NG2 glia generate new oligodendrocytes but few astrocytes in a murine experimental autoimmune encephalomyelitis model of demyelinating disease

Glial cells in amyotrophic lateral sclerosis

For more than twenty years glial cells have been implicated in the pathogenetic cascades for genetic and sporadic forms of ALS. The biological role of glia, including the principal CNS glia, astroglia and oligodendroglia, as well as the myeloid derived microglia, has uniformly led to converging data sets that implicate these diverse cells in the degeneration of neurons in ALS. Originating as studies in postmortem human brain implicating astroglia, the research progressed to strongly implicate microglia and contributors to CNS injury in all forms of ALS. Most recently and unexpectedly, oligodendroglia have also been shown in animal model systems and human brain to play an early role in the dysfunction and death of ALS neurons. These studies have identified a number of diverse cellular cascades that could be, or have already been, the target of therapeutic interventions. Understanding the temporal and regional role of these cells and the magnitude of their contribution will be important for future interventions. Employing markers of these cell types may also allow for future important patient subgrouping and pharmacodynamic drug development tools.

Neuroglia in ageing and disease

The proper operation of the mammalian brain requires dynamic interactions between neurones and glial cells. Various types of glial cells are susceptible to morpho-functional changes in a variety of brain pathological states, including toxicity, neurodevelopmental, neurodegenerative and psychiatric disorders. Morphological modifications include a change in the glial cell size and shape; the latter is evident by changes of the appearance and number of peripheral processes. The most blatant morphological change is associated with the alteration of the sheer number of neuroglia cells in the brain. Functionally, glial cells can undergo various metabolic and biochemical changes, the majority of which reflect upon homeostasis of neurotransmitters, in particular that of glutamate, as well as on defence mechanisms provided by neuroglia. Not only glial cells exhibit changes associated with the pathology of the brain but they also change with brain aging.

Synaptic integration by NG2 cells

NG2 expressing oligodendrocyte precursor cells stand out from other types of glial cells by receiving classical synaptic contacts from many neurons. This unconventional form of signaling between neurons and glial cells enables NG2 cells to receive information about the activity of presynaptic neurons with high temporal and spatial precision and has been postulated to be involved in activity-dependent myelination. While this still unproven concept is generally compelling, how NG2 cells may integrate synaptic input has hardly been addressed to date. Here we review the biophysical characteristics of synaptic currents and membrane properties of NG2 cells and discuss their capabilities to perform complex temporal and spatial signal integration and how this may be important for activity-dependent myelination.

White matter plasticity in adulthood

CNS white matter is subject to a novel form of neural plasticity which has been termed "myelin plasticity". It is well established that oligodendrocyte generation and the addition of new myelin internodes continue throughout normal adulthood. These new myelin internodes maybe required for the de novo myelination of previously unmyelinated axons, myelin sheath replacement, or even myelin remodeling. Each process could alter axonal conduction velocity, but to what end? We review the changes that occur within the white matter over the lifetime, the known regulators and mediators of white matter plasticity in the mature CNS, and the physiological role this plasticity may play in CNS function.

Increased carbonylation, protein aggregation and apoptosis in the spinal cord of mice with experimental autoimmune encephalomyelitis

Previous work from our laboratory implicated protein carbonylation in the pathophysiology of both MS (multiple sclerosis) and its animal model EAE (experimental autoimmune encephalomyelitis). Subsequent in vitro studies revealed that the accumulation of protein carbonyls, triggered by glutathione deficiency or proteasome inhibition, leads to protein aggregation and neuronal cell death. These findings prompted us to investigate whether their association can be also established in vivo. In the present study, we characterized protein carbonylation, protein aggregation and apoptosis along the spinal cord during the course of MOG (myelin-oligodendrocyte glycoprotein)(35-55) peptide-induced EAE in C57BL/6 mice. The results show that protein carbonyls accumulate throughout the course of the disease, albeit by different mechanisms: increased oxidative stress in acute EAE and decreased proteasomal activity in chronic EAE. We also show a temporal correlation between protein carbonylation (but not oxidative stress) and apoptosis. Furthermore, carbonyl levels are significantly higher in apoptotic cells than in live cells. A high number of juxta-nuclear and cytoplasmic protein aggregates containing the majority of the oxidized proteins are present during the course of EAE. The LC3 (microtubule-associated protein light chain 3)-II/LC3-I ratio is significantly reduced in both acute and chronic EAE indicating reduced autophagy and explaining why aggresomes accumulate in this disorder. Taken together, the results of the present study suggest a link between protein oxidation and neuronal/glial cell death in vivo, and also demonstrate impaired proteostasis in this widely used murine model of MS.

Early postnatal GFAP-expressing cells produce multilineage progeny in cerebrum and astrocytes in cerebellum of adult mice

Early postnatal GFAP-expressing cells are thought to be immature astrocytes. However, it is not clear if they possess multilineage capacity and if they can generate different lineages (astrocytes, neurons and oligodendrocytes) in the brain of adult mice. In order to identify the fate of astroglial cells in the postnatal brain, hGFAP-Cre-ER(T2) transgenic mice were crossed with the R26R Cre reporter mouse strains which exhibit constitutive expression of β-galactosidase (β-gal). Mice carrying the hGFAP-Cre-ER(T2)/R26R transgene were treated with Tamoxifen to induce Cre recombination in astroglial cells at postnatal (P) day 6 and Cre recombinase-expressing cells were identified by X-gal staining. Immunohistochemical staining was used to identify the type(s) of these reporter-tagged cells. Sixty days after recombination, X-gal-positive cells in different cerebral regions of the adult mice expressed the astroglial markers Blbp and GFAP, the neuronal marker NeuN, the oligodendrocyte precursor cell marker NG2 and the mature oligodendrocyte marker CC1. X-gal-positive cells in the cerebellum coexpressed the astroglial marker Blbp, but not the granule cell marker NeuN, Purkinje cell marker Calbindin or oligodendrocyte precursor cell marker NG2. Our genetic fate mapping data demonstrated that early postnatal GFAP-positive cells possessed multilineage potential and eventually differentiated into neurons, astrocytes, and oligodendrocyte precursor cells in the cerebrum and into astrocytes (including Bergmann glia) in the cerebellum of adult mice.

Ageing stem and progenitor cells: implications for rejuvenation of the central nervous system

The growing burden of the rapidly ageing global population has reinvigorated interest in the science of ageing and rejuvenation. Among organ systems, rejuvenation of the central nervous system (CNS) is arguably the most complex and challenging of tasks owing, among other things, to its startling structural and functional complexity and its restricted capacity for repair. Thus, the prospect of meaningful rejuvenation of the CNS has seemed an impossible goal; however, advances in stem cell science are beginning to challenge this assumption. This Review outlines these advances with a focus on ageing and rejuvenation of key endogenous stem and progenitor cell compartments in the CNS. Insights gleaned from studies of model organisms, chiefly rodents, will be considered in parallel with human studies.

Oligodendrocyte progenitor cell susceptibility to injury in multiple sclerosis

Remyelination in multiple sclerosis (MS) is often incomplete. In experimental models, oligodendrocyte progenitor cells (OPCs) rather than previously myelinating oligodendrocytes (OLs) are responsible for remyelination. This study compares the relative susceptibility of adult human OPCs and mature OLs to injury in actively demyelinating MS lesions and under in vitro stress conditions. In all lesions (n = 20), the number of OLs (Olig2 weak/NogoA positive) was reduced compared to control white matter (mean 38 ± 4% of control value). In 11 cases, OPC numbers (Olig2 strong; NogoA negative) were also decreased; in eight of these, the reduction was greater for OPCs than for OLs. In the other nine samples, OPC numbers were greater than control white matter, indicating ongoing OPC migration and/or proliferation. Analysis of co-cultures with rat dorsal root ganglia neurons confirmed that OPCs were more capable of contacting and ensheathing axons than OLs. In isolated culture under stress conditions (withdrawal of serum/glucose and/or antioxidants), OPCs showed increased cell death and reduced process extension compared to OLs. Under all culture conditions, OPCs up-regulated expression of genes in the extrinsic proapoptotic pathway, and had increased susceptibility to tumor necrosis factor-induced cell death as compared to OLs. Our data suggest that susceptibility of OPCs to injury within the MS lesion environment contributes to the limited remyelination in MS.

The road to remyelination in demyelinating diseases: current status and prospects for clinical treatment

Within CNS disorders, demyelinating diseases are among the most devastating and cost intensive due to long-term disabilities affecting relatively young patients. Multiple sclerosis, a chronic inflammatory demyelinating disease in which the persistent inhibitory microenvironment of the resident oligodendrocyte precursor cells abrogates regeneration of myelin sheaths, is the most prominent disease in the spectrum of demyelinating diseases. The essential goal is to stimulate creation of new myelin sheaths on the demyelinated axons, leading to restoration of saltatory conduction and resolving functional deficits. The past few decades witnessed significant efforts to understand the cellular interactions at the lesion site with studies suggesting efficient remyelination as a prerequisite for functional repair. Despite its proven efficacy in experimental models, immunosuppression has not had profound clinical consequences in multiple sclerosis, which argued for a paradigm shift in the design of therapeutics aiming to achieve remyelination. For example, targeting oligodendrocytes themselves may drive remyelination in the CNS. This group and others have demonstrated that natural autoreactive antibodies directed at oligodendrocyte progenitors participate in remyelination. Accordingly, the authors developed a recombinant autoreactive natural human IgM antibody with therapeutic potential for remyelination.

A TSPO ligand is protective in a mouse model of multiple sclerosis

Local production of neurosteroids such as progesterone and allopregnanolone confers neuroprotection in central nervous system (CNS) inflammatory diseases. The mitochondrial translocator protein (TSPO) performs a rate-limiting step in the conversion of cholesterol to pregnenolone and its steroid derivatives. Previous studies have shown that TSPO is upregulated in microglia and astroglia during neural inflammation, and radiolabelled TSPO ligands such as PK11195 have been used to image and localize injury in the CNS. Recent studies have shown that modulating TSPO activity with pharmacological ligands such as etifoxine can initiate the production of neurosteroids locally in the injured CNS. In this study, we examined the effects of etifoxine, a clinically available anxiolytic drug, in the development and progression of mouse experimental autoimmune encephalomyelitis (EAE), an experimental model for multiple sclerosis (MS). Our results showed that etifoxine attenuated EAE severity when administered before the development of clinical signs and also improved symptomatic recovery when administered at the peak of the disease. In both cases, recovery was correlated with diminished inflammatory pathology in the lumbar spinal cord. Modulation of TSPO activity by etifoxine led to less peripheral immune cell infiltration of the spinal cord, and increased oligodendroglial regeneration after inflammatory demyelination in EAE. Our results suggest that a TSPO ligand, e.g. etifoxine, could be a potential new therapeutic option for MS with benefits that could be comparable to the administration of systemic steroids but potentially avoiding the detrimental side effects of long-term direct use of steroids.

NG2 cells, a new trail for Alzheimer's disease mechanisms?

Background

Neuron Glial 2 (NG2) cells are glial cells known to serve as oligodendrocyte progenitors as well as modulators of the neuronal network. Altered NG2 cell morphology and up-regulation as well as increased shedding of the proteoglycan NG2 expressed on the cell surface have been described in rodent models of brain injury. Here we describe alterations in the human NG2 cell population in response to pathological changes characteristic of Alzheimer’s disease (AD).

Results

Immunohistological stainings of postmortem brain specimens from clinically diagnosed and postmortem verified AD patients and non-demented controls revealed reduced NG2 immunoreactivity as well as large numbers of NG2 positive astrocytes in individuals with high amyloid beta plaque load. Since fibrillar amyloid beta (Aβ)1-42 is the major component of AD-related senile plaques, we exposed human NG2 cells to oligomer- and fibril enriched preparations of Aβ1-42. We found that both oligomeric and fibrillar Aβ1-42 induced changes in NG2 cell morphology. Further, in vitro exposure to fibrillar Aβ1-42 decreased the NG2 concentrations in both cell lysates and supernatants. Interestingly, we also found significantly decreased levels of soluble NG2 in the cerebrospinal fluid (CSF) from clinically diagnosed AD patients compared to non-demented individuals. Additionally, the CSF NG2 levels were found to significantly correlate with the core AD biomarkers Aß1-42, T-tau and P-tau.

Conclusion

Our results demonstrate major alterations in the NG2 cell population in relation to AD pathology which highlights the NG2 cell population as a new attractive research target in the search for cellular mechanisms associated with AD pathogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/2051-5960-1-7) contains supplementary material, which is available to authorized users.

Oligodendrocyte progenitors balance growth with self-repulsion to achieve homeostasis in the adult brain

The adult CNS contains an abundant population of oligodendrocyte precursor cells (NG2⁺ cells) that generate oligodendrocytes and repair myelin, but how these ubiquitous progenitors maintain their density is unknown. Here we generated NG2-mEGFP mice and used in vivo two-photon imaging to study their dynamics in the adult brain. Time-lapse imaging revealed that NG2⁺ cells in the cortex are highly dynamic; they survey their local environment with motile filopodia, extend growth cones, and continuously migrate. They maintain unique territories through self-avoidance, and NG2⁺ cell loss through death, differentiation, or ablation triggered rapid migration and proliferation of adjacent cells to restore their density. NG2⁺ cells recruited to sites of focal CNS injury were similarly replaced by a proliferative burst surrounding the injury site. Thus, homeostatic control of NG2⁺ cell density through a balance of active growth and self-repulsion ensures that these progenitors are available to replace oligodendrocytes and participate in tissue repair.

The neural androgen receptor: a therapeutic target for myelin repair in chronic demyelination

Myelin regeneration is a major therapeutic goal in demyelinating diseases, and the failure to remyelinate rapidly has profound consequences for the health of axons and for brain function. However, there is no efficient treatment for stimulating myelin repair, and current therapies are limited to anti-inflammatory agents. Males are less likely to develop multiple sclerosis than females, but often have a more severe disease course and reach disability milestones at an earlier age than females, and these observations have spurred interest in the potential protective effects of androgens. Here, we demonstrate that testosterone treatment efficiently stimulates the formation of new myelin and reverses myelin damage in chronic demyelinated brain lesions, resulting from the long-term administration of cuprizone, which is toxic for oligodendrocytes. In addition to the strong effect of testosterone on myelin repair, the number of activated astrocytes and microglial cells returned to low control levels, indicating a reduction of neuroinflammatory responses. We also identify the neural androgen receptor as a novel therapeutic target for myelin recovery. After the acute demyelination of cerebellar slices in organotypic culture, the remyelinating actions of testosterone could be mimicked by 5α-dihydrotestosterone, a metabolite that is not converted to oestrogens, and blocked by the androgen receptor antagonist flutamide. Testosterone treatment also failed to promote remyelination after chronic cuprizone-induced demyelination in mice with a non-functional androgen receptor. Importantly, testosterone did not stimulate the formation of new myelin sheaths after specific knockout of the androgen receptor in neurons and macroglial cells. Thus, the neural brain androgen receptor is required for the remyelination effect of testosterone, whereas the presence of the receptor in microglia and in peripheral tissues is not sufficient to enhance remyelination. The potent synthetic testosterone analogue 7α-methyl-19-nortestosterone, which has been developed for long-term male contraception and androgen replacement therapy in hypogonadal males and does not stimulate prostate growth, also efficiently promoted myelin repair. These data establish the efficacy of androgens as remyelinating agents and qualify the brain androgen receptor as a promising drug target for remyelination therapy, thus providing the preclinical rationale for a novel therapeutic use of androgens in males with multiple sclerosis.

Oligodendrocyte dynamics in the healthy adult CNS: evidence for myelin remodeling

Oligodendrocyte precursors (OPs) continue to proliferate and generate myelinating oligodendrocytes (OLs) well into adulthood. It is not known whether adult-born OLs ensheath previously unmyelinated axons or remodel existing myelin. We quantified OP division and OL production in different regions of the adult mouse CNS including the 4-month-old optic nerve, in which practically all axons are already myelinated. Even there, all OPs were dividing and generating new OLs and myelin at a rate higher than can be explained by first-time myelination of naked axons. We conclude that adult-born OLs in the optic nerve are engaged in myelin remodeling, either replacing OLs that die in service or intercalating among existing myelin sheaths. The latter would predict that average internode length should decrease with age. Consistent with that, we found that adult-born OLs elaborated much shorter but many more internodes than OLs generated during early postnatal life.

Polydendrocytes in development and myelin repair

Polydendrocytes (NG2 cells) are a distinct type of glia that populate the developing and adult central nervous systems (CNS). In the adult CNS, they retain mitotic activity and represent the largest proliferating cell population. Genetic and epigenetic mechanisms regulate the fate of polydendrocytes, which give rise to both oligodendrocytes and astrocytes. In addition, polydendrocytes actively differentiate into myelin-forming oligodendrocytes in response to demyelination. This review summarizes the current knowledge regarding polydendrocyte development, which provides an important basis for understanding the mechanisms that lead to the remyelination of demyelinated lesions.

Neural and oligodendrocyte progenitor cells: transferrin effects on cell proliferation

NSC (neural stem cells)/NPC (neural progenitor cells) are multipotent and self-renew throughout adulthood in the SVZ (subventricular zone) of the mammalian CNS (central nervous system). These cells are considered interesting targets for CNS neurodegenerative disorder cell therapies, and understanding their behaviour in vitro is crucial if they are to be cultured prior to transplantation. We cultured the SVZ tissue belonging to newborn rats under the form of NS (neurospheres) to evaluate the effects of Tf (transferrin) on cell proliferation. The NS were heterogeneous in terms of the NSC/NPC markers GFAP (glial fibrillary acidic protein), Nestin and Sox2 and the OL (oligodendrocyte) progenitor markers NG2 (nerve/glia antigen 2) and PDGFRα (platelet-derived growth factor receptor α). The results of this study indicate that aTf (apoTransferrin) is able to increase cell proliferation of SVZ-derived cells in vitro, and that these effects were mediated at least in part by the TfRc1 (Tf receptor 1). Since OPCs (oligodendrocyte progenitor cells) represent a significant proportion of the proliferating cells in the SVZ-derived primary cultures, we used the immature OL cell line N20.1 to show that Tf was able to augment the proliferation rate of OPC, either by adding aTf to the culture medium or by overexpressing rat Tf in situ. The culture medium supplemented with ferric iron, together with aTf, increased the DNA content, while ferrous iron did not. The present work provides data that could have a potential application in human cell replacement therapies for neurodegenerative disease and/or CNS injury that require the use of in vitro amplified NPCs.

Myelin repair and functional recovery mediated by neural cell transplantation in a mouse model of multiple sclerosis

Cellular therapies are becoming a major focus for the treatment of demyelinating diseases such as multiple sclerosis (MS), therefore it is important to identify the most effective cell types that promote myelin repair. Several components contribute to the relative benefits of specific cell types including the overall efficacy of the cell therapy, the reproducibility of treatment, the mechanisms of action of distinct cell types and the ease of isolation and generation of therapeutic populations. A range of distinct cell populations promote functional recovery in animal models of MS including neural stem cells and mesenchymal stem cells derived from different tissues. Each of these cell populations has advantages and disadvantages and likely works through distinct mechanisms. The relevance of such mechanisms to myelin repair in the adult central nervous system is unclear since the therapeutic cells are generally derived from developing animals. Here we describe the isolation and characterization of a population of neural cells from the adult spinal cord that are characterized by the expression of the cell surface glycoprotein NG2. In functional studies, injection of adult NG2(+) cells into mice with ongoing MOG35-55-induced experimental autoimmune encephalomyelitis (EAE) enhanced remyelination in the CNS while the number of CD3(+) T cells in areas of spinal cord demyelination was reduced approximately three-fold. In vivo studies indicated that in EAE, NG2(+) cells stimulated endogenous repair while in vitro they responded to signals in areas of induced inflammation by differentiating into oligodendrocytes. These results suggested that adult NG2(+) cells represent a useful cell population for promoting neural repair in a variety of different conditions including demyelinating diseases such as MS.

Subventricular zone-derived oligodendrogenesis in injured neonatal white matter in mice enhanced by a nonerythropoietic erythropoietin derivative

Perinatal hypoxia-ischemia (HI) frequently causes white-matter injury, leading to severe neurological deficits and mortality, and only limited therapeutic options exist. The white matter of animal models and human patients with HI-induced brain injury contains increased numbers of oligodendrocyte progenitor cells (OPCs). However, the origin and fates of these OPCs and their potential to repair injured white matter remain unclear. Here, using cell-type- and region-specific genetic labeling methods in a mouse HI model, we characterized the Olig2-expressing OPCs. We found that after HI, Olig2+ cells increased in the posterior part of the subventricular zone (pSVZ) and migrated into the injured white matter. However, their oligodendrocytic differentiation efficiency was severely compromised compared with the OPCs in normal tissue, indicating the need for an intervention to promote their differentiation. Erythropoietin (EPO) treatment is a promising candidate, but it has detrimental effects that preclude its clinical use for brain injury. We found that long-term postinjury treatment with a nonerythropoietic derivative of EPO, asialo-erythropoietin, promoted the maturation of pSVZ-derived OPCs and the recovery of neurological function, without affecting hematopoiesis. These results demonstrate the limitation and potential of endogenous OPCs in the pSVZ as a therapeutic target for treating neonatal white-matter injury.

Separated at birth? The functional and molecular divergence of OLIG1 and OLIG2

The basic helix-loop-helix transcription factors oligodendrocyte transcription factor 1 (OLIG1) and OLIG2 are structurally similar and, to a first approximation, coordinately expressed in the developing CNS and postnatal brain. Despite these similarities, it was apparent from early on after their discovery that OLIG1 and OLIG2 have non-overlapping developmental functions in patterning, neuron subtype specification and the formation of oligodendrocytes. Here, we summarize more recent insights into the separate roles of these transcription factors in the postnatal brain during repair processes and in neurological disease states, including multiple sclerosis and malignant glioma. We discuss how the unique functions of OLIG1 and OLIG2 may reflect their distinct genetic targets, co-regulator proteins and/or post-translational modifications.

The astrocytic lineage marker calmodulin-regulated spectrin-associated protein 1 (Camsap1): phenotypic heterogeneity of newly born Camsap1-expressing cells in injured mouse brain

Calmodulin-regulated spectrin-associated protein 1 (Camsap1) has been recognized as a new marker for astrocytic lineage cells and is expressed on mature astrocytes in the adult brain (Yamamoto et al. [2009] J. Neurosci. Res. 87:503–513). In the present study, we found that newly born Camsap1-expressing cells exhibited regional heterogeneity in an early phase after stab injury of the mouse brain. In the surrounding area of the lesion site, Camsap1 was expressed on quiescent astrocytes. At 3 days after injury, Camsap1 immunoreactivity was upregulated on glial fibrillary acidic protein-immunoreactive (GFAP-ir) astrocytes. Some of these astrocytes incorporated bromodeoxyuridine (BrdU) together with re-expression of the embryonic cytoskeleton protein nestin. In the neighboring region of the lesion cavity, Camsap1 was expressed on GFAP-negative cells. At 3 days after injury, GFAP-ir astrocytes were absent around the lesion cavity. At this stage, NG2-ir cells immunopositive for Camsap1 and immunonegative for GFAP were distributed in border of the lesion cavity. By 10 days, Camsap1 immunoreactivity was exclusively detected on GFAP-ir reactive astrocytes devoid of NG2 immunoreactivity. BrdU pulse-chase labeling assay suggested the differentiation of Camsap1+/NG2+ cells into Camsap1+/GFAP+ astrocytes. In the subependymal zone of the lateral ventricle, Camsap1-ir cells increased after injury. Camsap1 immunoreactivity was distributed on ependymal and subependymal cells bearing various astrocyte markers, and BrdU incorporation was enhanced on such Camsap1-ir cells after injury. These results suggest that newly born reactive astrocytes are derived from heterogeneous Camsap1-expressing cells in the injured brain.

Olig2-lineage cells preferentially differentiate into oligodendrocytes but their processes degenerate at the chronic demyelinating stage of proteolipid protein-overexpressing mouse

In chronic demyelinating lesions of the central nervous system, insufficient generation of oligodendrocytes (OLs) is not due to a lack of oligodendrocyte precursor cells (OPCs), because the accumulation of OPCs and premyelinating OLs can be observed within these lesions. Here we sought to identify the basis for the failure of OLs to achieve terminal differentiation in chronic demyelinating lesions through the utilization of plp1-overexpressing (Plp(tg/-)) mice. These mice are characterized by progressive demyelination in young adults and chronic demyelinating lesions at more mature stages. We show that neural stem cells, which are the precursors of OL-lineage cells, are present in the Plp(tg/-) mouse brain and that their multipotentiality and ability to self-renew are comparable to those of wild-type adults in culture. Lineage-tracing experiments using a transgenic mouse line, in which an inducible Cre recombinase is knocked in at the Olig2 locus, revealed that Olig2-lineage cells preferentially differentiated into OPCs and premyelinating OLs, but not into astrocytes, in the Plp(tg/-) mouse brain. These Olig2-lineage cells matured to express myelin basic protein but after that their processes degenerated in the chronic demyelinating lesions of the Plp(tg/-) brain. These results indicate that in chronic demyelinated lesions more OL-lineage cells are produced as part of the repair process, but their processes degenerate after maturation.

Abnormal development of NG2+PDGFR-α+ neural progenitor cells leads to neonatal hydrocephalus in a ciliopathy mouse model

Hydrocephalus is a common neurological disorder leading to expansion of the cerebral ventricles and is associated with significant morbidity and mortality. Most neonatal cases are of unknown etiology and are likely to display complex inheritance involving multiple genes and environmental factors. Identifying molecular mechanisms for neonatal hydrocephalus and developing non-invasive treatment modalities are high priorities. Here we employ a hydrocephalic mouse model of the human ciliopathy Bardet-Biedl Syndrome (BBS) and identify a role for neural progenitors in the pathogenesis of neonatal hydrocephalus. We found that hydrocephalus in this mouse model is caused by aberrant PDGFRα signaling, resulting in increased apoptosis and impaired proliferation of NG2⁺PDGFRα⁺ neural progenitors. Targeting this pathway with lithium treatment rescued NG2⁺PDGFRα⁺ progenitor cell proliferation in BBS mutant mice, reducing ventricular volume. Our findings demonstrate that neural progenitors are critical in the pathogenesis of neonatal hydrocephalus and we identify novel therapeutic targets for this common neurological disorder.

Mechanical environment modulates biological properties of oligodendrocyte progenitor cells

Myelination and its regenerative counterpart remyelination represent one of the most complex cell-cell interactions in the central nervous system (CNS). The biochemical regulation of axon myelination via the proliferation, migration, and differentiation of oligodendrocyte progenitor cells (OPCs) has been characterized extensively. However, most biochemical analysis has been conducted in vitro on OPCs adhered to substrata of stiffness that is orders of magnitude greater than that of the in vivo CNS environment. Little is known of how variation in mechanical properties over the physiological range affects OPC biology. Here, we show that OPCs are mechanosensitive. Cell survival, proliferation, migration, and differentiation capacity in vitro depend on the mechanical stiffness of polymer hydrogel substrata. Most of these properties are optimal at the intermediate values of CNS tissue stiffness. Moreover, many of these properties measured for cells on gels of optimal stiffness differed significantly from those measured on glass or polystyrene. The dependence of OPC differentiation on the mechanical properties of the extracellular environment provides motivation to revisit results obtained on nonphysiological, rigid surfaces. We also find that OPCs stiffen upon differentiation, but that they do not change their compliance in response to substratum stiffness, which is similar to embryonic stem cells, but different from adult stem cells. These results form the basis for further investigations into the mechanobiology of cell function in the CNS and may specifically shed new light on the failure of remyelination in chronic demyelinating diseases such as multiple sclerosis.

Valproic acid induces the glutamate transporter excitatory amino acid transporter-3 in human oligodendroglioma cells

Glutamate transport in early, undifferentiated oligodendrocytic precursors has not been characterized thus far. Here we show that human oligodendroglioma Hs683 cells are not endowed with EAAT-dependent anionic amino acid transport. However, in these cells, but not in U373 human glioblastoma cells, valproic acid (VPA), an inhibitor of histone deacetylases, markedly induces SLC1A1 mRNA, which encodes for the glutamate transporter EAAT3. The effect is detectable after 8h and persists up to 120h of treatment. EAAT3 protein increase becomes detectable after 24h of treatment and reaches its maximum after 72-96h, when it is eightfold more abundant than control. The initial influx of d-aspartate increases in parallel, exhibiting the typical features of an EAAT3-mediated process. SLC1A1 mRNA induction is associated with the increased expression of PDGFRA mRNA (+150%), a marker of early oligodendrocyte precursor cells, while the expression of GFAP, CNP and TUBB3 remains unchanged. Short term experiments have indicated that the VPA effect is shared by trichostatin A, another inhibitor of histone deacetylases. On the contrary, EAAT3 induction is neither prevented by inhibitors of mitogen-activated protein kinases nor triggered by a prolonged incubation with lithium, thus excluding a role for the GSK3β/β-catenin pathway. Thus, the VPA-dependent induction of the glutamate transporter EAAT3 in human oligodendroglioma cells likely occurs through an epigenetic mechanism and may represent an early indicator of commitment to oligodendrocytic differentiation.

Cortical spreading depression shifts cell fate determination of progenitor cells in the adult cortex

Cortical spreading depression (SD) is propagating neuronal and glial depolarization and is thought to underly the pathophysiology of migraine. We have reported that cortical SD facilitates the proliferative activity of NG2-containing progenitor cells (NG2 cells) that give rise to oligodendrocytes and immature neurons under the physiological conditions in the adult mammalian cortex. Astrocytes have an important role in the maintenance of neuronal functions and alleviate neuronal damage after intense neuronal excitation, including SD and seizures. We here investigated whether SD promotes astrocyte generation from NG2 cells following SD stimuli. Spreading depression was induced by epidural application of 1 mol/L KCl solution in adult rats. We investigated the cell fate of NG2 cells following SD-induced proliferation using 5'-bromodeoxyuridine labeling and immunohistochemical analysis. Newly generated astrocytes were observed only in the SD-stimulated cortex, but not in the contralateral cortex or in normal cortex. The astrocytes were generated from proliferating NG2 cells. Astrogenesis depended on the number of SD stimuli, and was accompanied by suppression of oligodendrogenesis. These observations indicate that the cell fate of NG2 cells was shifted from oligodendrocytes to astrocytes depending on SD stimuli, suggesting activity-dependent tissue remodeling for maintenance of brain functions.

Intraventricular injection of FGF-2 promotes generation of oligodendrocyte-lineage cells in the postnatal and adult forebrain

FGF2 is considered a key factor in the generation of oligodendrocytes (OLs) derived from neural stem cells (NSCs) located within the subventricular zone (SVZ). Here, we have examined FGF2 signaling in the forebrain of postnatal and adult mice. Using qPCR of microdissected microdomains of the dorsal SVZ (dSVZ) and lateral SVZ (lSVZ), and prominin1-sorted NSCs purified from these microdomains, we show that transcripts for FGF receptor 1 (FGFR1) and FGFR2 are enriched in the dSVZ, from which OLs are largely derived, whereas FGFR3 are significantly enriched within prominen1-sorted NSC of the lSVZ, which mainly generate olfactory interneurons. We show that direct administration of FGF2 into the lateral ventricle increased the generation of oligodendrocyte progenitors (OPCs) throughout the SVZ, both within the dSVZ and ectopically in the lSVZ and ependymal wall of the SVZ. Furthermore, FGF2 stimulated proliferation of neural progenitors (NPs) and their differentiation into OPCs. The results indicate that FGF2 increased specification of OPCs, inducing NPs to follow an oligodendrocyte developmental pathway. Notably, FGF2 did not block OPC differentiation and increased the number of oligodendrocytes in the periventricular white matter (PVWM) and cortex. However, FGF2 markedly disrupted myelination in the PVWM. A key finding was that FGF2 had equivalent actions on the generation of OPCs and myelin disruption in postnatal and adult mice. This study demonstrates a central role for FGF2 in promoting oligodendrocyte generation in the developing and adult brain.

A review of the role of stem cells in the development and treatment of glioma

The neurosurgical management of patients with intrinsic glial cancers is one of the most rapidly evolving areas of practice. This has been fuelled by advances in surgical technique not only in cytoreduction but also in drug delivery. Further innovation will depend on a deeper understanding of the biology of the disease and an appreciation of the limitations of current knowledge. Here we review the controversial topic of cancer stem cells applied to glioma to provide neurosurgeons with a working overview. It is now recognised that the adult human brain contains regionally specified cell populations capable of self-renewal that may contribute to tumour growth and maintenance following accumulated mutational change. Tumour cells adapted to maintain growth demonstrate some stem-like characteristics and as such constitute a legitimate therapeutic target. Cellular reprogramming technologies raise the potential of developing stem cells as novel surgical tools to target disease and possibly ameliorate some of the consequences of treatment. Achieving these goals remains a significant challenge to neurosurgical oncologists, not least in challenging how we think about treating brain cancer. This review will briefly examine our understanding of adult stem cells within the brain, the evidence that they contribute to the development of brain tumours as tumour-initiating cells, and the potential implications for therapy. It will also look at the role stem cells may play in the future management of glioma.

NG2 and NG2-positive cells delineate focal cerebral infarct demarcation in rats

Focal cerebral ischemia induces cellular responses that may result in secondary tissue damage. We recently demonstrated multi-facetted spatial and temporal patterns of neuroinflammation by multimodal imaging. In the present study, we especially focus on the separation of vital and necrotic tissue, which enabled us to define a demarcation zone. Focal cerebral ischemia was induced via macrosphere embolization of the middle cerebral artery in Wistar rats. Subsequent cellular processes were investigated immunohistochemically from 3 to 56 days after onset of ischemia. We detected several infarct subareas: a necrotic infarct core and its margin adjacent to a nerve/glial antigen 2 (NG2)+ zone delineating it from a vital peri-infarct zone. Initially transition from necrotic to vital tissue was gradual; later on necrosis was precisely separated from vital tissue by a narrow NG2+ belt that was devoid of astrocytes, oligodendrocytes or neurons. Within this demarcation zone NG2+ cells associate with ionized calcium binding adaptor molecule 1 (Iba1) but not with GFAP, neuronal nuclear antigen (NeuN) or 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase). During further infarct maturation NG2 seemed to be positioned in the extracellular matrix (ECM) of the demarcation zone, whereas Iba1+ cells invaded the necrotic infarct core and GFAP+ cells built a gliotic containing belt between the lesion and NeuN+ unaffected tissue. Overall, our data suggested that NG2 proteoglycan expression and secretion hallmarked demarcation as a process that actively separated necrosis from vital tissue and therefore decisively impacts secondary neurodegeneration after ischemic stroke.

Olig2-dependent developmental fate switch of NG2 cells

NG2-expressing cells (NG2 cells or polydendrocytes) generate oligodendrocytes throughout the CNS and a subpopulation of protoplasmic astrocytes in the gray matter of the ventral forebrain. The mechanisms that regulate their oligodendrocyte or astrocyte fate and the degree to which they exhibit lineage plasticity in vivo have remained unclear. The basic helix-loop-helix transcription factor Olig2 is required for oligodendrocyte specification and differentiation. We have found that Olig2 expression is spontaneously downregulated in NG2 cells in the normal embryonic ventral forebrain as they differentiate into astrocytes. To further examine the role of Olig2 in NG2 cell fate determination, we used genetic fate mapping of NG2 cells in constitutive and tamoxifen-inducible Olig2 conditional knockout mice in which Olig2 was deleted specifically in NG2 cells. Constitutive deletion of Olig2 in NG2 cells in the neocortex and corpus callosum but not in ventral forebrain caused them to convert their fate into astrocytes, with a concomitant severe reduction in the number of oligodendrocytes and myelin. Deletion of Olig2 in NG2 cells in perinatal mice also resulted in astrocyte generation from neocortical NG2 cells. These observations indicate that the developmental fate of NG2 cells can be switched by altering a single transcription factor Olig2.

Polydendrocytes display large lineage plasticity following focal cerebral ischemia

Polydendrocytes (also known as NG2 glial cells) constitute a fourth major glial cell type in the adult mammalian central nervous system (CNS) that is distinct from other cell types. Although much evidence suggests that these cells are multipotent in vitro, their differentiation potential in vivo under physiological or pathophysiological conditions is still controversial.

To follow the fate of polydendrocytes after CNS pathology, permanent middle cerebral artery occlusion (MCAo), a commonly used model of focal cerebral ischemia, was carried out on adult NG2creBAC:ZEG double transgenic mice, in which enhanced green fluorescent protein (EGFP) is expressed in polydendrocytes and their progeny. The phenotype of the EGFP⁺ cells was analyzed using immunohistochemistry and the patch-clamp technique 3, 7 and 14 days after MCAo. In sham-operated mice (control), EGFP⁺ cells in the cortex expressed protein markers and displayed electrophysiological properties of polydendrocytes and oligodendrocytes. We did not detect any co-labeling of EGFP with neuronal, microglial or astroglial markers in this region, thus proving polydendrocyte unipotent differentiation potential under physiological conditions. Three days after MCAo the number of EGFP⁺ cells in the gliotic tissue dramatically increased when compared to control animals, and these cells displayed properties of proliferating cells. However, in later phases after MCAo a large subpopulation of EGFP⁺ cells expressed protein markers and electrophysiological properties of astrocytes that contribute to the formation of glial scar. Importantly, some EGFP⁺ cells displayed membrane properties typical for neural precursor cells, and moreover these cells expressed doublecortin (DCX) – a marker of newly-derived neuronal cells. Taken together, our data indicate that polydendrocytes in the dorsal cortex display multipotent differentiation potential after focal ischemia.

Wnt signaling in the pathogenesis of multiple sclerosis-associated chronic pain

Many multiple sclerosis (MS) patients develop chronic pain, but the underlying pathological mechanism is unknown. Mice with experimental autoimmune encephalomyelitis (EAE) have been widely used to model MS-related neurological complications, including CNS demyelination, neuroinflammation and motor impairments. Similar to MS patients, EAE mice also develop chronic pain. We are interested in elucidating the potential involvement of Wnt signaling in the pathogenesis of chronic pain in EAE mice. In this study, we characterized the expression of Wnt signaling proteins in the spinal cord dorsal horn (SCDH) of EAE mice, by immunoblotting and immunostaining. The EAE model was created by immunization of adult mice (C57BL/6, 10 weeks) with myelin oligodendrocyte glycoprotein (MOG) 35-55. Robust mechanical hyperalgesia and allodynia were developed in both fore- and hindpaws of the EAE mice. Wnt3a, a prototypical Wnt ligand for the canonical pathway, was significantly increased in the SCDH of the EAE mice. Another key protein in the canonical pathway, ß-catenin, was also significantly up-regulated. In addition, Wnt5a, a prototypic Wnt ligand for the non-canonical pathway, and its receptor (co-receptor) Ror2 were also up-regulated in the SCDH of the EAE mice. We further found that Wnt5a antagonist Box5 and β-catenin inhibitor indomethacin attenuated mechanical allodynia in the EAE mice. Our data collectively suggest that Wnt signaling pathways are up-regulated in the SCDH of the EAE mice and that aberrant activation of Wnt signaling contributes to the development of EAE-related chronic pain.

Role of the cellular prion protein in oligodendrocyte precursor cell proliferation and differentiation in the developing and adult mouse CNS

There are numerous studies describing the signaling mechanisms that mediate oligodendrocyte precursor cell (OPC) proliferation and differentiation, although the contribution of the cellular prion protein (PrP^c) to this process remains unclear. PrP^c is a glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein involved in diverse cellular processes during the development and maturation of the mammalian central nervous system (CNS). Here we describe how PrP^c influences oligodendrocyte proliferation in the developing and adult CNS. OPCs that lack PrP^c proliferate more vigorously at the expense of a delay in differentiation, which correlates with changes in the expression of oligodendrocyte lineage markers. In addition, numerous NG2-positive cells were observed in cortical regions of adult PrP^c knockout mice, although no significant changes in myelination can be seen, probably due to the death of surplus cells.

ApoTransferrin: dual role on adult subventricular zone-derived neurospheres

Neural stem and progenitor cells (NSC/NPCs) are multipotent self-renewing cells that are able to generate neurons, astrocytes and oligodendrocytes (OLs) within the adult central nervous system. We cultured NSC/NPCs from the rat subventricular zone as neurospheres (NS) and studied apoTransferrin (aTf) effects on oligodendroglial specification and maturation. Our findings suggest that aTf acts at different stages during progression from NSC to mature oligodendrocytes. On the one hand, an early event associated with the activation of NSC/NPCs proliferation and commitment toward the oligodendroglial fate, as indicated by increased BrdU incorporation, larger neurospheres production, and higher ability to generate OL precursors (OPCs) from undifferentiated cultures. On the other hand, aTf exposure during differentiating conditions favours OL maturation from OPCs by promoting OL morphological development. This evidence supports a key role of Tf on the generation of OL from NSC/NPCs and highlights its potential in demyelinating disorder treatment.

NG2 expression in rats with acute T10 spinal cord injury

Rat models of T₁₀ spinal cord injury were established with a clamp method. NG2 expression was detected with immunohistochemical staining and western blot. Ten days after spinal cord injury, the number of NG2-positive cells in the damaged areas and NG2 absorbance were both significantly increased. The findings indicate that acute T₁₀ spinal cord injury in rats can lead to upregulation of NG2 protein expression in damaged areas.

Adult neurogenesis in mammals--a theme with many variations

Investigations of adult neurogenesis in recent years have revealed numerous differences among mammalian species, reflecting the remarkable diversity in brain anatomy and function of mammals. As a mechanism of brain plasticity, adult neurogenesis might also differ due to behavioural specialization or adaptation to specific ecological niches. Because most research has focused on rodents and only limited data are available on other mammalian orders, it is hotly debated whether, in some species, adult neurogenesis also takes place outside of the well-characterized subventricular zone of the lateral ventricle and subgranular zone of the dentate gyrus. In particular, evidence for the functional integration of new neurons born in 'non-neurogenic' zones is controversial. Considering the promise of adult neurogenesis for regenerative medicine, we posit that differences in the extent, regional occurrence and completion of adult neurogenesis need to be considered from a species-specific perspective. In this review, we provide examples underscoring that the mechanisms of adult neurogenesis cannot simply be generalized to all mammalian species. Despite numerous similarities, there are distinct differences, notably in neuronal maturation, survival and functional integration in existing synaptic circuits, as well as in the nature and localization of neural precursor cells. We also propose a more appropriate use of terminology to better describe these differences and their relevance for brain plasticity under physiological and pathophysiological conditions. In conclusion, we emphasize the need for further analysis of adult neurogenesis in diverse mammalian species to fully grasp the spectrum of variation of this adaptative mechanism in the adult CNS.

NMDA receptor signaling in oligodendrocyte progenitors is not required for oligodendrogenesis and myelination

Oligodendrocyte precursor cells (OPCs) express NMDA receptors (NMDARs) and form synapses with glutamatergic neurons throughout the CNS. Although glutamate influences the proliferation and maturation of these progenitors in vitro, the role of NMDAR signaling in oligodendrogenesis and myelination in vivo is not known. Here, we investigated the consequences of genetically deleting the obligatory NMDAR subunit NR1 from OPCs and their oligodendrocyte progeny in the CNS of developing and mature mice. NMDAR-deficient OPCs proliferated normally, achieved appropriate densities in gray and white matter, and differentiated to form major white matter tracts without delay. OPCs also retained their characteristic physiological and morphological properties in the absence of NMDAR signaling and were able to form synapses with glutamatergic axons. However, expression of calcium-permeable AMPA receptors (AMPARs) was enhanced in NMDAR-deficient OPCs. These results suggest that NMDAR signaling is not used to control OPC development but to regulate AMPAR-dependent signaling with surrounding axons, pointing to additional functions for these ubiquitous glial cells.

Macroglial plasticity and the origins of reactive astroglia in experimental autoimmune encephalomyelitis

Accumulations of hypertrophic, intensely glial fibrillary acidic protein-positive (GFAP(+)) astroglia, which also express immunoreactive nestin and vimentin, are prominent features of multiple sclerosis lesions. The issues of the cellular origin of hypertrophic GFAP(+)/vimentin(+)/nestin(+) "reactive" astroglia and also the plasticities and lineage relationships among three macroglial progenitor populations-oligodendrocyte progenitor cells (OPCs), astrocytes and ependymal cells-during multiple sclerosis and other CNS diseases remain controversial. We used genetic fate-mappings with a battery of inducible Cre drivers (Olig2-Cre-ER(T2), GFAP-Cre-ER(T2), FoxJ1-Cre-ER(T2) and Nestin-Cre-ER(T2)) to explore these issues in adult mice with myelin oligodendrocyte glycoprotein peptide-induced experimental autoimmune encephalomyelitis (EAE). The proliferative rate of spinal cord OPCs rose fivefold above control levels during EAE, and numbers of oligodendroglia increased as well, but astrogenesis from OPCs was rare. Spinal cord ependymal cells, previously reported to be multipotent, did not augment their low proliferative rate, nor give rise to astroglia or OPCs. Instead, the hypertrophic, vimentin(+)/nestin(+), reactive astroglia that accumulated in spinal cord in this multiple sclerosis model were derived by proliferation and phenotypic transformation of fibrous astroglia in white matter, and solely by phenotypic transformation of protoplasmic astroglia in gray matter. This comprehensive analysis of macroglial plasticity in EAE helps to clarify the origins of astrogliosis in CNS inflammatory demyelinative disorders.

Myelin regeneration in multiple sclerosis: targeting endogenous stem cells

Regeneration of myelin sheaths (remyelination) after central nervous system demyelination is important to restore saltatory conduction and to prevent axonal loss. In multiple sclerosis, the insufficiency of remyelination leads to the irreversible degeneration of axons and correlated clinical decline. Therefore, a regenerative strategy to encourage remyelination may protect axons and improve symptoms in multiple sclerosis. We highlight recent studies on factors that influence endogenous remyelination and potential promising pharmacological targets that may be considered for enhancing central nervous system remyelination.

Cell therapy for multiple sclerosis

The spontaneous recovery observed in the early stages of multiple sclerosis (MS) is substituted with a later progressive course and failure of endogenous processes of repair and remyelination. Although this is the basic rationale for cell therapy, it is not clear yet to what degree the MS brain is amenable for repair and whether cell therapy has an advantage in comparison to other strategies to enhance endogenous remyelination. Central to the promise of stem cell therapy is the therapeutic plasticity, by which neural precursors can replace damaged oligodendrocytes and myelin, and also effectively attenuate the autoimmune process in a local, nonsystemic manner to protect brain cells from further injury, as well as facilitate the intrinsic capacity of the brain for recovery. These fundamental immunomodulatory and neurotrophic properties are shared by stem cells of different sources. By using different routes of delivery, cells may target both affected white matter tracts and the perivascular niche where the trafficking of immune cells occur. It is unclear yet whether the therapeutic properties of transplanted cells are maintained with the duration of time. The application of neural stem cell therapy (derived from fetal brain or from human embryonic stem cells) will be realized once their purification, mass generation, and safety are guaranteed. However, previous clinical experience with bone marrow stromal (mesenchymal) stem cells and the relative easy expansion of autologous cells have opened the way to their experimental application in MS. An initial clinical trial has established the probable safety of their intravenous and intrathecal delivery. Short-term follow-up observed immunomodulatory effects and clinical benefit justifying further clinical trials.

Cerebral cortex demyelination and oligodendrocyte precursor response to experimental autoimmune encephalomyelitis

Experimentally induced autoimmune encephalomyelitis (EAE) in mice provides an animal model that shares many features with human demyelinating diseases such as multiple sclerosis (MS). To what extent the cerebral cortex is affected by the process of demyelination and how the corollary response of the oligodendrocyte lineage is explicated are still not completely known aspects of EAE. By performing a detailed in situ analysis of expression of myelin and oligodendrocyte markers we have identified areas of subpial demyelination in the cerebral cortex of animals with conventionally induced EAE conditions. On EAE-affected cerebral cortices, the distribution and relative abundance of cells of the oligodendrocyte lineage were assessed and compared with control mouse brains. The analysis demonstrated that A2B5(+) glial restricted progenitors (GRPs) and NG2(+)/PDGFR-α(+) oligodendrocyte precursor cells (OPCs) were increased in number during "early" disease, 20 days post MOG immunization, whereas in the "late" disease, 39 days post-immunization, they were strongly diminished, and there was an accompanying reduction in NG2(+)/O4(+) pre-oligodendrocytes and GST-π mature oligodendrocytes. These results, together with the observed steady-state amount of NG2(-)/O4(+) pre-myelinating oligodendrocytes, suggested that oligodendroglial precursors attempted to compensate for the progressive loss of myelin, although these cells appeared to fail to complete the last step of their differentiation program. Our findings confirm that this chronic model of EAE reproduces the features of neocortex pathology in progressive MS and suggest that, despite the proliferative response of the oligodendroglial precursors, the failure to accomplish final differentiation may be a key contributing factor to the impaired remyelination that characterizes these demyelinating conditions.

NG2-glia as multipotent neural stem cells: fact or fantasy?

Cycling glial precursors-"NG2-glia"-are abundant in the developing and mature central nervous system (CNS). During development, they generate oligodendrocytes. In culture, they can revert to a multipotent state, suggesting that they might have latent stem cell potential that could be harnessed to treat neurodegenerative disease. This hope has been subdued recently by a series of fate-mapping studies that cast NG2-glia as dedicated oligodendrocyte precursors in the healthy adult CNS-though rare, neuron production in the piriform cortex remains a possibility. Following CNS damage, the repertoire of NG2-glia expands to include Schwann cells and possibly astrocytes-but so far not neurons. This reaffirms the central role of NG2-glia in myelin repair. The realization that oligodendrocyte generation continues throughout normal adulthood has seeded the idea that myelin genesis might also be involved in neural plasticity. We review these developments, highlighting areas of current interest, contention, and speculation.

Synapses between NG2 glia and neurons

NG2-expressing glia are precursors to oligodendrocytes and subpopulations of astrocytes. They are unique among glial cells in that they enter into synaptic specialisations with neurons throughout all areas of grey and white matter and at all ages. To date, the NG2 cells appear to represent a postsynaptic compartment, and synapses are formed with axons. With differentiation to oligodendrocytes, NG2 is downregulated and myelin antigens upregulated: this coincides with a loss of the synaptic contacts between neurons and NG2 glial cells. The functional roles of this glial-neuron synapse in regulation of differentiation into myelinating oligodendrocytes or additionally responding to and modulating neuronal network activity remain to be elucidated.