Shaping of Regional Differences in Oligodendrocyte Dynamics by Regional Heterogeneity of the Pericellular Microenvironment.

Oligodendrocyte precursor cells (OPCs) are glial cells that differentiate into mature oligodendrocytes (OLs) to generate new myelin sheaths. While OPCs are distributed uniformly throughout the gray and white matter in the developing and adult brain, those in white matter proliferate and differentiate into oligodendrocytes at a greater rate than those in gray matter. There is currently lack of evidence to suggest that OPCs comprise genetically and transcriptionally distinct subtypes. Rather, the emerging view is that they exist in different cell and functional states, depending on their location and age. Contrary to the normal brain, demyelinated lesions in the gray matter of multiple sclerosis brains contain more OPCs and OLs and are remyelinated more robustly than those in white matter. The differences in the dynamic behavior of OL lineage cells are likely to be influenced by their microenvironment. There are regional differences in astrocytes, microglia, the vasculature, and the composition of the extracellular matrix (ECM). We will discuss how the regional differences in these elements surrounding OPCs might shape their phenotypic variability in normal and demyelinated states.

The Impact of Fixation on the Detection of Oligodendrocyte Precursor Cell Morphology and Vascular Associations.

Oligodendrocyte precursor cells (OPCs) display numerous protrusions that extend into the surrounding parenchyma in the brain. Depending on the preparation of the tissue analyzed, these protrusions are more or less visible. We applied six different fixation methods and compared the effect of prolonged and stronger fixation on fluorescence intensity of platelet-derived growth factor receptor alpha, a surface marker of OPCs. Importantly, the fluorescence signal is mostly lost on protrusions as compared to the cell body, which has to be considered for specific analyses. Additionally, we show numerous contacts established between OPCs and the brain vasculature, which will contribute to the understanding of the interactions between these two elements.

Microglial neuropilin-1 promotes oligodendrocyte expansion during development and remyelination by trans-activating platelet-derived growth factor receptor.

Nerve-glia (NG2) glia or oligodendrocyte precursor cells (OPCs) are distributed throughout the gray and white matter and generate myelinating cells. OPCs in white matter proliferate more than those in gray matter in response to platelet-derived growth factor AA (PDGF AA), despite similar levels of its alpha receptor (PDGFRα) on their surface. Here we show that the type 1 integral membrane protein neuropilin-1 (Nrp1) is expressed not on OPCs but on amoeboid and activated microglia in white but not gray matter in an age- and activity-dependent manner. Microglia-specific deletion of Nrp1 compromised developmental OPC proliferation in white matter as well as OPC expansion and subsequent myelin repair after acute demyelination. Exogenous Nrp1 increased PDGF AA-induced OPC proliferation and PDGFRα phosphorylation on dissociated OPCs, most prominently in the presence of suboptimum concentrations of PDGF AA. These findings uncover a mechanism of regulating oligodendrocyte lineage cell density that involves trans-activation of PDGFRα on OPCs via Nrp1 expressed by adjacent microglia.

Life-long oligodendrocyte development and plasticity.

Oligodendrocyte precursor cells (OPCs) originate in localized germinal zones in the embryonic neural tube, then migrate and proliferate to populate the entire central nervous system, both white and gray matter. They divide and generate myelinating oligodendrocytes (OLs) throughout postnatal and adult life. OPCs express NG2 and platelet-derived growth factor receptor alpha subunit (PDGFRα), two functionally important cell surface proteins, which are also widely used as markers for OPCs. The proliferation of OPCs, their terminal differentiation into OLs, survival of new OLs, and myelin synthesis are orchestrated by signals in the local microenvironment. We discuss advances in our mechanistic understanding of paracrine effects, including those mediated through PDGFRα and neuronal activity-dependent signals such as those mediated through AMPA receptors in OL survival and myelination. Finally, we review recent studies supporting the role of new OL production and "adaptive myelination" in specific behaviours and cognitive processes contributing to learning and long-term memory formation. Our article is not intended to be comprehensive but reflects the authors' past and present interests.

Novel guanidine compounds inhibit platelet-derived growth factor receptor alpha transcription and oligodendrocyte precursor cell proliferation.

Oligodendrocyte precursor cells (OPCs), also known as NG2 cells or polydendrocytes, are distributed widely throughout the developing and mature central nervous system. They remain proliferative throughout life and are an important source of myelinating cells in normal and demyelinating brain as well as a source of glioma, the most common type of primary brain tumor with a poor prognosis. OPC proliferation is dependent on signaling mediated by platelet-derived growth factor (PDGF) AA binding to its alpha receptor (PDGFRα). Here, we describe a group of structurally related compounds characterized by the presence of a basic guanidine group appended to an aromatic core that is effective in specifically repressing the transcription of Pdgfra but not the related beta receptor (Pdgfrb) in OPCs. These compounds specifically and dramatically reduced proliferation of OPCs but not that of astrocytes and did not affect signal transduction by PDGFRα. These findings suggest that the compounds could be further developed for potential use in combinatorial treatment strategies for neoplasms with dysregulated PDGFRα function.

The effects of developmental and current niches on oligodendrocyte precursor dynamics and fate.

Oligodendrocyte precursor cells (OPCs), whose primary function is to generate myelinating oligodendrocytes, are distributed widely throughout the developing and mature central nervous system. They originate from several defined subdomains in the embryonic germinal zones at different developmental stages and in the adult. While many phenotypic differences have been observed among OPCs in different anatomical regions and among those arising from different germinal zones, we know relatively little about the molecular and cellular mechanisms by which the historical and current niches shape the behavior of oligodendrocyte lineage cells. This minireview will discuss how the behavior of oligodendrocyte lineage cells is influenced by the developmental niches from which subpopulations of OPCs emerge, by the current niches surrounding OPCs in different regions, and in pathological states that cause deviations from the normal density of oligodendrocyte lineage cells and myelin.

The Effects of D-aspartate on Neurosteroids, Neurosteroid Receptors, and Inflammatory Mediators in Experimental Autoimmune Encephalomyelitis.

OBJECTIVE: Experimental autoimmune encephalomyelitis (EAE) is a widely used model for multiple sclerosis. The present study has been designed to compare the efficiencies of oral and intraperitoneal (IP) administration of D-aspartate (D-Asp) on the onset and severity of EAE, the production of neurosteroids, and the expression of neurosteroid receptors and inflammatory mediators in the brain of EAE mice. METHODS: In this study, EAE was induced in C57BL/6 mice treated with D-Asp orally (D-Asp-Oral) or by IP injection (D-Asp-IP). On the 20th day, brains (cerebrums) and cerebellums of mice were evaluated by histological analyses. The brains of mice were analyzed for: 1) Neurosteroid (Progesterone, Testosterone, 17ß-estradiol) concentrations; 2) gene expressions of cytokines and neurosteroid receptors by reverse transcription polymerase chain reaction, and 3) quantitative determination of D-Asp using liquid chromatography-tandem mass spectrometry. Further, some inflammatory cytokines and matrix metalloproteinase-2 (MMP-2) were identified in the mouse serum using enzyme-linked immunosorbent assay kits. RESULTS: Our findings demonstrated that after D-Asp was administered, it was taken up and accumulated within the brain. Further, IP injection of D-Asp had more beneficial effects on EAE severity than oral gavage. The concentration of the testosterone and 17ß-estradiol in D-Asp-IP group was significantly higher than that of the control group. There were no significant differences in the gene expression of cytokine and neurosteroid receptors between control, D-Asp-IP, and D-Asp-Oral groups. However, IP treatment with D-Asp significantly reduced C-C motif chemokine ligand 2 and MMP-2 serum levels compared to control mice. CONCLUSION: IP injection of D-Asp had more beneficial effects on EAE severity, neurosteroid induction and reduction of inflammatory mediators than oral gavage.

Organotypic Slice Cultures to Study Oligodendrocyte Proliferation, Fate, and Myelination.

Oligodendrocyte development and myelination are processes in the central nervous system that are regulated by cell intrinsic and extrinsic mechanisms. Organotypic slice cultures provide a simple method for studying factors that affect oligodendrocyte proliferation, differentiation, and myelination in the context of the local cellular environment. Here we show that major glial cell types and neurons are preserved in slice cultures from postnatal mouse forebrain, and their morphological characteristics are retained. We further demonstrate that cellular processes requiring interactions with neighboring cells such as myelination can proceed in slice culture.

Age-Dependent Decline in Fate Switch from NG2 Cells to Astrocytes After Olig2 Deletion.

NG2 cells are a resident glial progenitor cell population that is uniformly distributed throughout the developing and mature mammalian CNS. Those in the postnatal CNS generate exclusively myelinating and non-myelinating oligodendrocytes and are thus equated with oligodendrocyte precursor cells. Prenatally, NG2 cells in the ventral gray matter of the forebrain generate protoplasmic astrocytes as well as oligodendrocytes. The fate conversion from NG2 cells into protoplasmic astrocytes is dependent on downregulation of the key oligodendrocyte transcription factor Olig2. We showed previously that constitutive deletion of Olig2 in NG2 cells converts NG2 cells in the neocortex into protoplasmic astrocytes at the expense of oligodendrocytes. In this study, we show that postnatal deletion of Olig2 caused NG2 cells in the neocortex but not in other gray matter regions to become protoplasmic astrocytes. However, NG2 cells in the neocortex became more resistant to astrocyte fate switch over the first 3 postnatal weeks. Fewer NG2 cells differentiated into astrocytes and did so with longer latency after Olig2 deletion at postnatal day 18 (P18) compared with deletion at P2. The high-mobility group transcription factor Sox10 was not downregulated for at least 1 month after Olig2 deletion at P18 despite an early transient upregulation of the astrocyte transcription factor NFIA. Furthermore, inhibiting cell proliferation in slice culture reduced astrocyte differentiation from Olig2-deleted perinatal NG2 cells, suggesting that cell division might facilitate nuclear reorganization needed for astrocyte transformation.SIGNIFICANCE STATEMENT NG2 cells are glial progenitor cells that retain a certain degree of lineage plasticity. In the normal postnatal neocortex, they generate mostly oligodendrocyte lineage cells. When the oligodendrocyte transcription factor Olig2 is deleted in NG2 cells in the neocortex, they switch their fate to protoplasmic astrocytes. However, the efficiency of the fate switch decreases with age over the first 3 postnatal weeks and is reduced when cell proliferation is inhibited. As the neocortex matures, sustained expression of the oligodendrocyte lineage-specific key transcription factor Sox10 becomes less dependent on Olig2. Together, our findings suggest a gradual stabilization of the oligodendrocyte lineage genes and loss of lineage plasticity during the first 3 weeks after birth, possibly due to nuclear reorganization.