Differentiation of glial precursor cells from developing rat brain in vitro

Identification of two novel glial-restricted cell populations in the embryonic telencephalon arising from unique origins

Background

Considerably less attention has been given to understanding the cellular components of gliogenesis in the telencephalon when compared to neuronogenesis, despite the necessity of normal glial cell formation for neurological function. Early proposals of exclusive ventral oligodendrocyte precursor cell (OPC) generation have been challenged recently with studies revealing the potential of the dorsal telencephalon to also generate oligodendrocytes. The identification of OPCs generated from multiple regions of the developing telencephalon, together with the need of the embryonic telencephalon to provide precursor cells for oligodendrocytes as well as astrocytes in ventral and dorsal areas, raises questions concerning the identity of the precursor cell populations capable of generating macroglial subtypes during multiple developmental windows and in differing locations.

Results

We have identified progenitor populations in the ventral and dorsal telencephalon restricted to the generation of astrocytes and oligodendrocytes. We further demonstrate that the dorsal glial progenitor cells can be generated de novo from the dorsal telencephalon and we demonstrate their capacity for in vivo production of both myelin-forming oligodendrocytes and astrocytes upon transplantation.

Conclusion

Based on our results we offer a unifying model of telencephalic gliogenesis, with the generation of both oligodendrocytes and astrocytes from spatially separate, but functionally similar, glial restricted populations at different developmental times in the dorsal and ventral CNS.

The tripotential glial-restricted precursor (GRP) cell and glial development in the spinal cord: generation of bipotential oligodendrocyte-type-2 astrocyte progenitor cells and dorsal-ventral differences in GRP cell function

We have found that the tripotential glial-restricted precursor (GRP) cell of the embryonic rat spinal cord can give rise in vitro to bipotential cells that express defining characteristics of oligodendrocyte-type-2 astrocyte progenitor cells (O2A/OPCs). Generation of O2A/OPCs is regulated by environmental signals and is promoted by platelet-derived growth factor (PDGF), thyroid hormone (TH) and astrocyte-conditioned medium. In contrast to multiple observations indicating that oligodendrocyte precursor cells in the embryonic day 14 (E14) spinal cord are ventrally restricted, GRP cells are already present in both the dorsal and ventral spinal cord at E13.5. Ventral-derived GRP cells, however, were more likely to generate O2A/OPCs and/or oligodendrocytes than were their dorsal counterparts when exposed to TH, PDGF, or even bone morphogenetic protein-4. The simplest explanation of our results is that oligodendrocyte generation occurs as a result of generation of GRP cells from totipotent neuroepithelial stem cells, of O2A/OPCs from GRP cells and, finally, of oligodendrocytes from O2A/OPCs. In this respect, the responsiveness of GRP cells to modulators of this process may represent a central control point in the initiation of this critical developmental sequence. Our findings provide an integration between the earliest known glial precursors and the well-studied O2A/OPCs while opening up new questions concerning the intricate spatial and temporal regulation of precursor cell differentiation in the CNS.

Re-evaluation of nestin as a marker of oligodendrocyte lineage cells

Maturation of oligodendrocyte progenitors (O2A) is characterized by morphological changes and the sequential expression of specific antigens leading to the formation of myelin membrane. Monoclonal antibodies A2B5, A007, anti-vimentin, and anti-galactocerebroside, recognize oligodendroglia at different stages of development. The neuroepithelial precursor marker nestin is also expressed by the oligodendroglial lineage; we have used enriched populations of progenitors isolated from neonatal rat brain cultures to further examine the cellular distribution of this intermediate filament protein. The phenotypic distribution of nestin positive cells among the oligodendrocyte lineage showed that 65% reacted with A2B5, whereas only 5% were A007(+), and 4% galactocerebroside(+). The remaining 25% of the cells were not labeled and had small cellular bodies devoid of processes, characteristic of the pre-O2A progenitor. Further analysis of the nestin(+) population showed that the majority of the cells were also vimentin(+). Antibody-dependent complement mediated cytolysis of A2B5(+) (O2A cells) and galactocerebroside(+) (mature oligodendrocytes) cells left a population of nestin(+) cells that were induced to proliferate in the presence of growth factors and to differentiate into A2B5(+) and galactocerebroside(+) cells. Proliferating cells maintained in the presence of platelet-derived growth factor or basic fibroblast growth factor retained nestin expression along with A2B5. By contrast, in serum-free medium nestin expression decreased while postmitotic cells acquired A007 and galactocerebroside. Our results suggest that nestin expression is a marker of pre-O2A cells that is maintained in proliferating glial progenitors, but is quickly down-regulated in postmitotic oligodendrocytes (A007(+)/galacto-cerebroside(+)) along with A2B5 and vimentin. However, other glial cells including type 2 astrocytes and some amoeboid microglia also share nestin expression.

Serum and CNTF stimulate oligodendroglia and reduce fiber outgrowth from striatal cultures

Organotypic slice cultures of newborn rat striatal tissue displayed an exceptionally dense and fasciculated outgrowth of GABAergic fibers when grown in a chemically defined medium, compared to serum-containing medium. The enhanced fiber growth was not the result of an increased density of GABAergic neurons in the cultures, but coincided with a marked reduction of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase)-immunoreactive cells within and around the cultures. An inverse, causal relationship between the number of CNPase-positive cells, presumably of oligodendroglial lineage, and GABAergic fiber outgrowth was further evidenced by the observation that addition of ciliary neurotrophic factor (CNTF) to the chemically defined medium resulted in both an increase in CNPase-positive cells and a decrease in GABAergic fiber outgrowth. The observations suggest that CNTF and serum indirectly inhibit axonal growth by stimulating oligodendroglial cells.

In vitro differentiation of neural progenitor cells from prenatal rat brain: common cell surface glycoprotein on three glial cell subsets

Glial progenitor cell differentiation and cell lineage relationships during brain development are complex hierarchical processes depending on genetic programming, cell-cell interactions, and microenvironmental factors. The identification of precursor cell-specific antigens provides a tool for the study of both normal development and deviations from lineage-specific differentiation associated with malignant transformation. Monoclonal antibody (mAb) RB13-6 recognizes a 130-kDa cell surface glycoprotein (gp130RB13-6) expressed by a subset of 9OAcGD3-positive glial precursor cells scattered in the rat neuroepithelium on prenatal day (PRD) 13. During prenatal development the fraction of gp130RB13-6-positive fetal brain cells (FBC) decreased from about 18% (PRD 14) to about 1.5% (PRD 22), coinciding with increasing fractions of more mature cell types, as indicated by the elevated expression of p24RB21-15, another cell surface determinant specified by mAb RB21-15 (Kindler-Röhrborn et al.; Differentiation 30:53-60, 1985) and other neural cell type-specific markers. Accordingly, gp130RB13-6 positive precursor cells were localized in the ventricular zones throughout brain development. Concomitant with their formation and in the adult rat brain, ependymal layers lining the ventricular surface, choroid plexus, and the leptomeninges were intensely labeled by anti-gp130RB13-6 mAb. As visualized by confocal laser scanning microscopy of FBC cultures from PRD 13, gp130RB13-6 was coexpressed with the RC1 antigen by progenitor cells morphologically resembling radial glia cells. In addition, a very small subpopulation of astrocytes coexpressing gp130RB13-6 and glial fibrillary acidic protein (GFAP; < 5%) occurred 3 days after seeding. Primary FBC cultures from PRD 18 contained an increased subset of astrocytes coexpressing gp130RB13-6 and GFAP (approximately 25% of all gp130RB13-6 expressing cells), apparently generated from gp130RB13-6-positive precursors. Corresponding to in vivo conditions, ciliated ependymal cells but also microglial cells/macrophages and leptomeningeal cells showed strong expression of gp130RB13-6 in culture. We thus present a new glycoprotein on the cell surfaces of a glial progenitor cell subset for further studies of cell lineage relationships between radial glia cells, astrocytes, and ependymal cells.

Expression of glial antigens in mouse astrocytes: species differences and regulation in vitro

Expression of developmentally regulated antigens was used to characterize glial cells in cultures from embryonic mouse cerebral cortex. Over 90% of the cells had a flat morphology, and about 50% of these flat cells also expressed the ganglioside GD3. Up to 40% of all the GD3 expressing cells also expressed A2B5 antigen. Flat cells expressing either glial fibrillary acidic protein (GFAP), or GD3 or both were present at all times in vitro. These three populations of flat cells could not be further distinguished on the basis of NG2 or fibronectin expression, or with respect to their responses to the mitogens FGF-2, PDGF, or EGF. The glial cultures also contain a small number (approximately 5%) of process bearing cells with the morphological and immunocytochemical characteristics of oligodendrocyte precursors. The expression of GD3 by flat cells changed with time in culture as the fraction of flat cells expressing only GD3 declined and the fraction of cells expressing GFAP (with or without GD3) increased. The data are consistent with those flat cells expressing only GD3 being astrocyte precursors. Furthermore, between 1 and 3 weeks in vitro GD3/GFAP cells lose GD3 while retaining GFAP. Cells expressing only GFAP could be induced to express GD3 and A2B5 by treatment with FGF-2. The widespread and regulated expression of GD3 and A2B5 by murine glia is different from the restricted pattern of expression previously reported for these antigens in rat brain cell cultures. These results demonstrate that expression of GD3 and A2B5 by murine astrocytes depends on both culture age and extracellular signals and that these gangliosides are not markers for cell lineage in the mouse.

Ventriculomegaly, delayed myelination, white matter hypoplasia, and "periventricular" leukomalacia: how are they related?

Preterm infants, including some who have sustained intracranial hemorrhage, appear to be at increased risk of lateral ventricular enlargement. Although some occurrences might be due to an impairment of cerebrospinal fluid flow or absorption, many instances of ventriculomegaly without accompanying macrocephaly reflect diffuse white matter damage resulting in diminished (i.e., hypoplastic) white matter or an inadequate density of axons. Perinatally acquired widespread white matter damage is sometimes associated with the focal white matter necrosis. We hypothesize that in some infants both ventriculomegaly and delayed myelination are consequences of disturbances to myelinogenesis that result from an impairment of cells destined to become oligodendroglia or of disturbances to rapidly growing axons. The vulnerability of developing white matter in preterm newborns might, in part, reflect the diminished availability of growth/ survival factors, or a vulnerability to toxins or physiologic perturbations. Awareness that some ventriculomegaly reflects widely distributed white matter damage should prevent overtreatment of what might appear to be hydrocephalus, but is not due to impaired cerebrospinal fluid dynamics. Increased understanding of the phenomena leading to ventriculomegaly related to paucity of white matter should lead to successful efforts to prevent white matter damage in preterm newborns.

Do oligodendrocytes divide?

Remyelination occurs in the adult central nervous system following a wide variety of experimental and naturally occurring demyelinating conditions, including multiple sclerosis. Remyelination is preceded by the appearance of new oligodendrocytes. These new cells may be generated from glial precursor cells, or from pre-existing differentiated oligodendrocytes that re-enter the cell cycle, which may first dedifferentiate, or both processes may occur. The evidence for the source of new oligodendrocytes following toxic or immune-mediated lesions is reviewed. Good evidence exists that fully differentiated oligodendrocytes can incorporate [3H]thymidine but this may be a rare event. Most of the evidence points towards glial precursor cells as the source of new oligodendrocytes in the adult, but definitive experiments have not yet been done. Research strategies, using our current knowledge and techniques, are outlined for solving this problem.