Putative neural stem cells expressing both CNP-GFP (green) and NG2 (red) are present in the adult CNS.

The brain has come alive with the sightings of stem cells, but the molecular handles on those cells are few. On page 169, Belachew et al. identify and characterize a new population of neural stem cells (NSCs) that can generate neurons in the postnatal mouse brain.

In previous work, the researchers identified cells that express CNP in transgenic mice using a CNP-GFP marker. CNP is made in immature oligodendrocytes, so it is not surprising that the GFP-positive cells also express the proteoglycan NG2, which is also found in oligodendrocytes. But NG2+ cells are also present in parts of the CNS that contain NSCs, and are the major cell type that continues to divide in the adult CNS. The significance of these facts has remained unclear, as the function of NG2 in cell types other than oligodendrocytes had never been investigated.

Now, Belachew et al. have cultured the CNP-GFP+/NG2+ cells, and found that the cells quickly turn into a variety of neural cell types, including astrocytes, oligodendrocytes, and electrically active neurons. When examined in their natural context, CNP-GFP+/NG2+ cells are highly proliferative and give rise to a variety of cell types in different parts of the brain. In the hippocampus, one place where neurons are born through adulthood, the CNP-GFP+/NG2+ cells turn into neurons. In contrast to previously identified GFAP+ NSCs, which are thought to give rise to excitatory neurons, CNP-GFP+/NG2+ cells give rise to inhibitory neurons. So there may be several types of stem cells in the brain, generating distinct subsets of adult-born neurons.

A large hindrance to NSC research has been the lack of markers for stem cells. Use of the CNP and NG2 markers will allow researchers to explore the full repertoire of these stem cells and to test ways to manipulate their behavior. If the cells can be coaxed into migrating to the right places and stimulated to proliferate and differentiate, they might eventually be used to repair a damaged CNS. ▪