BAX contributes to apoptotic-like death following neonatal hypoxia-ischemia: Evidence for distinct apoptosis pathways

Background: Hypoxic-ischemic (H-I) injury to the neonatal brain has been sh own to result in rapid cell death with features of acute excitotoxicity/nec rosis as well as prominent delayed cell death with features of apoptosis su ch as marked caspase-3 activation. BAX, a pro-apoptotic molecule, has been shown to be required for apoptotic neuronal cell death during normal develo pment but the contribution of endogenous BAX in cell death pathways followi ng H-I injury to the developing or adult brain has not been studied. Materials and Methods: Bax +/+, +/-, and -/- mice at post-natal day 7 were subjected to unilateral carotid ligation followed by exposure to 45 minutes of 8% oxygen. At different timepoints following H-I, brain tissue was stud ied by conventional histology, immunohistochemistry, immunofluorescence, We stern blotting, and enzymatic assay to determine the extent and type of cel l injury as well as the amount of caspase activation. Results: We found that bax -/- mice had significantly less (38%) hippocampa l tissue loss than mice expressing bax. Some of the remaining cell death in bax -/- mice, however, still had features of apoptosis including evidence of nuclear shrinkage and caspase-3 activation. Though bax -/- mice had sign ificantly decreased caspase-3 activation as compared to bax expressing mice following H-I, the density of cells with activated caspase-8 in the CA3 re gion of the hippocampus did not differ between bax +/- and bax -/- mice. Conclusions: These findings demonstrate that endogenous BAX plays a role in regulating cell death in the central nervous system (CNS) following neonat al H-I. a model of cerebral palsy. in addition, while BAX appears to modula te the caspase-3 activation following neonatal H-I, caspase-8 which is link ed to death receptor activation, may contribute to apoptotic-like neuronal death in a BAX-independent manner.