Placement of the BCL2 Family Member BAX in the Death Pathway of Sympathetic Neurons Activated by Trophic Factor Deprivation

Abstract

The BCL2 family member BAX is required for the induction of apoptosis in neonatal sympathetic neurons after NGF withdrawal.Bax-deficient sympathetic neurons are NGF-independent for survival. To characterize the physiological state of neurons protected by BAX deficiency and to place BAX within the death pathway, we determine which of the molecular changes induced by NGF deprivation depend on BAX and compare the results with those for neurons protected by caspase inhibition. We find that neurons deficient in bothBaxandBcl2resist NGF-deprivation similar toBax-deficient neurons discounting a role for BCL2 in the mechanism by whichBaxdeficiency causes trophic factor independence. We identify two new molecular changes, phosphorylation of c-Jun on Ser63and α-spectrin proteolysis, which precede and accompany apoptosis, respectively. Early reversible changes induced by NGF withdrawal, such as decreased protein synthesis and glucose uptake, increased c-Jun phosphorylation, increased steady statec-junmRNA levels, and cellular atrophy, occur both in wild type andBax-deficient neurons and thus are BAX-independent. In contrast to neurons protected by caspase inhibition, noc-fosinduction occurs inBax-deficient neurons. Terminal irreversible events of apoptosis such as caspase-mediated α-spectrin proteolysis are prevented by bothBax-deficiency and caspase inhibition. This places BAX downstream or in a different pathway of the early changes and upstream of the terminal events such as those leading toc-fosinduction and caspase activation. This order indicates that the physiological state of NGF-deprived neurons protected byBaxdeficiency may be less perturbed than that of caspase inhibitor-saved neurons.