Overlapping and divergent actions of estrogen and the neurotrophins on cell fate and p53-dependent signal transduction in conditionally immortalized cerebral cortical neuroblasts

The developing cerebral cortex undergoes overlapping periods of neurogenesi s, suicide, and differentiation to generate the mature cortical plate. The following experiments examined the role of the gonadal hormone estrogen in comparison to the neurotrophins, in the regulation of p53-dependent cortica l cell fate. To synchronize choices between neurogenesis, apoptosis, and ne ural differentiation, embryonic rat cerebral cortical neuroblasts were cond itionally immortalized with the SV40 large T antigen containing the tsA58/U 19 temperature-sensitive mutations. At the nonpermissive temperature, cessa tion of large T antigen expression was accompanied by induction of p53, as well as the p53-dependent proteins, wild-type p53-activated fragment-1/Cdk (cyclin-dependent kinase)-interacting protein-1 (p21/Waf1), Bcl (B-cell lym phoma)-associated protein (Bax), and murine double minute 2 (MDM2), that le ad to cell cycle-arrest, suicide, and p53 inhibition, respectively. Simulta neously, neuroblasts exit cell cycle and die apoptotically or differentiate primarily into astrocytes and immature postmitotic neuroblasts. At the non permissive temperature, estrogen specifically induced an antagonist-indepen dent increase in phosphorylated p53 expression, while increasing p21/Waf1 a nd decreasing Bar. Coincidentally, estrogen rapidly increased and then decr eased MDM2 relative to controls, suggesting temporal modulation of p53 func tion. Both estrogen and neurotrophins prevented DNA fragmentation, a marker for apoptosis. However, estrogen also induced a transient increase in rele ased lactate dehydrogenase, suggesting that estrogen simultaneously induced rapid cell death in a subpopulation of cells. In contrast to the neurotrop hins, estrogen also increased cell proliferation. Both estrogen and the neu rotrophins supported neuronal differentiation. However, in contrast to the neurotrophins, estrogen only supported the expression of a subset of oligod endrocytic markers. These results suggest that estrogen and the neurotrophi ns support overlapping and distinct aspects of differentiation in the devel oping cerebral cortex.