ANALYSIS OF THE MECHANISM OF LOSS OF TROPHIC FACTOR DEPENDENCE ASSOCIATED WITH NEURONAL MATURATION - A PHENOTYPE INDISTINGUISHABLE FROM BAXDELETION

During development, sympathetic neurons are critically dependent on ne rve growth factor (NGF) for survival. Neurons isolated from the superi or cervical ganglia (SCG) of embryonic rodents and maintained for 1 we ek in vitro undergo programmed cell death in response to NGF deprivati on. As the cells mature in vitro and in vivo, however, these neurons d evelop a resistance to NGF deprivation and become much less acutely de pendent on NGF for survival. Using an in vitro model of neuronal matur ation, we confirmed that SCG neurons maintained in culture for 3-4 wee ks did not experience a dramatic loss in viability after NGF removal, yet they did undergo the initial biochemical and genetic changes elici ted by NGF deprivation of young neurons. NGF deprivation of mature neu rons produced rapid decreases in glucose uptake and protein and RNA sy nthesis rates, increased phosphorylation of c-Jun, and an increase in c-jun mRNA. Mature neurons, however, experienced a block in the cell d eath program before the final stages of the pathway activated in young neurons, which includes the induction of c-fos mRNA and characteristi c apoptotic nuclear changes. This maturation-induced block was indisti nguishable by these criteria from the block produced by Bax deficiency . Expression of Bax in mature neurons restored the apoptotic pathway, such that after NGF removal, Bax-overexpressing mature neurons resumed the apoptotic program, including the induction of c-Fos and passage t hrough a caspase checkpoint. Thus, a block in the apoptotic program at or near the BAX checkpoint accounts for the decreased dependence of m ature neurons on neurotrophic factor to maintain survival.