Even though active neurogenesis in adult mammals is well known and occurs in some regions of our brain, our capacity to replace neurons after an injury or lost is quite limited. In contrast, other vertebrates such as zebrafish and salamanders can regenerate their central nervous system much better. In these animals radial glia like cells (GFAP+) function as neuronal progenitors during regeneration. In the newt brain there are regions (hot spots) in which active neurogenesis is observed during homeostasis in intact animals. However, it is also possible to trigger neuronal regeneration in regions in which neurogenesis is not detected in normal conditions. A recent paper from the laboratory of András Simon (http://www.ncbi.nlm.nih.gov/pubmed/24749074) addresses the heterogeneity of the ependymoglia cells within and outside of the constitutively active niches in the newt telencephalon.
As a first step to isolate neural stem cells (NSCs) the authors tested whether brain cells from different regions were able to form neurospheres in vitro, a typical assay to determine the NSC nature. Neurospheres were indeed formed and included GFAP+ cells that proliferated. Upon media changes cells expressing differentiated neuronal markers were found in those neurospheres indicating that GFPA+ cells have stem cell properties.
In a previous study these authors showed that proliferating ependymoglia cells were mainly localized in hot spots in the newt brain. Here, they addressed the characterization of the heterogeneity of these cells. By analyzing the expression of glutamine synthetase (GS) they could distinguish two different population s of ependymoglia cells. Type 1 GFAP+ cells were positive for GS whereas type 2 GFAP+ cells did not express GS. Type 2 (GS-) cells were found in clusters in hot spots and represent about 32% of the ependymoglia cells. Most of the proliferating ependymoglia cells in hot spots (about 85%) are type 2 cells. The remaining proliferating cells in hot spots (about 15%) are type 1 (GS+). In contrast, in non-hot spots type 2 cells represent only 0,3% of the ependymoglia cells, whereas most of the proliferating cells in these regions are of type 1 (about 90% of the proliferating ependymoglia cells). Thus, type 1 and 2 are found in hot spots but type 2 is practically absent from non-hot spots.
As Notch signaling has a well-known role in the regulation of neural stem and progenitor cells the authors characterized then the expression Notch receptor in these 2 populations of ependymoglia cells in hot spots and non-hot spots. In hot spots their observations are consistent with type 1 cells being GFAP+/GS+/Notch1+ and type 2 GFAP+/GS-/Notch1-. In agreement with this, they found that 94% of proliferating cells in hot spots were Notch1-, whereas most of the proliferating ependymoglia cells in non-hot spots were Notch1+.
Next, as type 2 cells are mainly localized in hot spots the authors hypothesized that they could have a stem cell nature. However, what they found was that a majority of stem cells were in fact type 1 ependymoglia cells. By doing pulse chase experiments with BrdU to detect the long-term label retention characteristic of stem cells together with treatments with AraC (a method to selectively eliminate transit-amplifying cells that divide more frequently than slowly dividing stem cells) the authors concluded that type 1 ependymoglia cells (found in most of the ventricle wall of the telencephalon) have stem cell properties whereas type 2 cells in the hot spots would be transit-amplifying cells.
Then, the authors analyzed how these distinct cell populations responded to the ablation of cholinergic neurons in hot spots and non-hot spots. Upon ablation they found an increase in the proliferation of type 1 and type 2 cells in the hot spots. Remarkably, they also found that in non-hot spots there appeared type 2 cells as well as cells positive for PSA-NCAM, a marker of immature neurons that is not detected in homeostasis in these regions. These last results suggested that neuronal ablation gave rise to the appearance of new neurogenic regions.
Finally, the authors analyzed how interfering with Notch signaling affected the behavior of type 1 and type 2 cells in homeostasis and during regeneration. Upon treatment with the Notch inhibitor DAPT, they concluded that in homeostasis type 1 (stem cell) proliferation is not sensitive to Notch signaling whereas type 2 (transit-amplifying) proliferation is Notch sensitive. During regeneration, type 1 cells in the hot spots are still insensitive to Notch signaling; however, the increase in proliferation of stem cells in non-hot spots is dependent on Notch signaling.
In summary, the authors report here the identification of two distinct subpopulations among the ependymoglia cells in the newt telencephalon: type 1 with stem cell properties and type 2 with transit-amplifying ones. Remarkably, although in the newt telencephalon there are active hot spots, type 1 cells are also found in most of the ventricle wall, which could account for the high neuroregenerative capacity of these animals. In fact, neuronal ablation leads to the appearance of new neurogenic niches in non-hot spots, in which there is an increase in the proliferation of type 1 cells (Notch dependent) as well as the appearance of type 2 and neuronal precursors.