Astrocyte differentiation, which occurs late in brain development, is largely dependent on the activation of a transcription factor, STAT3. We show that astrocytes, as judged by glial fibrillary acidic protein (GFAP) expression, never emerge from neuroepithelial cells on embryonic day (E) 11.5 even when STAT3 is activated, in contrast to E14.5 neuroepithelial cells. A CpG dinucleotide within a STAT3 binding element in the GFAP promoter is highly methylated in E11.5 neuroepithelial cells, but is demethylated in cells responsive to the STAT3 activation signal to express GFAP. This CpG methylation leads to inaccessibility of STAT3 to the binding element. We suggest that methylation of a cell type-specific gene promoter is a pivotal event in regulating lineage specification in the developing brain.

We show that when telencephalic neural progenitors are briefly exposed to bone morphogenetic protein 2 (BMP2) in culture, their developmental fate is changed from neuronal cells to astrocytic cells. BMP2 significantly reduced the number of cells expressing microtubule-associated protein 2, a neuronal marker, and cells expressing nestin, a marker for undifferentiated neural precursors, but BMP2 increased the number of cells expressing S100-␤, an astrocytic marker. In telencephalic neuroepithelial cells, BMP2 up-regulated the expression of negative helix-loop-helix (HLH) factors Id1, Id3, and Hes-5 (where Hes is homologue of hairy and Enhancer of Split) that inhibited the transcriptional activity of neurogenic HLH transcription factors Mash1 and neurogenin. Ectopic expression of either Id1 or Id3 (where Id is inhibitor of differentiation) inhibited neurogenesis of neuroepithelial cells, suggesting an important role for these HLH proteins in the BMP2-mediated changes in the neurogenic fate of these cells. Because gliogenesis in the brain and spinal cord, derived from implanted neural stem cells or induced by injury, is responsible for much of the failure of neuronal regeneration, this work may lead to a therapeutic strategy to minimize this problem. P recise mechanisms by which neurogenesis and gliogenesis are regulated in the central nervous system (CNS) remain to be elucidated. Fetal telencephalic neuroepithelial cells contain neural precursors that give rise to the neuronal lineage and the glial lineage, which includes astrocytes and oligodendrocytes (1, 2). The fate of neural precursors in the developing brain is believed to be determined by intrinsic cellular programs and by external cues, including the cytokines (1, 2). Bone morphogenetic protein 2 (BMP2) is a pleiotropic cytokine (3) that is active in many tissues including the CNS (for review, see ref. 4). The action of BMP2 is mediated by heterotetrameric serine͞ threonine kinase receptors and the downstream transcription factors Smad1, -5, or -8. After these transcription factors are phosphorylated on serines, they form a complex with a common mediator, Smad4, and the complex is translocated into the nucleus to activate transcription of specific genes (5-7). Inhibitory Smad proteins, Smad6 and Smad7, repress the action of BMP2 by inhibiting the receptor-mediated phosphorylation of Smad1, -5, or -8 or by competing with Smad4 for the binding to Smad1, -5, and -8 (5-10). BMP2 can promote telencephalic neuroepithelial cells to differentiate as astrocyte (11,12). We have further demonstrated (12, 13) that BMP2 and leukemia inhibitory factor (LIF) act synergistically to induce neuroepithelial cells to become astrocytes by forming a complex of the respective downstream transcription factors, Smads and STAT3, bridged by p300. In the present study, we demonstrate that BMP2 not only promotes astrocyte differentiation in cooperation with LIF but also alters the neurogenic cell fate of telencephalic progenitors so that they develop into astrocytic cells. We found tha...