Neural precursor cells differentiating in the absence of Rb exhibit delayed terminal mitosis deregulated E2F 1 and 3 activity

The severe neurological deficit in embryos carrying null mutations for the retinoblastoma (Rb) gene suggests that Rb plays a crucial role in neurogene sis. While developing neurons undergo apoptosis in vivo neural precursor ce lls cultured from Rb-deficient embryos appear to differentiate and survive. To determine whether Rb is an essential regulator of the intrinsic pathway modulating terminal mitosis we examined the terminal differentiation of pr imary cortical progenitor cells and bFGF-dependent neural stem cells derive d from Rb-deficient mice. Although Rb -/- neural precursor cells are able t o differentiate in vitro we show that these cells exhibit a significant del ay in terminal mitosis relative to wild-type cells. Furthermore, Br -/- cel ls surviving in vitro exhibit and upregulation of p107 that is found in com plexes with E2F3. This suggests that p107 may partially compensate for the loss of Rb in neural precursor cells. Functional ablation of Rb family prot eins by adenovirus-mediated delivery of an E1A N-terminal mutant results in apoptosis in Rb-deficient cells, consistent with the interpretation that o ther Rb family proteins may facilitate differentiation and survival. While p107 is upregulated and interacts with the putative Rb target E2F3 in neura l precursor cells, our results indicate that it clearly cannot restore norm al E2F regulation. Rb-deficient cells exhibit a significant enhancement of E2F 1 and 3 activity throughout differentiation concomitant with the aberra nt expression of E2F-inducible genes. In these studies we show that Rb is e ssential for the regulation of E2F 1 and 3 activity as well as the onset of terminal mitosis in neural precursor cells. (C) 1999 Academic Press.