bcl-2 prolongs neuronal survival during hypoxia-induced apoptosis

In vivo models of hypoxic-ischemic brain injury have shown altered expression of a number of genes that are important in regulating neuronal survival. However, it is not clear as to whether hypoxia alone can alter the expression of genes regulating neuronal survival. We hypothesized that (1) hypoxia alone alters the expression of bcl-2 in neurons, (2) the severity and duration of hypoxia influence bcl-2 expression, and (3) the alteration of bcl-2 expression has an important role in regulating neuronal survival during hypoxia. Embryonic rat neocortical neurons cultured for 7-10 days were exposed to 0.1, 1, or 3% oxygen for various durations and were removed for analyses at 24-h intervals. Under all hypoxic conditions, neurons exhibited morphologic changes, as assessed by electron microscopy and Annexin V staining, consistent with apoptosis. Immunoblot and immunofluorescence analyses revealed an increase in neuronal bcl-2 protein during hypoxic exposure. Quantitative immunofluorescence analyses of bcl-2 immunostained neurons indicated that expression of bcl-2 was altered by the duration and severity of hypoxia. Attenuation of bcl-2 expression by antisense oligonucleotides decreased the proportion of surviving neurons by approximately 50% after 48 h of exposure to 0.1% oxygen. We conclude that observed increase in bcl-2, in part, plays an important role in neuronal survival during exposure to hypoxia.