We have used bipotent postnatal cortical oligodendroglialastroglial progenitor cells to examine the role of inductive signals in astroglial lineage commitment. O-2A progenitor cells undergo progressive oligodendroglial differentiation when cultured in serum-free medium, but differentiate into astrocytes in medium supplemented with FBS. We now report that the bone morphogenetic proteins (BMPs), a major subclass of the transforming growth factor ␤ (TGF␤) superfamily, promote the selective, dosedependent differentiation of O-2As into astrocytes with concurrent suppression of oligodendroglial differentiation. This astroglialinductive action is not sanctioned by other members of the TGF␤ superfamily. Astroglial differentiation requires only very brief initial exposure to the BMPs and is accompanied by increased cellular survival and accelerated exit from cell cycle. Dual-label immunofluorescence microscopy documents that O-2A progenitor cells express a complement of BMP type I and type II receptor subunits required for signal transduction. Furthermore, expression of BMP2 in vivo reaches maximal levels during the period of gliogenesis. These results suggest that the BMPs act as potent inductive factors in postnatal glial lineage commitment that initiate a stable program of astroglial differentiation.

There is increasing evidence that the neurotrophins, particularly nerve growth factor (NGF) and neurotrophin-3 (NT-3), play a role in the regulation of glial development in the CNS. Recent studies have shown that the proliferation of optic nerve-derived O2A progenitors (OLPs) is potentiated by NT-3 in combination with platelet-derived growth factor, whereas NT-3 alone supports the survival of their differentiated progeny (Barres et al., 1994). In this study, we have examined the expression of the high-affinity neurotrophin receptors (trks) and the low-affinity nerve growth factor receptor p75 in developing oligodendrocytes (OLs). In addition, we have examined the effects of NGF and NT-3 on proliferation and survival of OLPs and OLs, respectively. TrkC, the high-affinity NT-3 receptor, and trkA, the high-affinity NGF receptor, are both expressed from the early OLP through the mature OL stage. The truncated form of trkB, lacking the tyrosine kinase domain, and the low-affinity neurotrophin receptor p75 are expressed at low levels in OLPs and are upregulated in mature OLs. NGF and NT-3 both induced the phosphorylation of mitogen-activated protein kinase (MAPK) in OLPs and in OLs. In both OLPs and OLs, NT-3 sustained the activation of MAPK more than NGF. NT-3 enhanced the proliferation of OLPs and supported the survival of OLs. By contrast, unless coadministered with FGF-2, NGF did not exhibit mitogenic effects on OLPs but did enhance the survival of differentiated OLs. Our data demonstrate the presence of functional trkA and trkC in developing OLs and indicate that both NGF and NT-3 have a broad spectrum of developmental actions on cells of the OL lineage.

Multipotent neural progenitor species present within developing and adult periventricular generative zones can give rise to all of the major cellular elements of the brain. Although lineage specification during development has been thought to be restricted to these generative zones, we have utilized quantitative immunoselection techniques to isolate an enriched population of multipotent neural progenitor cells that express polysialylated neural cell adhesion molecule (PSA-NCAM) from postnatal day 2 cerebral cortex independent of generative zones. This population of cerebral cortical progenitor cells exhibited robust proliferation in response to epidermal growth factor and subsequently gave rise to clonally derived neurons, astrocytes, and oligodendrocytes. Quantitative regional analysis further demonstrated that while the multipotent cells derived from the cerebral cortex uniformly expressed PSA-NCAM, multipotent cells derived from generative zones contained equal proportions of PSA-NCAM-positive and -negative multipotent progenitor cells. The generation of individual cellular lineages from cortical multipotent progenitors could be enhanced by specific cytokines that are expressed within the cerebral cortex. Further, while oligodendroglial progenitor cells derived from cortical multipotent progenitors exhibited responsiveness to platelet-derived growth factor (PDGF) and neurotrophin-3 (NT-3), primary cultures of cortical oligodendroglial progenitors were responsive to PDGF but not to NT-3. These observations suggest that in addition to glial progenitors that commit to a specific lineage prior to migration from generative zones, there is within the cerebral cortex a separate pool of multipotent cells that are capable of generating mature glial progeny in response to specific environmental cues. Therapeutic interventions aimed at differentiation of endogenous cerebral pools of multipotent progenitors may provide a novel strategy for amelioration of the sequelae of environmental and genetic insults to the postnatal cerebrum.