The retinoblastoma protein (pRb) has been proposed to regulate cell cycle progression in part through its ability to interact with enzymes that modify histone tails and create a repressed chromatin structure. We created a mutation in the murine Rb1 gene that disrupted pRb's ability to interact with these enzymes to determine if it affected cell cycle control. Here, we show that loss of this interaction slows progression through mitosis and causes aneuploidy. Our experiments reveal that while the LXCXE binding site mutation does not disrupt pRb's interaction with the Suv4-20h histone methyltransferases, it dramatically reduces H4-K20 trimethylation in pericentric heterochromatin. Disruption of heterochromatin structure in this chromosomal region leads to centromere fusions, chromosome missegregation, and genomic instability. These results demonstrate the surprising finding that pRb uses the LXCXE binding cleft to control chromatin structure for the regulation of events beyond the G 1 -to-S-phase transition.

The mechanisms through which cell-cycle control and cell-fate decisions are coordinated in proliferating stem cell populations are largely unknown. Here, we show that E2f3 isoforms, which control cell-cycle progression in cooperation with the retinoblastoma protein (pRb), have critical effects during developmental and adult neurogenesis. Loss of either E2f3 isoform disrupts Sox2 gene regulation and the balance between precursor maintenance and differentiation in the developing cortex. Both isoforms target the Sox2 locus to maintain baseline levels of Sox2 expression but antagonistically regulate Sox2 levels to instruct fate choices. E2f3-mediated regulation of Sox2 and precursor cell fate extends to the adult brain, where E2f3a loss results in defects in hippocampal neurogenesis and memory formation. Our results demonstrate a mechanism by which E2f3a and E2f3b differentially regulate Sox2 dosage in neural precursors, a finding that may have broad implications for the regulation of diverse stem cell populations.

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 Rb pathway is best characterized for its role in regulating cell cycle progression through E2F-mediated transcriptional regulation of classical cell cycle machinery target genes. Recently, however, accumulating in vivo and in vitro evidence is emerging to suggest that Rb and E2F are capable of regulating expression of atypical target genes with functions other than cell cycle regulation in cell-type-specific manners (reviewed in reference 35). In vivo, several studies have emerged that implicate Rb and E2F interaction in novel processes beyond well-characterized roles in cell cycle regulation (10; for a review, see reference 6). In the nervous system, in particular, we have recently shown that an Rb-E2F3 interaction mediates migration of a subpopulation of GABAergic interneurons (34). In the same study, we also observed deregulation of a number of genes with known roles in neuronal migration in cell populations lacking Rb, suggesting a role for E2F3 in regulating transcription of novel targets (34). A second cell cycle-independent role for E2F3a in regulating Rb-mediated interneuron differentiation was also reported in the retina (9). Thus far, in vivo studies have failed to identify the mechanism through which these cell cycle-independent processes occur.In parallel, in vitro several microarray studies examining changes in gene expression in response to various models of deregulated E2F expression have each identified groups of overlapping novel target genes with well-characterized roles in differentiation, development, and migration (5,15,25,31,39,41,60). More recently, chromatin immunoprecipitation (ChIP)-on-chip studies have identified putative E2F binding site...

Over the last decade significant advances have been made toward reprogramming the fate of somatic cells, typically by overexpression of cell lineage-determinant transcription factors. As key regulators of cell fate, the SOX family of transcription factors has emerged as potent drivers of direct somatic cell reprogramming into multiple lineages, in some cases as the sole overexpressed factor. The vast capacity of SOX factors, especially those of the SOXB1, E and F subclasses, to reprogram cell fate is enlightening our understanding of organismal development, cancer and disease, and offers tremendous potential for regenerative medicine and cell-based therapies. Understanding the molecular mechanisms through which SOX factors reprogram cell fate is essential to optimize the development of novel somatic cell transdifferentiation strategies.

E2F transcription factors and their regulatory partners, the pocket proteins (PPs), have emerged as essential regulators of stem cell fate control in a number of lineages. In mammals, this role extends from both pluripotent stem cells to those encompassing all embryonic germ layers, as well as extra-embryonic lineages. E2F/PP-mediated regulation of stem cell decisions is highly evolutionarily conserved, and is likely a pivotal biological mechanism underlying stem cell homeostasis. This has immense implications for organismal development, tissue maintenance, and regeneration. In this article, we discuss the roles of E2F factors and PPs in stem cell populations, focusing on mammalian systems. We discuss emerging findings that position the E2F and PP families as widespread and dynamic epigenetic regulators of cell fate decisions. Additionally, we focus on the ever expanding landscape of E2F/PP target genes, and explore the possibility that E2Fs are not simply regulators of general ‘multi-purpose’ cell fate genes but can execute tissue- and cell type-specific gene regulatory programs.