RB regulates the production and the survival of newborn neurons in the embryonic and adult dentate gyrus.

In mammals, hippocampal dentate gyrus granule cells (DGCs) constitute a particular neuronal population produced both during embryogenesis and adult life, and play key roles in neural plasticity and memory. However, the molecular mechanisms regulating neurogenesis in the dentate lineage throughout development and adulthood are still not well understood. The Retinoblastoma protein (RB), a transcriptional repressor primarily involved in cell cycle control and cell death, plays crucial roles during cortical development but its function in the formation and maintenance of DGCs remains unknown. Here, we show that loss of RB during embryogenesis induces massive ectopic proliferation and delayed cell cycle exit of young DGCs specifically at late developmental stages but without affecting stem cells. This phenotype was partially counterbalanced by increased cell death. Similarly, during adulthood, loss of RB causes ectopic proliferation of newborn DGCs and dramatically impairs their survival. These results demonstrate a crucial role for RB in the generation and the survival of DGCs in the embryonic and the adult brain. © 2016 Wiley Periodicals, Inc.

Rb/E2F regulates expression of neogenin during neuronal migration.

The Rb/E2F pathway has long been appreciated for its role in regulating cell cycle progression. Emerging evidence indicates that it also influences physiological events beyond regulation of the cell cycle. We have previously described a requirement for Rb/E2F mediating neuronal migration; however, the molecular mechanisms remain unknown, making this an ideal system to identify Rb/E2F-mediated atypical gene regulation in vivo. Here, we report that Rb regulates the expression of neogenin, a gene encoding a receptor involved in cell migration and axon guidance. Rb is capable of repressing E2F-mediated neogenin expression while E2F3 occupies a region containing E2F consensus sites on the neogenin promoter in native chromatin. Absence of Rb results in aberrant neuronal migration and adhesion in response to netrin-1, a known ligand for neogenin. Increased expression of neogenin through ex vivo electroporation results in impaired neuronal migration similar to that detected in forebrain-specific Rb deficiency. These findings show direct regulation of neogenin by the Rb/E2F pathway and demonstrate that regulation of neogenin expression is required for neural precursor migration. These studies identify a novel mechanism through which Rb regulates transcription of a gene beyond the classical E2F targets to regulate events distinct from cell cycle progression.

The p107/E2F pathway regulates fibroblast growth factor 2 responsiveness in neural precursor cells.

We have previously shown that p107, a member of the retinoblastoma (Rb) cell cycle regulatory family, has a unique function in regulating the pool of neural precursor cells. As the pool of progenitors is regulated by a limiting supply of trophic factors, we asked if the Rb/E2F pathway may control the size of the progenitor population by regulating the levels of growth factors or their receptors. Here, we demonstrate that fibroblast growth factor 2 (FGF2) is aberrantly upregulated in the brains of animals lacking Rb family proteins and that the gene encoding the FGF2 ligand is directly regulated by p107 and E2F3. Chromatin immunoprecipitation assays demonstrated that E2F3 and p107 occupy E2F consensus sites on the FGF2 promoter in the context of native chromatin. To evaluate the physiological consequence of FGF2 deregulation in both p107 and E2F3 mutants, we measured neural progenitor responsiveness to growth factors. Our results demonstrate that E2F3 and p107 are each mediators of FGF2 growth factor responsiveness in neural progenitor cells. These results support a model whereby p107 regulates the pool of FGF-responsive progenitors by directly regulating FGF2 gene expression in vivo. By identifying novel roles for p107/E2F in regulating genes outside of the classical cell cycle machinery targets, we uncover a new mechanism whereby Rb/E2F mediates proliferation through regulating growth factor responsiveness.