behavior of a murine oligodendrocyte precursor cell line following transplantation into the demyelinated and nondemyelinated central nervous system

Understanding the basic mechanisms that control CNS remyelination is of direct clinical relevance. Suitable model systems include the analysis of naturally occurring and genetically generated mouse mutants and the transplantation of oligodendrocyte precursor cells (OPCs) following experimental demyelination. However, aforementioned studies were exclusively carried out in rats and little is known about the in vivo behavior of transplanted murine OPCs. Therefore in the present study, we (i) established a model of ethidium bromide-induced demyelination of the caudal cerebellar peduncle (CCP) in the adult mouse and (ii) studied the distribution and marker expression of the murine OPC line BO-1 expressing the enhanced green fluorescent protein (eGFP) 10 and 17 days after stereotaxic implantation. Injection of ethidium bromide (0.025%) in the CCP resulted in a severe loss of myelin, marked astrogliosis, and mild to moderate axonal alterations. Transplanted cells formed an invasive and liquorogenic metastasizing tumor, classified as murine giant cell glioblastoma. Transplanted BO-1 cells displayed substantially reduced CNPase expression as compared to their in vitro phenotype, low levels of MBP and GFAP, prominent upregulation of NG2, PDGFRα, nuclear p53, and an unaltered expression of signal transducer and activator of transcription (STAT)-3. Summarized environmental signaling in the brain stem was not sufficient to trigger oligodendrocytic differentiation of BO-1 cells and seemed to block CNPase expression. Moreover, the lack of the remyelinating capacity was associated with tumor formation indicating that BO-1 cells may serve as a versatile experimental model to study tumorigenesis of glial tumors.

The remyelination Philosopher's Stone: stem and progenitor cell therapies for multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease that leads to oligodendrocyte loss and subsequent demyelination of the adult central nervous system (CNS). The pathology is characterized by transient phases of recovery during which remyelination can occur as a result of resident oligodendroglial precursor and stem/progenitor cell activation. However, myelin repair efficiency remains low urging the development of new therapeutical approaches that promote remyelination activities. Current MS treatments target primarily the immune system in order to reduce the relapse rate and the formation of inflammatory lesions, whereas no therapies exist in order to regenerate damaged myelin sheaths. During the last few years, several transplantation studies have been conducted with adult neural stem/progenitor cells and glial precursor cells to evaluate their potential to generate mature oligodendrocytes that can remyelinate axons. In parallel, modulation of the endogenous progenitor niche by neural and mesenchymal stem cell transplantation with the aim of promoting CNS progenitor differentiation and myelination has been studied. Here, we summarize these findings and discuss the properties and consequences of the various molecular and cell-mediated remyelination approaches. Moreover, we address age-associated intrinsic cellular changes that might influence the regenerative outcome. We also evaluate the extent to which these experimental treatments might increase the regeneration capacity of the demyelinated human CNS and hence be turned into future therapies.

RhoA-inhibiting NSAIDs promote axonal myelination after spinal cord injury

Nonsteroidal anti-inflammatory drugs (NSAIDs) are extensively used to relieve pain and inflammation in humans via cyclooxygenase inhibition. Our recent research suggests that certain NSAIDs including ibuprofen suppress intracellular RhoA signal and improve significant axonal growth and functional recovery following axonal injury in the CNS. Several NSAIDs have been shown to reduce generation of amyloid-beta42 peptide via inactivation of RhoA signal, supporting potent RhoA-repressing function of selected NSAIDs. In this report, we demonstrate that RhoA-inhibiting NSAIDs ibuprofen and indomethacin dramatically reduce cell death of oligodendrocytes in cultures or along the white matter tracts in rats with a spinal cord injury. More importantly, we demonstrate that treatments with the RhoA-inhibiting NSAIDs significantly increase axonal myelination along the white matter tracts following a traumatic contusion spinal cord injury. In contrast, non-RhoA-inhibiting NSAID naproxen does not have such an effect. Thus, our results suggest that RhoA inactivation with certain NSAIDs benefits recovery of injured CNS axons not only by promoting axonal elongation, but by enhancing glial survival and axonal myelination along the disrupted axonal tracts. This study, together with previous reports, supports that RhoA signal is an important therapeutic target for promoting recovery of injured CNS and that RhoA-inhibiting NSAIDs provide great therapeutic potential for CNS axonal injuries in adult mammals.

Enhancing remyelination in disease--can we wrap it up?

Demyelinating disorders of the central nervous system are among the most crippling neurological diseases affecting patients at various stages of life. In the most prominent demyelinating disease, multiple sclerosis, the regeneration of myelin sheaths often fails due to a default of the resident stem/precursor cells (oligodendrocyte precursor cells) to differentiate into mature myelin forming cells. Significant advances have been made in our understanding of the molecular and cellular processes involved in remyelination. Furthermore, important insight has been gained from studies investigating the interaction of stem/precursor cells with the distinct environment of demyelinating lesions. These suggest that successful regeneration depends on a signalling environment conducive to remyelination, which is provided in the context of acute inflammation. However, multiple sclerosis lesions also contain factors that inhibit the differentiation of oligodendrocyte precursor cells into myelinating oligodendrocytes. The pattern by which remyelination inducers and inhibitors are expressed in multiple sclerosis lesions may determine a window of opportunity during which oligodendrocyte precursor cells can successfully differentiate. As the first molecules aiming at promoting remyelination are about to enter clinical trials, this review critically evaluates recent advances in our understanding of the biology of oligodendrocyte precursor cells and of the stage-dependent molecular pathology of multiple sclerosis lesions relevant to the regeneration of myelin sheaths. We propose a model that may help to provide cues for how remyelination can be therapeutically enhanced in clinical disease.

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

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

Cell Therapy From Bench to Bedside Translation in CNS Neurorestoratology Era

Recent advances in cell biology, neural injury and repair, and the progress towards development of neurorestorative interventions are the basis for increased optimism. Based on the complexity of the processes of demyelination and remyelination, degeneration and regeneration, damage and repair, functional loss and recovery, it would be expected that effective therapeutic approaches will require a combination of strategies encompassing neuroplasticity, immunomodulation, neuroprotection, neurorepair, neuroreplacement, and neuromodulation. Cell-based restorative treatment has become a new trend, and increasing data worldwide have strongly proven that it has a pivotal therapeutic value in CNS disease. Moreover, functional neurorestoration has been achieved to a certain extent in the CNS clinically. Up to now, the cells successfully used in preclinical experiments and/or clinical trial/treatment include fetal/embryonic brain and spinal cord tissue, stem cells (embryonic stem cells, neural stem/progenitor cells, hematopoietic stem cells, adipose-derived adult stem/precursor cells, skin-derived precursor, induced pluripotent stem cells), glial cells (Schwann cells, oligodendrocyte, olfactory ensheathing cells, astrocytes, microglia, tanycytes), neuronal cells (various phenotypic neurons and Purkinje cells), mesenchymal stromal cells originating from bone marrow, umbilical cord, and umbilical cord blood, epithelial cells derived from the layer of retina and amnion, menstrual blood-derived stem cells, Sertoli cells, and active macrophages, etc. Proof-of-concept indicates that we have now entered a new era in neurorestoratology.

Ethanol alters the expressions of c-Fos and myelin basic protein in differentiating oligodendrocytes

Myelination occurs in the central nervous system of the human fetus, adolescents, and young adults. Ethanol interferes with myelination in part by altering the composition of the myelin sheath. Here we show that ethanol also affected the expression of the transcription factor c-Fos in differentiating oligodendrocytes (OLGs). Central glial-4 OLG progenitors were induced to differentiate in the absence and presence of 100 mM ethanol, and ethanol-caused changes in the levels of c-Fos and myelin basic protein (MBP) were determined by Western blot analysis at selected developmental stages. The relatively high c-Fos level in progenitors did not immediately decrease to a low level at the onset of differentiation but displayed a downregulation at a later developmental stage. Ethanol delayed the developmental c-Fos downregulation maintaining c-Fos at a 45% higher level at 2 days of differentiation (DoD). Ethanol also decreased the rate of the burst of MBP expression that occurred between 1 and 2 DoD, reducing the MBP level by 47% at 2 DoD. The ethanol-caused delays of c-Fos downregulation and MBP upregulation were both blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIM). Likewise, treatment of OLGs with a low 5-nM concentration of the PKC activator by 12-O-tetradecanoylphorbol-13-acetate mimicked the ethanol effects on the expression of both proteins, effects that were also counteracted by BIM. The results indicate that ethanol-caused delays of the stage-specific c-Fos downregulation and the inhibition of MBP expression both occur through a PKC-mediated mechanism. The ethanol-caused delay in c-Fos downregulation may disrupt normal timing for expression of genes involved in OLG differentiation, and the inhibited MBP expression may alter the myelin sheath composition.

Transplantation-mediated strategies to promote axonal regeneration following spinal cord injury

Devastating central nervous system injuries and diseases continue to occur in spite of the tremendous efforts of various prevention programs. The enormity and annual escalation of healthcare costs due to them require that therapeutic strategies be responsibly developed. The dysfunctions that occur after injury and disease are primarily due to neurotransmission damage. The last two decades of both experimental and clinical research have demonstrated that neural and non-neural tissue and cell transplantation is a viable option for ameliorating dysfunctions to markedly improve quality of life. Moreover, significant progress has been made with tissue and cell transplantation in studies of pathophysiology, plasticity, sprouting, regeneration, and functional recovery. This article will review information about the ability and potential, particularly for traumatic spinal cord injury, that neural and non-neural tissue and cell transplantation has to replace lost neurons and glia, to reconstruct damaged neural circuitry, and to restore neurotransmitters, hormones, neurotrophic factors, and neurotransmission. Donor tissues and cells to be discussed include peripheral nerve, fetal spinal cord and brain, central and peripheral nervous systems' glia, stem cells, those that have been genetically engineered, and non-neural ones. Combinatorial approaches and clinical research are also reviewed.

In vitro characterization of a murine oligodendrocyte precursor cell line (BO-1) following spontaneous immortalization

The understanding of oligodendrocyte differentiation is crucial for designing therapies of demyelinating diseases. Oligodendrocyte precursor cells are of particular interest in this context, because of their remyelinating potential. Permanent cell lines, which are a versatile tool for studying oligodendrocyte physiology, have been so far mainly established from the rat CNS. In the present study, we describe a novel murine oligodendrocyte precursor cell line (BO-1) established by spontaneous immortalization using light microscopy, immunocytochemical phenotyping and genetic analysis. BO-1 cells displayed a bi- to multipolar morphology and expressed early oligodendrocytic lineage markers, such as A2B5 and NG-2. Expression of pre-oligodendrocyte (O4, CNPase) and mature oligodendrocyte markers (e.g. myelin basic protein) was found in about 30% and 1.5% of the cells, respectively. Addition of serum, known to promote type-2 astrocyte differentiation, significantly increased the number of GFAP-positive cells, while thyroid hormones, (T3/T4) known to foster oligodendrocyte differentiation, did not substantially alter the antigenic and gene expression of myelin markers. This deficiency might be related to the high intrinsic proliferation rate of BO-1 cells that was unaltered upon removal of mitogenic factors. Expression of O4 and CNPase in BO-1 cells could be significantly increased by co-culture with primary astrocytes suggesting that the differentiating potential of BO-1 cells was influenced by environmental factors and may have to be fully explored in future studies. In summary, the novel murine BO-1 cell line shares several characteristics with oligodendrocyte precursor cells but displays a restricted differentiation into mature oligodendrocytes.

Glial cells: old cells with new twists

Based on their characteristics and function--migration, neural protection, proliferation, axonal guidance and trophic effects--glial cells may be regarded as probably the most versatile cells in our body. For many years, these cells were considered as simply support cells for neurons. Recently, it has been shown that they are more versatile than previously believed--as true stem cells in the nervous system--and are important players in neural function and development. There are several glial cell types in the nervous system: the two most abundant are oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Although both of these cells are responsible for myelination, their developmental origins are quite different. Oligodendrocytes originate from small niche populations from different regions of the central nervous system, while Schwann cells develop from a stem cell population (the neural crest) that gives rise to many cell derivatives besides glia and which is a highly migratory group of cells.

NG2 cells generate both oligodendrocytes and gray matter astrocytes

NG2 glia constitute a fourth major glial cell type in the mammalian central nervous system (CNS) that is distinct from other cell types. Although circumstantial evidence suggests that some NG2 glia differentiate into oligodendrocytes, their in vivo fate has not been directly examined. We have used the bacterial artificial chromosome (BAC) modification technique to generate transgenic mice that express DsRed or Cre specifically in NG2-expressing (NG2+) cells. In NG2DsRedBAC transgenic mice, DsRed was expressed specifically in NG2+ cells throughout the postnatal CNS. When the differentiation potential of NG2+ cells in vitro was examined using DsRed+NG2+ cells purified from perinatal transgenic brains, the majority of the cells either remained as NG2+ cells or differentiated into oligodendrocytes. In addition, DsRed+NG2+ cells also differentiated into astrocytes. The in vivo fate of NG2 glia was examined in mice that were double transgenic for NG2creBAC and the Cre reporter Z/EG. In the double transgenic mice, the Cre reporter EGFP was detected in myelinating oligodendrocytes and in a subpopulation of protoplasmic astrocytes in the gray matter of ventrolateral forebrain but not in fibrous astrocytes of white matter. These observations suggest that NG2+ cells are precursors of oligodendrocytes and some protoplasmic astrocytes in gray matter.

The selective RNA-binding protein quaking I (QKI) is necessary and sufficient for promoting oligodendroglia differentiation

Quaking I (QKI) is a selective RNA-binding protein essential for myelination of the central nervous system. Three QKI isoforms with distinct C termini and subcellular localization, namely QKI-5, QKI-6, and QKI-7, are expressed in oligodendroglia progenitor cells (OPCs) prior to the initiation of myelin formation and implicated in promoting oligodendrocyte lineage development. However, the functional requirement for each QKI isoform and the mechanisms by which QKI isoforms govern OPC development still remain elusive. We report here that exogenous expression of each QKI isoform is sufficient to enhance differentiation of OPCs with different efficiency, which is abolished by a point mutation that abrogates the RNA binding activity of QKI. Reciprocally, small interfering RNA-mediated QKI knockdown blocks OPC differentiation, which can be partly rescued by QKI-5 and QKI-6 but not by QKI-7, indicating the differential requirement of QKI isoform function in advancing OPC differentiation. Furthermore, we found that abrogation of OPC differentiation, as a result of QKI deficiency, is not due to altered proliferation capacity or cell cycle progression. These results indicate that QKI isoforms are necessary and sufficient for promoting OPC development, which must involve direct influence of QKI on differentiation/maturation of OPCs independent of cell cycle exit, likely via regulating the expression of the target mRNAs of QKI that support OPC differentiation.

Developing oligodendrocytes express functional GABA(B) receptors that stimulate cell proliferation and migration

GABA(B) receptors (GABA(B)Rs) are involved in early events during neuronal development. The presence of GABA(B)Rs in developing oligodendrocytes has not been established. Using immunofluorescent co-localization, we have identified GABA(B)R proteins in O4 marker-positive oligodendrocyte precursor cells (OPCs) in 4-day-old mouse brain periventricular white matter. In culture, OPCs, differentiated oligodendrocytes (DOs) and type 2 astrocytes (ASTs) express both the GABA(B1abcdf) and GABA(B2) subunits of the GABA(B)R. Using semiquantitative PCR analysis with GABA(B)R isoform-selective primers we found that the expression level of GABA(B1abd) was substantially higher in OPCs or ASTs than in DOs. In contrast, the GABA(B2) isoform showed a similar level of expression in OPCs and DOs, and a significantly higher level in ASTs. This indicates that the expression of GABA(B1) and GABA(B2) subunits are under independent control during oligodendroglial development. Activation of GABA(B)Rs using the selective agonist baclofen demonstrated that these receptors are functionally active and negatively coupled to adenylyl cyclase. Manipulation of GABA(B)R activity had no effect on OPC migration in a conventional agarose drop assay, whereas baclofen significantly increased OPC migration in a more sensitive transwell microchamber-based assay. Exposure of cultured OPCs to baclofen increased their proliferation, providing evidence for a functional role of GABA(B)Rs in oligodendrocyte development. The presence of GABA(B)Rs in developing oligodendrocytes provides a new mechanism for neuronal-glial interactions during development and may offer a novel target for promoting remyelination following white matter injury.

The adult “paraplegic” rat: treatment with cell graftings

Spinal cord injury often results in irreversible and permanent neurologic deficits below the lesion level. Nowadays, treatment is limited to drugs and/or physiotherapy aimed at compensating disability. New experimental studies focus on the transplantation of cells capable of surviving, regenerating tissue, recovering functions and/or improving symptoms. A review of such type of studies on spinal cord reconstruction published between 1991 and 2004 is presented. In the latter years, cell transplantation appeared as the most promising approach in spinal cord regeneration research. To date, this promise has not been maintained, despite the appearance of new attractive cell populations for grafting, such as neural stem cells. The demonstration that stem cells exist in the adult brain and that they can be isolated and expanded in vitro offers the possibility to test such interesting cells in the paraplegic rat. Some neurotrophic factors can facilitate axonal regeneration and neuronal survival. Therefore, the development of strategies, such as implanting neural stem cells engineered to secrete neurotrophic factors directly in the lesion site, could be important to promote regeneration in the injured spinal cord. Despite all the strategies used till now, the problem of the paraplegic rat remains. Only the solution of such problem will authorize studies in higher mammals and, finally, the clinical application in human patients. The paraplegic adult rat with a T8 spinal cord transection should be considered the standard experimental model to be used in spinal cord reconstruction studies. Function and anatomic results are undisputed only after spinal cord transection.

Glial grafting for demyelinating disease

Remyelination of demyelinated central nervous system (CNS) axons is considered as a potential treatment for multiple sclerosis, and it has been achieved in experimental models of demyelination by transplantation of pro-myelinating cells. However, the experiments undertaken have not addressed the need for tissue-type matching in order to achieve graft-mediated remyelination since they were performed in conditions in which the chance for graft rejection was minimized. This article focuses on the factors determining survival of allogeneic oligodendrocyte lineage cells and their contribution to the remyelination of demyelinating CNS lesions. The immune status of the CNS as well as the suitability of different models of demyelination for graft rejection studies are discussed, and ways of enhancing allogeneic oligodendrocyte-mediated remyelination are presented. Finally, the effects of glial graft rejection on host remyelination are described, highlighting the potential benefits of the acute CNS inflammatory response for myelin repair.

Oligodendrocyte progenitor cell (OPC) transplantation is unlikely to offer a means of preventing X-irradiation induced damage in the CNS

Oligodendrocyte lineage cells [oligodendrocytes and their parent cells, the oligodendrocyte progenitor cells (OPCs)] are depleted by X-irradiation and progenitor cell transplantation has been proposed as a therapeutic strategy to counteract radiation induced myelopathy. Previous studies have demonstrated that oligodendrocyte progenitor cell (OPC) depletion is a prerequisite for establishing transplanted OPCs in normal tissue. One can therefore predict that the extent and timing of OPC depletion and regeneration following X-irradiation will be crucial factors in determining the feasibility of this therapeutic approach. To address this issue, we have examined the time course of OPC depletion and regeneration following a range of X-irradiation doses (5 to 40 Gy), and its relationship to establishing transplanted OPCs in X-irradiated tissue. Doses above 10 Gy resulted in rapid death of OPCs. With doses up to 20 Gy, surviving X-irradiated OPCs were capable of robust regeneration, restoring normal densities within 6 weeks. Transplanted OPCs could only be established in tissue that had been exposed to > or =20 Gy. Since 20 Gy is close to the ED50 for radiation necrosis, our findings demonstrate the limitation of OPC replacement strategies.

Effects of glial transplantation on functional recovery following acute spinal cord injury

Numerous efforts have been made to maximize the efficacy of treatment for spinal cord injury (SCI). Recently, oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells have been reported to remyelinate focal areas of demyelinated spinal cord in adult rats. We conducted a study to investigate the therapeutic potential of transplantation of O-2A cells in a rat model of acute SCI. SCI was induced with an NYU Impactor at T9 of rats. O-2A cells labeled with bromodeoxyuridine (BrdU) were transplanted into sites of SCI at 1 week after the induction of SCI. At 6 weeks after cell transplantation, a behavioral test showed significant functional improvement in animals that had received O-2A-cell transplants as compared to animals given cell-culture medium alone. An electrophysiological study revealed that the transplants did not improve the amplitude or latency of somatosensory evoked potentials, but a recording of motor evoked potentials showed that the latency of these potentials in the O-2A-cell-transplant group was significantly shorter than that in the group treated with cell-culture medium. Following transplantation of BrdU-labeled O-2A cells, cells positive for BrdU were detected at and near sites of SCI. Cells labeled for both BrdU and 2',3' -cyclic nucleotide-3-phosphodiesterase were also detected, showing that the transplanted O-2A cells differentiated into oligodendrocytes. By contrast, cells labeled for BrdU and glial fibrillary acidic protein, or for neuronal nuclei antigen, were not detected. Furthermore, a tract-tracing study showed that numbers of retrogradely labeled neurons increased in areas of the brain stem after O-2A-cell transplantation. The study data showed that after being transplanted into an animal with SCI, O-2A cells migrated to the area adjacent to the site of injury and differentiated into oligodendrocytes. The behavioral test and the electrophysiological and morphological studies showed that transplantation of O-2A cells may play an important role in functional recovery and the regeneration of axons after SCI.

The case for a central nervous system (CNS) origin for the Schwann cells that remyelinate CNS axons following concurrent loss of oligodendrocytes and astrocytes

In certain experimental and naturally occurring pathological situations in the central nervous system (CNS), demyelinated axons are remyelinated by Schwann cells. It has always been assumed that these Schwann cells are derived from Schwann cells associated with peripheral nerves. However, it has become apparent that CNS precursors can give rise to Schwann cells in vitro and following transplantation into astrocyte-free areas of demyelination in vivo. This paper compares the behaviour of remyelinating Schwann cells following transplantation of peripheral nerve derived Schwann cells over, and into, astrocyte-depleted areas of demyelination to that which follows transplantation of CNS cells and that seen in normally remyelinating ethidium bromide induced demyelinating lesions. It concludes that while the examination of normally remyelinating lesions can not resolve the origin of the remyelinating Schwann cells, the results from transplantation studies provide strong evidence that the Schwann cells that remyelinate CNS axons are most likely generated from CNS precursors. In addition these studies also indicate that the precursors that give rise to these Schwann cells are the same cells that give rise to remyelinating oligodendrocytes.

CNS axons retain their competence for myelination throughout life

An important question relevant to developing remyelination therapies is whether axons that remain without myelin sheaths after an episode of demyelination retain myelination competence. To resolve this, we have developed a model of transplantation into the nerve fibre layer of the adult rat retina, where the axons are unmyelinated. In the adult, these axons can be myelinated by transplantation of both the oligodendrocyte progenitor cells (OPCs) and an OPC line (CG4). The extent of myelination achieved following transplantation of OPCs is the same in young adult recipients (2 months old) as that which occurs in old adult recipients (12-18 months old), indicating that there are no changes in axons remaining unmyelinated for many months that would prevent effective remyelination. This finding suggests that chronically demyelinated regions of axons such as those in seen in multiple sclerosis are likely to remain competent to be remyelinated.

Oligodendrocyte progenitor migration in response to injury of glial monolayers requires the polysialic neural cell-adhesion molecule

Injury to the nervous system results in reactive astrogliosis that is a critical determinant of neuronal regeneration. To analyze glial responses to mechanical injury and the role of the polysialic neural cell adhesion molecule (PSA-NCAM) in this process, we established primary glia cultures from newborn rat cerebral cortex. Scratching a confluent monolayer of primary glial cells resulted in two major events: rapid migration of oligodendrocyte progenitor-like (O-2A) cells into the wounded area and development of polarized morphology of type 1 astrocytes at the wound edge. Migrating O-2A progenitors had a bipolar morphology and exhibited A2B5 and O4 immunolabeling. Once these cells were established inside the wounded area, they lost A2B5 immunoreactivity and differentiated into glial fibrillary acidic protein-positive astrocytes. Migrating O-2A cells expressed PSA-NCAM, but type 1 astrocytes at the wound edge did not. Treatment of wounded cultures with Endo-N, which specifically removes PSA from the surface of cells, resulted in a significant decrease in O-2A cell migration into the wounded area and completely blocked the wound closure. Video time-lapse analysis showed that, in the presence of Endo-N, O-2A cells remained motile and migrated short distances but did not move away from the monolayer. These results demonstrate that O-2A progenitors contribute to reactive astrogliosis in culture and that PSA-NCAM is involved in this process by regulating cell migration.

Promotion of axonal regeneration in the injured CNS

Molecules that are found in the extracellular environment at a CNS lesion site, or that are associated with myelin, inhibit axon growth. In addition, neuronal changes--such as an age-dependent reduction in concentrations of cyclic AMP--render the neuron less able to respond to axotomy by a rapid, forward, actin-dependent movement. An alternative mechanism, based on the protrusive forces generated by microtubule elongation or the anterograde transport of cytoskeletal elements, may underlie a slower form of axon elongation that happens during regeneration in the mature CNS. Therapeutic approaches that restore the extracellular CNS environment or the neuron's characteristics back to a more embryonic state increase axon regeneration and improve functional recovery after injury. These advances in the understanding of regeneration in the CNS have major implications for neurorehabilitation and for the use of axonal regeneration as a therapeutic approach to disorders of the CNS such as spinal-cord injury.

Functional metabotropic glutamate receptors are expressed in oligodendrocyte progenitor cells

We investigated the expression of metabotropic glutamate receptor (mGluR) isoforms in CG-4 rodent oligodendroglial progenitor cells (OPC) and rat brain oligodendrocytes. Our RT-PCR analysis detected mRNAs for mGluR3 and mGluR5 isoforms in OPCs. Although neurons express both mGluR5a and mGluR5b splice variants, only mGluR5a was identified in OPCs. Antibodies to mGluR2/3 and mGluR5 detected the corresponding receptor proteins in immunoblots of OPC membrane fractions. Furthermore, immunocytochemical analysis identified mGluR5 in oligodendrocyte marker O4-positive OPCs. The expression of mGluR5 was also demonstrated in oligodendrocyte marker (O4 and O1) positive cells in white matter of postnatal 4- and 7-day-old rat brain sections using immunofluorescent double labelling and confocal microscopy. The mGluR5 receptor function was assessed in CG-4 OPCs with fura-2 microfluorometry. Application of the mGluR1/5 specific agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced calcium oscillations, which were inhibited by the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP). The DHPG induced calcium oscillations required Ca2+ release from intracellular stores. In OPCs the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) decreased forskolin-stimulated cAMP synthesis, indicating the presence of functional mGluR3. The newly identified mGluR3 and mGluR5a may be involved in the differentiation of oligodendrocytes, myelination and the development of white matter damage.

In vivo magnetic resonance tracking of magnetically labeled cells after transplantation

During the last few years, the therapeutic use of stem and progenitor cells as a substitute for malfunctioning endogenous cell populations has received considerable attention. Unlike their current use in animal models, the introduction of therapeutic cells in patients will require techniques that can monitor their tissue biodistribution noninvasively. Among the different imaging modalities, magnetic resonance (MR) imaging offers both near-cellular (i.e., 25- to 50-mu) resolution and whole-body imaging capability. In order to be visualized, cells must be labeled with an intracellular tracer molecule that can be detected by MR imaging. Methods have now been developed that make it possible to incorporate sufficient amounts of superparamagnetic iron oxide into cells, enabling their detection in vivo using MR imaging. This is illustrated for (neural stem cell-derived) magnetically labeled oligodendroglial progenitors, transplanted in the central nervous system of dysmyelinated rats. Cells can be followed in vivo for at least 6 weeks after transplantation, with a good histopathologic correlation including the formation of myelin. Now that MR tracking of magnetically labeled cells appears feasible, it is anticipated that this technique may ultimately become an important tool for monitoring the efficacy of clinical (stem) cell transplantation protocols.

A novel seven transmembrane receptor induced during the early steps of astrocyte differentiation identified by differential expression

The rat glial progenitor cell line CG-4 can be induced to differentiate into either oligodendrocytes or type-2 astrocytes. In order to identify genes whose expression varies coincident with such phenotypic differentiation, we employed representational difference analysis (RDA) of mRNA. Here, we report 38 cDNAs induced in type-2 astrocytes, oligodendrocytes, or both differentiated states. Among these were known transcription factors, membrane receptors, extracellular matrix proteins, secreted signaling modulators, chromatin regulators and myelin sheath components. In addition several novel genes were identified; among these was a gene induced during the very early stages of astrocyte differentiation that we have named Ieda (induced early in differentiating astrocytes). Several Ieda transcripts were detected by RT-PCR, and appeared to be produced by alternative splicing and promoter usage. The protein deduced from the longest Ieda mRNA exhibited sequence features characteristic of G-protein coupled receptors, including seven putative transmembrane domains, while the shorter Ieda transcripts encoded proteins that lacked several transmembrane segments. In the adult rat, Ieda transcripts were found exclusively in brain and testis. In the developing rat brain, Ieda expression was first detected at embryonic day 16, that is two days before the first appearance of mature astrocytes. Thus, this approach has yielded a potential source of markers for differentiation states of these two cellular types as well as genes predicted to be functionally involved in the differentiation process itself.

A quantitative morphometric analysis of rat spinal cord remyelination following transplantation of allogenic Schwann cells

Quantitative morphometric techniques were used to assess the extent and pattern of remyelination produced by transplanting allogenic Schwann cells into demyelinated lesions in adult rat spinal cords. The effects of donor age, prior culturing of donor cells, prior lesioning of donor nerves, and host immunosuppression were evaluated by transplanting suspensions of 30,000 acutely dissociated or cultured Schwann cells from neonatal, young adult, or aged adult rat sciatic nerves into X-irradiation and ethidium bromide-induced demyelinated dorsal column lesions, with or without co-transplantation of neonatal optic nerve astrocytes. Three weeks after transplantation, spinal cords were processed for histological analysis. Under all Schwann cell transplant protocols, large areas containing many Schwann cell-like myelinated axon profiles could be readily observed throughout most of the lesion length. Within these "myelin-rich" regions, the vast majority of detectable axons showed a peripheral-like pattern of myelination. However, interaxonal spacing also increased, resulting in densities of myelinated axons that were more similar to peripheral nerve than intact dorsal columns. Freshly isolated Schwann cells remyelinated more axonal length than cultured Schwann cells, and cells from younger donors remyelinated slightly more axon length than cells from older donors, but all Schwann cell transplant protocols remyelinated tens of thousands of millimeters of axon length and remyelinated axons at similar densities. These results indicate that Schwann cells prepared under a variety of conditions are capable of eliciting remyelination, but that the density of remyelinated axons is much lower than the myelinated axon density in intact spinal cords.

Differentiation of bipolar CG-4 line oligodendrocytes is associated with regulation of CREB, MAP kinase and PKC signalling pathways

Undifferentiated bipolar CG-4 cell line oligodendrocytes provide a model system for the O-2A progenitor cell from which oligodendrocytes are derived both in vivo and in vitro. The exchange of neuroblastoma conditioned basal media for basal media causes differentiation of undifferentiated bipolar CG-4 cells into multipolar oligodendrocyte-like cells whilst replacement with basal media containing 20% foetal bovine serum favours the formation of type-2 astrocyte-like cells. Here, we demonstrate that activation of these differentiation pathways correlates with distinct changes both in cell metabolism and in signal transduction. Exchange of neuroblastoma conditioned media for basal media correlates with stimulation of basal metabolic activity, reduced phosphorylation of p44/42 MAP kinase and reduced phosphorylation of the transcription factor CREB. In contrast, differentiation with basal medium containing 20% foetal bovine serum (FBS), into type 2 astrocyte-like cells, correlates with reduction in basal metabolic activity, increased phosphorylation of p44/42 MAP kinase and increased phosphorylation of the transcription factor CREB. Inhibition of protein kinase C blocked both the metabolic and morphological changes associated with differentiation towards mature multipolar oligodendrocyte-like cells. Inhibition of PKA and MEK did not effect metabolic activity. The rapid return of neuroblastoma conditioned basal media to cells treated with basal media, increased phosphorylation of CREB and MAP kinase. These results demonstrate that protein kinase C and p44/42 MAP kinase signalling pathways are modulated during bipolar CG-4 cell differentiation and demonstrate that the transcription factor CREB may play a pivotal role in differentiation along oligodendrocyte-or astrocyte-lineages.

Modification of radiation myelopathy by the transplantation of neural stem cells in the rat

In a novel approach, neural stem cells were transplanted to ameliorate radiation-induced myelopathy in the spinal cords of rats. A 12-mm section of the cervical spinal cord (T2-C2) of 5-week-old female Sprague-Dawley rats was locally irradiated with a single dose of 22 Gy of (60)Co gamma rays. This dose is known to produce myelopathy in all animals within 6 months of irradiation. After irradiation, the animals were subdivided into three groups, and at 90 days after irradiation, neural stem cells or saline (for controls) were injected into the spinal cord, intramedullary, at two sites positioned 6 mm apart on either side of the center of the irradiated length of spinal cord. The injection volume was 2 microl. Group I received a suspension of MHP36 cells, Group II MHP15 cells, and Group III (controls) two injections of 2 microl saline. All rats received 10 mg/kg cyclosporin (10 mg/ml) daily i.p. to produce immunosuppression. All animals that received saline (Group III) developed paralysis within 167 days of irradiation. The paralysis-free survival rates of rats that received transplanted MHP36 and MHP15 cells (Groups I and II) were 36.4% and 32% at 183 days, respectively. It was concluded that transplantation of neural stem cells 90 days after irradiation significantly (P = 0.03) ameliorated the expression of radiation-induced myelopathy in the spinal cords of rats.

Embryonic-derived glial-restricted precursor cells (GRP cells) can differentiate into astrocytes and oligodendrocytes in vivo

We have isolated and characterized a unique glial-restricted precursor cell (GRP) from the embryonic spinal cord. Clonal analysis demonstrated that these cells are able to generate oligodendrocytes and two distinct type of astrocytes (type 1 and type 2) when exposed to appropriate signals in vitro. We now show that many aspects of these cells are retained in vivo. GRP cells are restricted to the glial lineage in vivo as they seem to be unable to generate neuronal phenotypes in an in vivo neurogenic environment. GRP cells survive and migrate in the neonatal and adult brain. Transplanted GRP cells differentiate into myelin-forming oligodendrocytes in a myelin-deficient background and also generate immature oligodendrocytes in the normal neonatal brain. In addition, GRP cells also consistently generated glial fibrillary protein-expressing cells in the neonatal and adult brain, a property not consistently expressed by other glial precursor cells like the O-2A/OPC cells. We suggest that the lineage restriction of GRP cells and their ability to generate both oligodendrocytes and astrocytes in vivo together with their embryonic character that allows for extensive in vitro expansion of the population makes the cell useful for clinical application.

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.

Ethanol exerts different effects on myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase expression in differentiating CG-4 oligodendrocytes

Evidence suggests that abnormal myelination is one factor contributing to the neuoropathology associated with fetal alcohol syndrome. We investigated the potential teratogenic effects of ethanol (EtOH) on myelin formation by determining its effects on the developmentally regulated increased expression of myelin basic protein (MBP) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in differentiating CG-4 oligodendrocytes (OLGs). By using CG-4 OLGs in vitro we identified processes altered by ethanol actions exerted directly on OLGs. During the first 8 days of development, EtOH decreased the expression of the major structural 18.5 and 14 kDa MBP isoforms by at least 40% at 4 days of development. EtOH concentrations between 25 and 75 mM inhibited MBP expression in a dose-dependent manner. Adding or withdrawing EtOH on specific days of differentiation reversibly modulated the expression of MBP, and the degree of inhibition was directly related to the length of ethanol exposure. As little as two consecutive days of EtOH exposure either early or late during development caused at least a 20% inhibition, however, no short critical time window of EtOH vulnerability for the inhibition was observed. The ethanol effect was selective for MBP expression, as EtOH did not alter the developmentally-regulated increased expression of CNP isozymes or enzyme activity. The results indicate that one factor contributing to the development of fetal alcohol syndrome may be defective myelination resulting from delayed and decreased MBP expression.

Tracking oligodendrocytes during development and regeneration

Over the past decade, advances in strategies to tag cells have opened new avenues for examining the development of myelin-forming glial cells and for monitoring transplanted cells in animal models of myelin insufficiency. The strategies for labelling glial cells have encompassed a range of genetic modifications as well as methods for directly attaching labels to cells. Genetically modified oligodendrocytes have been engineered to express enzymatic (e.g., beta-galactosidase, alkaline phosphatase), naturally fluorescent (e.g., green fluorescent protein), and antibiotic resistance (e.g., neomycin, zeomycin) reporters. Genes have been introduced in vivo and in vitro with viral or plasmid vectors to somatically label glial cells. To generate germ-line transmission of tagged oligodendrocytes, transgenic mice have been created both by direct injection into mouse fertilized eggs and by "knock-in" of reporters targetted to myelin gene loci in embryonic stem cells. Each experimental approach has advantages and limitations that need to be considered for individual applications. The availability of tagged glial cells has expanded our basic understanding of how oligodendrocytes are specified from stem cells and should continue to fill in the gaps in our understanding of how oligodendrocytes differentiate, myelinate, and maintain their myelin sheaths. Moreover, the ability to select oligodendrocytes by virtue of their acquired antibiotic resistance has provided an important new tool for isolating and purifying oligodendrocytes. Tagged glial cells have also been invaluable in evaluating cell transplant therapies in the nervous system. The tracking technologies that have driven these advances in glial cell biology are continuing to evolve and present new opportunities for examining oligodendrocytes in living systems. Microsc. Res. Tech. 52:766-777, 2001. Published 2001 Wiley-Liss, Inc.

Depletion of endogenous oligodendrocyte progenitors rather than increased availability of survival factors is a likely explanation for enhanced survival of transplanted oligodendrocyte progenitors in X-irradiated compared to normal CNS

Oligodendrocyte progenitors (OPs) survive and migrate following transplantation into adult rat central nervous system (CNS) exposed to high levels of X-irradiation but fail to do so if they are transplanted into normal adult rat CNS. In the context of developing OP transplantation as a potential therapy for repairing demyelinating diseases it is clearly of some importance to understand what changes have occurred in X-irradiated CNS that permit OP survival. This study addressed two alternative hypotheses. Firstly, X-irradiation causes an increase in the availability of OP survival factors, allowing the CNS to support a greater number of progenitors. Secondly, X-irradiation depletes the endogenous OP population thereby providing vacant niches that can be occupied by transplanted OPs. In situ hybridization was used to examine whether X-irradiation causes an increase in mRNA expression of five known OP survival factors, CNTF, IGF-I, PDGF-A, NT-3 and GGF-2. The levels of expression of these factors at 4 and 10 days following exposure of the adult rat spinal cord to X-irradiation remain the same as the expression levels in normal tissue. Using intravenous injection of horseradish peroxidase, no evidence was found of X-irradiation-induced change in blood-brain barrier permeability that might have exposed X-irradiated tissue to serum-derived survival factors. However, in support of the second hypothesis, a profound X-irradiation-induced decrease in the number of OPs was noted. These data suggest that the increased survival of transplanted OPs in X-irradiated CNS is not a result of the increases in the availability of the OP survival factors examined in this study but rather the depletion of endogenous OPs creating 'space' for transplanted OPs to integrate into the host tissue.

Transient expression of the NG2 proteoglycan by a subpopulation of activated macrophages in an excitotoxic hippocampal lesion

Cells that express the NG2 proteoglycan (NG2+ cells) constitute a large glial population in the normal mature rodent brain. They can differentiate into oligodendrocytes but are distinct from mature oligodendrocytes, astrocytes, microglia, and neurons. Changes in NG2+ cells were examined in kainic acid-induced excitotoxic lesions of the hippocampus, and the relationship between NG2+ cells and reactive astrocytes and microglia was investigated between 1 and 90 days after lesioning. Two types of reactive NG2+ cells with altered morphology and increased NG2 immunoreactivity were observed in the lesion. Early changes, consisting of an increase in NG2 immunoreactivity and the number of processes, were apparent 24 h after lesioning and persisted through 3 months. These cells were distinct from reactive astrocytes or activated microglia/macrophages. A second type of reactive NG2+ cells appeared 2 weeks after injection, following an influx of macrophages. They had large, round cell bodies with short processes and expressed the microglia/macrophage antigens OX42 and ED1. Single cells coexpressing NG2 and macrophage/microglial antigens could be isolated from the lesion. The number of NG2+/OX42+ cells gradually declined and disappeared by 3 months after injection. They did not express glial fibrillary acidic protein or the alpha receptor for platelet-derived growth factor, indicating that they are distinct from astrocytes or oligodendrocyte progenitor cells. Cells that coexpressed NG2 and OX42 were never observed in hippocampal slice cultures treated with kainic acid, suggesting that NG2+/OX42+ cells are not derived from endogenous resident brain cells. These findings demonstrate that NG2 expression is transiently upregulated on activated macrophages/microglia that appear during the chronic stage in an excitotoxic lesion in the adult CNS.

Effects of transient ethanol exposure on the incorporation of [(3)H]ethanolamine into plasmalogen in the differentiating CG-4 oligodendrocyte cell line

We investigated the potential teratogenic effects of ethanol (EtOH) on myelination by monitoring its effects on the labeling of the myelin-typical lipid, ethanolamine plasmalogen (EPl), in the CG-4 cell line of differentiating oligodendrocytes (OLGs). On 5 different days during the first 8 days of OLG development, cells were labeled for 24 hr with [(3)H]ethanolamine to label EPl and diacyl-ethanolamine phosphoglycerols (diacyl-EPG), and the amount of labeled lipid expressed on each day was determined in the presence and absence of 25-120 mM EtOH. At early stages of development, a lower amount of [(3)H]EPl per cell was found in cells exposed to EtOH. The ratio of [(3)H]EPl to [(3)H]diacyl-EPG in cells exposed to 25, 50, or 120 mM EtOH was decreased by 50% after 4 days of differentiation compared with that in control cells. By adding or withdrawing EtOH at specific days of differentiation, we showed that EtOH inhibited the increased labeling of EPl if it was present for the first 48 hr of differentiation, and subsequent withdrawal failed to relieve the inhibition. Addition of EtOH anytime after the first day of differentiation did not inhibit the increased labeling of EPl. The results show that the increased labeling of EPl in differentiating OLGs resulted from an EtOH-sensitive, developmentally programmed, transient process active only during the first 2 days of differentiation.

Porcine neural progenitors require commitment to the oligodendrocyte lineage prior to transplantation in order to achieve significant remyelination of demyelinated lesions in the adult CNS

Glial cell transplantation is a potential therapy for human demyelinating disease, though obtaining large numbers of oligodendrocyte precursors from nonrodent species is currently problematic. Culturing of multipotent neural progenitors may provide a solution to this problem, because these cells can be expanded in vitro whilst retaining the ability to differentiate into both neurons and glial cells. In order to investigate the myelinating capability of multipotent neural progenitors, we isolated cells from the porcine subventricular zone, a region rich in neural progenitors, and transplanted them into areas of persistent demyelination in the spinal cord of immunosuppressed rats, created by the injection of ethidium bromide and subsequent exposure to 40 Gy X-irradiation. Porcine SVZ cells were transplanted either within 12 h of isolation or after 7 days in B104-conditioned medium. Freshly isolated cells did not mature into myelinating oligodendrocytes following transplantation and instead remained as clusters of undifferentiated progenitors. However, cells exposed to B104-conditioned medium prior to transplantation were able to effect complete remyelination of the demyelinated axons. This suggests that neural progenitors must be manipulated in vitro for commitment to the oligodendrocyte lineage prior to transplantation if significant remyelination is to be achieved.

The regulation of proliferation and differentiation in oligodendrocyte progenitor cells by alphaV integrins

We have previously shown that oligodendrocyte progenitor cells exhibit developmental switching between alphav-associated beta integrin subunits to sequentially express alphavbeta1, alphavbeta3 and alphavbeta5 integrins during differentiation in vitro. To understand the role that alphavveta3 integrin may play in regulating oligodendrocyte progenitor cell behaviour, cells of the rat cell line, CG-4, were genetically engineered to constitutively express alphavbeta3 integrin by transfection with full-length human beta3 integrin subunit cDNA. Time-lapse videomicroscopy showed no effect of beta3 expression on cell migration but revealed enhanced proliferation on vitronectin substrata. Comparison of mitotic indices, as measured by 5-bromo-2′-deoxyuridine incorporation, confirmed that human beta3 integrin-expressing cells exhibited enhanced proliferation, as compared to both vector-only transfected, and wild-type CG-4 cells when switched to differentiation medium from growth medium, but only in cultures grown on vitronectin and not on poly-D-lysine. The effects on proliferation were inhibited by a function-blocking antibody specifically directed against the human beta3 integrin subunit. Human beta3 integrin-expressing cells also exhibited reduced differentiation. This differentiation could be reduced still further by a function-blocking monoclonal antibody against alphavbeta5 integrin, as could differentiation in the wild-type CG-4 cells. Taken together, these results suggest that alphavbeta3 integrin may regulate oligodendroglial cell proliferation and that both downregulation of alphavbeta3 integrin expression and signalling through alphavbeta5 integrin may be critical to continued differentiation in vitro.