DUAL ROLES OF THE RETINOBLASTOMA PROTEIN IN CELL-CYCLE REGULATION ANDNEURON DIFFERENTIATION

To assess the functions of the retinoblastoma protein (RB) during norm al development, we have analyzed mouse embryos that lack a functional copy of the retinoblastoma gene (genotype: Rb-1(Delta 20)/Rb-1(Delta 2 0)). Our findings demonstrate that RB plays an important role in the r egulation of the neuronal cell cycle, In mutant embryos, dividing cell s are found well outside of the normal neurogenic regions in both the central and peripheral nervous systems. In addition to abnormal cell c ycle regulation, however, the mutant embryos show two less expected ph enotypes. First, many of the ectopically dividing cells die by apoptos is 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.O appears t o be near normal, nerve cells, especially in sensory ganglia, do not m ature properly. Their morphology is stunted and expression of neuronal beta II tubulin is greatly reduced. Preferential reduction in the exp ression of TrkA, TrkB, and the low-affinity neurotrophin receptor p75( LNGFR) may be relevant to neuronal cell death and lack of neuronal dif ferentiation seen in the mutant embryos. Primary cultures of dorsal ro ot and trigeminal ganglion cells from later stage mutant embryos revea l a decrease in neuronal cell survival and in neurite outgrowth even i n the presence of the appropriate neurotrophins. Taken together, these results suggest that the p110(RB) protein not only regulates progress ion through the cell cycle but is also important for cell survival and differentiation.