The survival of dorsal root ganglion (DRG) neurons, both in vivo and i
n vitro, is dependent on the availability of nerve growth factor (NGF)
for a transient period early in development after which these neurons
become independent of NGF for survival. The precise molecular mechani
sm by which developing DRG neurons gain independence from NGF has not
been determined. We used an in vitro model of DRG neuronal development
to test hypotheses that independence from NGF in mature DRG neurons c
ould be caused by developmental regulation of either elements of the N
GF withdrawal signal transduction pathway or of proteins important for
activation of the apoptosis output pathway. Interruption of phosphoti
dylinositol-3 kinase activation, by treatment with the specific inhibi
tor LY294002, resulted in apoptosis in immature but not mature DRG neu
rons in a manner similar to that observed with NGF withdrawal. Further
downstream along the signal transduction pathway, c-JUN phosphorylati
on occurred in both immature and mature DRG neurons after NGF withdraw
al or treatment with LY294002, despite the fact that the older neurons
did not undergo apoptosis. In contrast, the ratio of expression of th
e proapoptotic gene bax to antiapoptotic gene bcl-x(L) was many times
higher in immature than mature neurons, both in vivo and in vitro. Tak
en together, these results strongly suggest that developmental regulat
ion of NGF withdrawal-induced apoptosis in DRG occurs via control of t
he relative level of expression of members of the bcl-2 gene family.