Bcl-xL is an antiapoptotic regulator for postnatal CNS neurons

Bcl-xL is a death-inhibiting member of the Bcl-2/Ced9 family of proteins which either promote or inhibit apoptosis. Gene targeting has revealed that Bcl-xL is required for neuronal survival during brain development; however, Bcl-xL knock-out mice do not survive past embryonic day 13.5, precluding an analysis of Bcl-xL function at later stages of development. Bcl-xL expression is maintained at a high level postnatally in the CNS, suggesting that it may also regulate neuron survival in the postnatal period. To explore functions of Bcl-xL related to neuron survival in postnatal life, we generated transgenic mice overexpressing human Bcl-xL under the control of a pan-neuronal promoter. A line that showed strong overexpression in brainstem and a line that showed overexpression in hippocampus and cortex were chosen for analysis. We asked whether overexpression of Bcl-xL influences neuronal survival in the postnatal period by studying two injury paradigms that result in massive neuronal apoptosis. In the standard neonatal facial axotomy paradigm, Bcl-xL overexpression had substantial effects, with survival of 65% of the motor neurons 7 d after axotomy, as opposed to only 15% in nontransgenic littermates. To investigate whether Bcl-xL regulates survival of CNS neurons in the forebrain, we used a hypoxia-ischemia paradigm in neonatal mice. We show here that hypoxia-ischemia leads to substantial apoptosis in the hippocampus and cortex of wild-type neonatal mice. Furthermore, we show that overexpression of Bcl-xL is neuroprotective in this paradigm. We conclude that levels of Bcl-xL in postnatal neurons may be a critical determinant of their susceptibility to apoptosis.