Endogenous progenitors remyelinate demyelinated axons in the adult CNS.

Remyelination occurs in demyelinated CNS regions in diseases such as multiple sclerosis. Identification of the cell type(s) responsible for this remyelination, however, has been elusive. Here, we examine one potential source of remyelinating oligodendrocytes-immature, cycling cells endogenous to adult white matter-and demonstrate that this population responds to demyelination by differentiating into myelinating oligodendrocytes. Dividing cells in subcortical white matter of adult rats were labeled by stereotactic injection of a replication-deficient lacZ-encoding retrovirus (BAG). Following a focal demyelination induced with lysolecithin, many of the BAG-labeled cells differentiated into myelinating oligodendrocytes engaging in repair of the lesion. Identification of endogenous cells capable of remyelination provides a target for the study of CNS repair processes in demyelinating diseases.

Heterogeneity of cycling glial progenitors in the adult mammalian cortex and white matter.

The heterogeneity and differentiation potential of mitotically active cells in the adult brain were studied by labeling adult rats with BrdU, and isolating an enriched population of cycling cells from neocortex and from subcortical white matter. The majority of this population isolated from either brain region labeled with O4, an early oligodendrocyte marker. In tissue culture, these O4(+) progenitors acquired galactocerebroside, a glycolipid of mature oligodendrocytes, but not GFAP, an intermediate filament of astrocytes. A minority population expressed the intermediate filament protein, vimentin, but not O4. This population expressed GFAP after several days in culture. A third population of cycling cells, expressing the gangliosides labeled with the A2B5 antibody, represented a minority population in subcortical white matter, but one of the major cycling populations in cortex, with substantial overlap with O4. Small populations of cycling NG2(+) cells also were observed. Thus, the cycling cells in the adult brain are heterogeneous, and the majority appear to belong to glial lineages.

In vivo characterization of endogenous proliferating cells in adult rat subcortical white matter.

Proliferating cells in adult rat subcortical white matter were characterized in vivo using stereotactic injections of a replication-deficient retrovirus containing the construct for beta galactosidase (BAG); BAG was deposited into the cingulum at the level of the septal nuclei. Morphological profiles, generated using Xgal substrate to visualize labeled cells, revealed a population of simple, immature cells. The antigenic profile, generated immunohistochemically with cell-specific markers 2 or 30 days post injection (dpi), showed a population of cells that primarily expressed nestin or an oligodendrocyte-specific glutathione-S-transferase isoform, Yp (GST-Yp) at 2 dpi and nestin, GST-Yp or Rip at 30 dpi. Occasionally, labeled cells differentiated in vivo into myelinating oligodendrocytes 30 dpi. Labeled cells did not express the astrocyte markers GFAP, GST-Yb, or S100 beta at 2 or 30 dpi. Comparisons of cell distribution 2 and 30 dpi indicated the non-migratory nature of these cells. Cell distribution patterns and nearest neighbor analyses confirmed the emergence of clusters of labeled cells 30 dpi, which bromodeoxyuridine (BrdU) incorporation studies suggested arose from continued proliferation of some labeled cells. In vivo characterization of proliferating cells in the adult revealed a non-migratory, primarily undifferentiated population of cells.

Fate determination and migration of progenitors in the postnatal mammalian CNS.

Using replication-deficient retrovirus to transfer marker genes into immature cells, we have characterized spatial and temporal patterns of glial progenitor migration and differentiation in the early postnatal rat forebrain and cerebellum, and interneuron differentiation in the cerebellum. Progenitors do not migrate randomly, but follow discrete paths, largely confined to a coronal plane in forebrain and a sagittal plane in cerebellum. Radial glia provide one substrate for migration. In vitro studies suggest that radial glia contribute a permissive pathway along which progenitors in an immature, migratory state. Local environmental cues that progenitors encounter during migration may influence fate decisions substantially. Not all progenitors differentiate; some remain in an immature, proliferative state in which they do not complete differentiation, but can be induced to do so by pathological conditions.