Dual roles of the retinoblastoma protein in cell cycle regulation and neuron differentiation.

Abstract

To assess the functions of the retinoblastoma protein (RB) during normal development, we have analyzed mouse embryos that lack a functional copy of the retinoblastoma gene (genotype: Our findings demonstrate that RB plays an important role in the regulation of the neuronal cell cycle. In mutant embryos, dividing cells are found well outside of the normal neurogenic regions in both the central and peripheral nervous systems. In addition to abnormal cell cycle regulation, however, the mutant embryos show two less expected phenotypes. First, many of the ectopically dividing cells die by apoptosis shortly after their entrance into S phase. In sensory ganglia, most nerve cells die by this process, beginning at about the same time as normal target-related neuronal death. Second, although the expression of certain differentiation markers such as N-CAM and Brn-3.0 appears to be near normal, nerve cells, especially in sensory ganglia, do not mature properly. Their morphology is stunted and expression of neuronal [HI tubulin is greatly reduced. Preferential reduction in the expression of TrkA, TrkB, and the low-affinity neurotrophin receptor p75 LNGFR may be relevant to neuronal cell death and lack of neuronal differentiation seen in the mutant embryos. Primary cultures of dorsal root and trigeminal ganglion cells from later stage mutant embryos reveal a decrease in neuronal cell survival and in neurite outgrowth even in the presence of the appropriate neurotrophins. Taken together, these results suggest that the p110 RB protein not only regulates progression through the cell cycle but is also important for cell survival and differentiation.

The retinoblastoma gene family is differentially expressed during embryogenesis

“…Consistent with the documented neuronal defects in RB 7/7 mice (Lee et al, 1994), we have detected RB1 transcripts in the nervous system at all stages examined, from E8.5 to E17.5 (Figure 2a ± j). Folding of the neural plate starts in the area of the somites at around E8 and proceeds bidirectionally, ending at the cranial and caudal neuropores.…”

“…In contrast to adult mice where RB1 is widely expressed (Bernards et al, 1989), the expression of RB1 during embryogenesis was restricted to speci®c cell lineages including the nervous system, liver, muscles and lens. RB 7/7 mice show phenotypic defects in all these tissues (Jacks et al, 1992;Lee et al, 1994;Morgenbesser et al, 1994;Zacksenhaus et al, 1996a). RB1 was also expressed in the follicles of vibrissae at E14.5 ( Figure 2g), and in the thymus at E16.5 (Figure 2i) where no defect has yet been demonstrated in the absence of pRb.…”

“…The liver of RB 7/7 embryos is small and exhibits reduced cellularity, with many immature nucleated red blood cells (Jacks et al, 1992). In the nervous system, ectopic DNA synthesis and increased cell death in post-mitotic areas lead to abnormal di erentiation (Lee et al, 1994). Development of the lens of RB 7/7 embryos is also impaired (Morgenbesser et al, 1994).…”

Immunohistochemical Expression of Cell Cycle Proteins E2F-1, Cdk-2, Cyclin E, p27kip1, and Ki-67 in Normal Placenta and Gestational Trophoblastic Disease

“…Immunostaining for the presence of pRb often provides ambiguous information, because methods to accurately distinguish between partially phosphorylated and completely dephosphorylated states are not currently available. Furthermore, in addition to down-regulating cell-cycle progression, pRb has also been found to play a key role in promoting cell cycle exit during terminal embryonic differentiation in many tissues (11,27). The expression patterns of cyclin E and Cdk2 would indeed suggest that pRb plays a role in trophoblastic proliferations, because these proteins are known to phosphorylate/inactivate pRb and allow cell cycle progression.…”

pRb and p107 regulate E2F activity during lens fiber cell differentiation

“…Mouse strains lacking a functional Rb gene (Rb7/7) die during embryogenesis, with defects in both hepatic erythropoiesis and neuronal development (Clarke et al, 1992;Lee et al, 1994;Jacks et al, 1992). In regions of the central and peripheral nervous system where only post-mitotic cells are found in wild type animals, both mitotic and apoptotic cells are found in Rb7/7 mice (Lee et al, 1994). Rb de®ciency also inhibits di erentiation of the occular lens (Pan and Griep, 1994;Morgenbesser et al, 1994;Mahon et al, 1987;Fromm et al, 1994).…”