The Role of Atypical E2fs in the Maintenance and Development of the Ependymal Cell Barrier

The discovery of neural stem cells within the adult CNS has indicated an enormous potential in facilitating neuronal regeneration after injury. Studies from our laboratory have suggested that manipulation of the Rb/E2f pathway can directly impact embryonic and adult neurogenesis, thereby having tremendous potential for neuronal regeneration therapies. Recently, two new members of the Rb/E2f pathway have been discovered, the atypical E2fs: E2f7 and E2f8. Initial studies have suggested that atypical E2fs regulate diverse processes such as cell proliferation, DNA-stress response, apoptosis, however, their importance in the brain are unknown. To analyze their function during brain development, we crossed Nestin-Cre mice with mice bearing a conditional E2f7/E2f8 allele to delete both E2f7 and E2f8 in neural precursor cells. Whereas cortical development was largely unaffected by E2f7/E2f8 deficiency, we observed an enlargement of the lateral ventricles occurring postnatally, a brain condition named ventriculomegaly. We then looked at the ependymal cells, which are the cells lining the wall of the lateral ventricles, to determine if these cells were affected by the absence of atypical E2fs. We found progressive denaturation of the ependymal cell layer, distortion of the ependymal motile cilia and reactive astrocytes within the layer. We identified Gli3, a component of the Sonic hedgehog pathway (Shh), as a target for E2fs, including atypical E2fs. We unravelled a novel mechanism by which atypical E2fs regulate the expression of Gli3, leading to up-regulation of Numb/NumbL, which in consequence destabilizes cadherins organization within the ependymal cell layer. In conclusion, our work suggests that E2f7 and E2f8 transcription factors play an essential role in maintaining the ependymal cell barrier