CNS myelin sheath is stochastically built by homotypic fusion of myelin membranes within the bounds of an oligodendrocyte process.

Myelin - the multilayer membrane that envelops axons - is a facilitator of rapid nerve conduction. Oligodendrocytes form CNS myelin; the prevailing hypothesis being that they do it by extending a process that circumnavigates the axon. It is pertinent to ask how myelin is built because oligodendrocyte plasma membrane and myelin are compositionally different. To this end, we examined oligodendrocyte cultures and embryonic avian optic nerves by electron microscopy, immuno-electron microscopy and three-dimensional electron tomography. The results support three novel concepts. Myelin membranes are synthesized as tubules and packaged into "myelinophore organelles" in the oligodendrocyte perikaryon. Myelin membranes are matured in and transported by myelinophore organelles within an oligodendrocyte process. The myelin sheath is generated by myelin membrane fusion inside an oligodendrocyte process. These findings abrogate the dogma of myelin resulting from a wrapping motion of an oligodendrocyte process and open up new avenues in the quest for understanding myelination in health and disease.

The genetic signature of perineuronal oligodendrocytes reveals their unique phenotype.

Oligodendrocytes--best known for assembling central nervous system myelin--can be categorized as precursors, myelin-forming cells and non-myelinating perineuronal cells. Perineuronal oligodendrocytes have been well characterized morphologically and ultrastructurally, but knowledge about their function remains scanty. It has been proposed that perineuronal oligodendrocytes support neurons and, following injury, transform into myelin-synthesizing cells. Recent findings implicating perineuronal oligodendrocytes in cytoarchitectural abnormalities in the prefrontal cortex of schizophrenia and other psychiatric disorders shed new light on these cells. We have obtained the genetic signature of perineuronal oligodendrocytes by identifying gene expression differences between oligodendrocyte subpopulations using cell-specific tags, microarray technology, quantitative time-resolved polymerase chain reaction and bioinformatics tools. We show that perineuronal cells are the progeny of oligodendrocyte progenitors and, hence, are members of the oligodendrocyte lineage. Physiologically they exhibit a novel phenotype. Their expression of PDGFR-αß and its growth factor ligand PDGF-CC sets them apart from members of their lineage as this receptor precludes their response to the same growth factors that act on myelinating cells. Their coordinate expression and context-specific usage of transcription factors Olig2, Ascl1 and Pax6, together with the prominent presence of transcription factors Pea3, Lhx2 and Otx2--not hitherto linked to the oligodendrocyte lineage--suggested a cell with features that blur the boundary between a neuron and a glial cell. But they also maintain a reservoir of untranslated transcripts encoding major myelin proteins presumably for a demyelinating episode. This first molecular characterization of perineuronal oligodendrocytes revealed the striking difference between the myelinating and non-myelinating phenotypes.

NOVOcan: a molecular link among selected glial cells.

The nervous system is generated from cells lining the ventricular system. Our understanding of the fate potentials and lineage relationships of these cells is being re-evaluated, both because of recent demonstrations that radial glia can generate neurons and because of the identification of fate-determining genes. A variety of intrinsic and extrinsic molecules, including proteoglycans, regulate embryonic and postnatal brain development. Using probes modeled after species conserved domains of heparan sulfate proteoglycans, we cloned a novel gene called novocan, raised monoclonal antibodies against a segment of the predicted amino acid sequence of the expressed protein (NOVOcan) and used the antibodies to establish the cell and tissue localization of NOVOcan in postnatal rat brains by immunohistochemistry. NOVOcan was expressed in cells lining the ventricles, including a variety of radial glia during early postnatal development. Later, as radial glia disappeared and ependymal cells appeared, NOVOcan was detected in ependymal cells and in tanycytes, a specialized form of ependymal cell resembling radial glia. NOVOcan was absent in two known progeny of radial glia, mature astrocytes and neurons. Whereas NOVOcan was also absent in mature oligodendrocytes (OLGs), it was present in OLG precursors in developing white matter. These studies set the stage for determining the roles of NOVOcan in brain cell lineage patterns as well as in other aspects of development.

Oligodendrocytes express an alpha/beta-interferon-susceptible Mx gene: molecular characterization of the encoded protein.

Interferons (INFs) are cytokines that exert a plethora of biological activities by interacting with receptors and inducing the synthesis of effector proteins. One such effector is an antiviral protein called Mx, widely distributed among vertebrates. Mx proteins belong to the superfamily of large GTPases, which includes dynamins, products of the Drosophila shibire gene, the yeast Vps1p, and others. Oligodendrocytes (OLGs) synthesize, assemble, and maintain myelin in the central nervous system. In an effort to identify genes implicated in establishing a myelinogenic phenotype, we isolated a 2,494 bp cDNA that encodes a protein of 654 amino acids, which are 95% identical to ovine Mx (from endometrium of pregnant sheep) and 83% similar to human MxA. Cultured OLGs express the mRNA and protein. We present structural features of this protein and speculate on the origin of its induction and the functional significance of its presence in OLGs. The finding that OLGs express an IFN-susceptible protein raises interesting possibilities in light of the evidence that beta-IFN is therapeutically beneficial for the treatment of multiple sclerosis.