Anna Ivanova

Evidence for a second wave of oligodendrogenesis in the postnatal cerebral cortex of the mouse.

The existing view is that cortical oligodendrocytes (OLs) in rodents are born from the cortical subventricular zone (SVZ) after birth, but recent data suggest that many forebrain oligodendrocyte progenitor cells (OPCs) are specified much earlier (between E9.5 and E13.5 in the mouse) in the ventricular zone of the ventral forebrain under the control of sonic hedgehog (Shh) and migrate into the cortex afterward. We examined expression of specific early OL markers (PDGFRα, PLP/DM20, Olig2, and NG2) in the developing forebrain to clarify this issue. We propose that OPCs colonize the developing cortex in two temporally distinct waves. The gray matter is at least partially populated by a first wave of OPCs that arises in the medial ganglionic eminence and the entopeduncular area and spreads into the cortex via the developing cortical plate. The cerebral cortex benefits from the second wave of OPCs coming from residential SVZ. In the second wave, there might be two different types of precursor cells: PLP/DM20+ cells populating only inner layers and PDGFRα+ cells, which might eventually myelinate the outer regions as well.

Mutant PLP/DM20 cannot be processed to secrete PLP-related oligodendrocyte differentiation/survival factor

Most of the mutations within the PLP gene result in degeneration of oligodendrocytes and this is believed to be caused by intracellular trafficking defects. Previous studies have demonstrated that cells expressing the wild type PLP gene release a factor promoting differentiation/survival of oligodendrocyte and that this factor is the C-terminal portion of the protein itself. In this study we asked how the naturally occurring mutations of the PLP gene (jimpy, jimpy msd, and rumpshaker) affect this activity. We developed a transient expression system for retroviral production and transduction that enabled the expression of mutant PLP/DM20 cDNAs in NIH3T3 cells. None of the NIH3T3 cells producing mutant PLP/DM20s secreted the PLP-related factor that increases the number of oligodendrocytes. Since it has been shown that rumpshaker DM20 can be transported to the cell surface, but its folding is incorrect, absence of secretion of this factor is more heavily attributable to incorrect protein folding than to the defect in the PLP/DM20 trafficking.

In vitro analysis of early development of mouse oligodendrocyte

Development of oligodendrocyte (OL) was immunohistochemically studied in the embryonic chick retina with monoclonal antibodies 04, 01 and anti-myelin basic protein (MBP) antibody. The 04-positive (04+) cells in the retina were first observed at El0 in the optic nerve layer, close to the optic fissure. Most of them were unipolar or asymmetrical bipolar in shape with a growth cone, arranged along the ganglion cell axons, while a small number of the labeled cells extended short radial processes interiorly. In the El2 retina, the number of radially-oriented 04+ cells increased in the area close to the optic fissure. The labeled cells were also detected in the middle part of the retina, which still showed an unipolar shape. At E14, 04+ cells were positioned in or interior to the ganglion cell layer and extended fine processes into the optic nerve layer. A limited number of 04+ cells were also Ol+. 04+/01+ cells increased in number as embryo developed. MBP+ cell were observed El6 onward. Four days after an intraventricular injection of DiI at E6, DiI+/O4+ cells were detected in the ganglion cell layer in the El0 retina. The present results demonstrate that retinal OL in the chick is probably immigrants from the optic nerve and that they may share a same cell lineage to OL in the optic nerve.

S5-2 Development and differentiation of normal and mutant oligodemdrocytes

During embryonic stages, the glutamate transporter GLAST is expressed in the radial glia, whose cell body is localized in the ventricular zone and its radial process extends toward the pial surface of the brain and spinal cord. At embryonic day 15, the GLASTpositive radial glia begins to migrate from ventricular zone. Concomitant with the migration, localization of the GlAST shifts from the radial processes to astrocytic cell membrane. In postnatal development, the GLAST remarkably increases the level of expression in the cerebellar Bergmann astroglia which associates with migrating granule cells and with synapses of Purkinje cells, whereas it is substantially down-regulated in most other astrocytes. Therefore, the GLAST will be a useful molecular marker to investigate the morphodifferentiation from radial glia to astrocyte and also the intimate neuro-glial relationship in the cerebellum.