We conclude that ectopic activation of the Hh pathway is sufficient to respecify dorsal SVZ neuronal progeny. We demonstrate that the Hedgehog pathway has a critical function in the regional specification of stem IDH tumor cells of the adult brain. We found that Smo is expressed by stem cells throughout the SVZ. However, high Hh pathway activation is more prevalent in the ventral SVZ and is sufficient to respecify

dorsal SVZ stem cells when ectopically induced. In the forebrain, we found that Shh is the only Hh isoform expressed. Interestingly, Shh production occurs in neurons that are located very close to the ventral subventricular zone. These results indicate that specification of subpopulations of neural stem cells and their progeny in the adult brain is actively regulated by Hh signaling. It has previously been suggested that the Hh pathway functions in multiple cell types within the SVZ. Genetic and pharmacologic studies have argued that this pathway is important in stem cell self-renewal, the

generation of transit-amplifying progeny, and neuroblast migration to the OB (Machold et al., 2003, Palma et al., 2005, Balordi and Fishell, 2007a and Balordi and Fishell, 2007b). Our results indicate that astrocytes of the SVZ are the major Hh-responsive population within the SVZ: expression of Smo protein, which is required for a cell to respond to Hh ligand, is restricted to the GFAP-positive population (including type B1 cells) in this region. Intriguingly, Smo NVP-BKM120 supplier expression labels a subset of GFAP-expressing SVZ cells, suggesting either that Smo is only expressed

in a subset of stem cells, or that it is a more specific marker of stem cells than GFAP, which is also expressed by some astrocytes in non-germinal regions (Garcia et al., 2010). We searched for components of the Hh signaling pathway that might be differentially Histone demethylase expressed in the subregions of the SVZ. We did not observe significant differences in the expression of Smo, Gli2, or Gli3 between the ventral and dorsal SVZ, but found that Gli1 expression is much higher in the ventral SVZ. Lineage tracing of these cells using Gli1-CreERT2 mice indicates that progenitors responding to high levels of Shh are located in the ventral SVZ and primarily generate deep granule interneurons. Moreover, Hh signaling appears to be instructive in cell fate decisions: ablation of Shh expression reduced the production of ventrally-derived progeny, and ectopic activation of the pathway resulted in the respecification of dorsally-derived progeny to a ventral fate. These results are surprising, as previous studies using viral targeting and cell culture indicated that cell identity was largely encoded using a cell-intrinsic program: transplantation of dorsal cells to a ventral location, even after intervening time in culture, was not sufficient to respecify their progeny ( Merkle et al., 2007).