[P1.51]: Rb/E2F regulation of neogenin modulates netrin‐1 induced neuronal migration

Abstract

Every year, a significant numbers of Canadians are affected by psychiatric and neurological disorders such as depression, autism and schizophrenia. The possibility of promoting neurogenesis following the onset of these diseases is an extremely appealing therapeutic option. The functionally intertwined Retinoblastoma (pRb) and E2F protein families are well known for their essential roles in regulating cell cycle entry; however we have described expanded roles for these proteins in regulating multiple aspects of neural development. Determining the mechanism that regulates these processes is essential for our understanding of how neural development proceeds. We show that the absence of the cell cycle regulatory protein E2F4 leads to a deficit in neural stem cell numbers and a severe impairment of self renewal. Additionally, E2F4 deficiency results in a loss of ventral telencephalic structures, a phenotype with striking similarity to animals lacking the Sonic Hedgehog (Shh) gene. We have previously shown that loss of both Shh expression and ventral telencephalic structures are rescued by interbreeding E2F4 s mutant with mice heterozygous for Gli3, a negative regulator of the Shh pathway. Preliminary data using ChIP-on Chip has identified Gli3 as a regulatory target for the E2F4 transcription factor. These findings suggest that E2F4 negatively regulates Gli3 expression and that in the absence of E2F4, Gli3 aberrantly represses Shh expression, consequently impeding ventral telencephalic patterning. In conclusion, these findings suggest that E2F4 is an essential regulator of neural stem cell renewal and telencephalic patterning by regulating the activity of the Shh pathway through repression of Gli3 expression.In order to effectively harness the potential of endogenous neural stem cells for therapeutic application the mechanisms which govern their proliferation, fate decisions and integration into their proper neural circuit need to be fully elucidated. Appreciated for its role in regulating cell cycle progression, it is now emerging that the pRb/E2F pathway is capable of mediating events in neural development beyond cell cycle regulation. The exact molecular mechanisms through which these roles are achieved remain unknown. We have previously described a requirement for the interaction between pRb and E2F3 to mediate neuronal migration during nervous system development, and identified neogenin as specifically deregulated in the absence of pRb. Through in-vitro explant cultures and ex-vivo electroporation, we observe a novel role for neogenin and its netrin ligand, in mediating migration from the ganglionic eminence. Conditional pRb neural precursors cells exhibit an increased propensity to adhere to substrate bound netrin-1 suggesting aberrant adhesion as the defective process in tangential migration. We also display an interaction between E2F3 and a conserved nonconsensus E2F motif in the promoter region of neogenin. By identifying a receptor ligand pathway disrupted in pRb deficiency, these results contribute to our h...