Oligodendrocytes in mouse corpus callosum are coupled via gap junction channels formed by connexin47 and connexin32

According to previously published ultrastructural studies, oligodendrocytes in white matter exhibit gap junctions with astrocytes, but not among each other, while in vitro oligodendrocytes form functional gap junctions. We have studied functional coupling among oligodendrocytes in acute slices of postnatal mouse corpus callosum. By whole‐cell patch clamp we dialyzed oligodendrocytes with biocytin, a gap junction‐permeable tracer. On average 61 cells were positive for biocytin detected by labeling with streptavidin‐Cy3. About 77% of the coupled cells stained positively for the oligodendrocyte marker protein CNPase, 9% for the astrocyte marker GFAP and 14% were negative for both CNPase and GFAP. In the latter population, the majority expressed Olig2 and some NG2, markers for oligodendrocyte precursors. Oligodendrocytes are known to express Cx47, Cx32 and Cx29, astrocytes Cx43 and Cx30. In Cx47‐deficient mice, the number of coupled cells was reduced by 80%. Deletion of Cx32 or Cx29 alone did not significantly reduce the number of coupled cells, but coupling was absent in Cx32/Cx47‐double‐deficient mice. Cx47‐ablation completely abolished coupling of oligodendrocytes to astrocytes. In Cx43‐deficient animals, oligodendrocyte‐astrocyte coupling was still present, but coupling to oligodendrocyte precursors was not observed. In Cx43/Cx30‐double deficient mice, oligodendrocyte‐to‐astrocyte coupling was almost absent. Uncoupled oligodendrocytes showed a higher input resistance. We conclude that oligodendrocytes in white matter form a functional syncytium predominantly among each other dependent on Cx47 and Cx32 expression, while astrocytic connexins expression can promote the size of this network. © 2010 Wiley‐Liss, Inc.     

Basic fibroblast growth factor down-regulates myelin basic protein gene expression and alters myelin compaction of mature oligodendrocytes in vitro

The effects of basic fibroblast growth factor (bFGF) on myelin basic protein (MBP) gene expression and myelin-like membrane formation were investigated in oligodendrocyte cultures containing mainly mature oligodendrocytes expressing MBP. These cultures were obtained by selective detachment of the cells of the oligodendrocyte lineage from 40-day-old mixed cultures derived from newborn rat brain. They were further purified by a 3-day pretreatment with cytosine arabinoside (ARA-C) in order to kill cycling cells. After withdrawal of ARA-C, daily treatment of the cells with bFGF for 3 days induced a drastic decrease in MBP mRNA level compared to control cultures treated only with ARA-C. Moreover, the percentage of oligodendrocytes labelled with anti-MBP antibodies decreased by 50%, as well as that of oligodendrocytes expressing myelin oligodendrocyte glycoprotein (MOG), whereas proteolipid protein (PLP) immunolabelled cells were less affected. At the ultrastructural level, myelin-like membranes were still abundant in the ARA-C-and bFGF-treated cultures, but they were conspicuously uncompacted compared to cultures only pretreated with ARA-C. These results bring the first evidence that bFGF is able to down-regulate myelin protein gene expression in mature oligodendrocytes and to alter myelin structure. They imply that if bFGF is secreted after a demyelinating lesion of the central nervous system (CNS), this plasticity of mature oligodendrocytes will allow final remyelination of axons to complete only after this factor has returned to low levels. © 1995 Wiley-Liss, Inc.     

Myelin sheath thickness in the CNS is regulated near the axon

Two myelin sheaths sharing a common outer tongue process are illustrated in a thin section electron micrograph from mouse optic nerve. Although they derive from the same outer tongue process, and therefore from the same oligodendrocyte, the two sheaths are of different thickness (6 and 9 turns). The example demonstrates that myelin sheath thickness is regulated independently for each axon, at a site very near the axon.     

Astrocytes—Friends or foes in multiple sclerosis?

In multiple sclerosis (MS), the presence of demyelinating plaques has concentrated researchers' minds on the role of the oligodendrocyte in its pathophysiology. Recently, with the rediscovery of early and widespread loss of axons in the disease, new emphasis has been put on the role of axons and axon-oligodendrocyte interactions in MS. Despite the fact that, in 1904, Müller claimed that MS was a disease of astrocytes, more recently, astrocytes have taken a back seat, except as the cells that form the final glial scar after all hope of demyelination is over. However, perhaps it is time for the return of the astrocyte to popularity in the pathogenesis of MS, with recent reports on the dual role of astrocytes in aiding degeneration and demyelination, by promoting inflammation, damage of oligodendrocytes and axons, and glial scarring, but also in creating a permissive environment for remyelination by their action on oligodendrocyte precursor migration, oligodendrocyte proliferation, and differentiation. We review these findings to try to provide a cogent view of astrocytes in the pathology of MS.     

GSK3β negatively regulates oligodendrocyte differentiation and myelination in vivo

Glycogen synthase kinase 3β (GSK3β) is an essential integrating molecule for multiple proliferation and differentiation signals that regulate cell fate. Here, we have examined the effects of inhibiting GSK3β on the development of oligodendrocytes (OLs) from their oligodendrocyte precursors (OP) in vivo by injection into the lateral ventricle of postnatal mice and ex vivo in organotypic cultures of isolated intact rodent optic nerve. Our results show that a range of GSK3β inhibitors (ARA-014418, lithium, indirubin, and L803-mt) increase OPs and OLs and promote myelination. Inhibition of GSK3β stimulates OP proliferation and is prosurvival and antiapoptotic. The effects of GSK3β inhibition in OPs is via the canonical Wnt signaling pathway by stimulating nuclear translocation of β-catenin. However, direct comparison of the effects of Wnt3a and GSK3β inhibition in optic nerves shows that they have opposing actions on OLs, whereby GSK3β inhibition strikingly increases OL differentiation, whereas Wnt3a inhibits OL differentiation. Notably, GSK3β inhibition overrides the negative effects of Wnt3a on OLs, indicating novel GSK3β signaling mechanisms that negatively regulate OL differentiation. We identify that two mechanisms of GSK3β inhibition are to stimulate cAMP response element binding (CREB) and decrease Notch1 signaling, which positively and negatively regulate OL differentiation and myelination, respectively. A key finding is that GSK3β inhibition has equivalent effects in the adult and stimulates the regeneration of OLs and remyelination following chemically induced demyelination. This study identifies GSK3β as a profound negative regulator of OL differentiation that contributes to inefficient regeneration of OLs and myelin repair in demyelination.     

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.