Bcl-XL-Caspase-9 Interactions in the Developing Nervous System: Evidence for Multiple Death Pathways

Programmed cell death is critical for normal nervous system development and is regulated by Bcl-2 and Caspase family members. Targeted disruption of bcl-x L , an antiapoptotic bcl-2 gene family member, causes massive death of immature neurons in the developing nervous system whereas disruption of caspase-9 , a proapoptotic caspase gene family member, leads to decreased neuronal apoptosis and neurodevelopmental abnormalities. To determine whether Bcl-X L and Caspase-9 interact in an obligate pathway of neuronal apoptosis, bcl-x/caspase-9 double homozygous mutants were generated. The increased apoptosis of immature neurons observed in Bcl-X L -deficient embryos was completely prevented by concomitant Caspase-9 deficiency. In contrast, bcl-x − /− / caspase-9 − /− embryonic mice exhibited an expanded ventricular zone and neuronal malformations identical to that observed in mice lacking only Caspase-9. These results indicate both epistatic and independent actions of Bcl-X L and Caspase-9 in neuronal programmed cell death. To examine Bcl-2 and Caspase family-dependent apoptotic pathways in telencephalic neurons, we compared the effects of cytosine arabinoside (AraC), a known neuronal apoptosis inducer, on wild-type, Bcl-X L -, Bax-, Caspase-9-, Caspase-3-, and p53-deficient telencephalic neurons in vitro . AraC caused extensive apoptosis of wild-type and Bcl-X L -deficient neurons. p53- and Bax-deficient neurons showed marked protection from AraC-induced death, whereas Caspase-9- and Caspase-3-deficient neurons showed minimal or no protection, respectively. These findings contrast with our previous investigation of AraC-induced apoptosis of telencephalic neural precursor cells in which death was completely blocked by p53 or Caspase-9 deficiency but not Bax deficiency. In total, these results indicate a transition from Caspase-9- to Bax- and Bcl-X L -mediated neuronal apoptosis.