The role of Bcl-2 and Bax in hypoxia-induced caspase-3 activation in the fetal superior colliculus: in vivo optical

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The fetal brain is known to be survival from severe hypoxia during delivery, although its physiological mechanism remains unclear. We studied the roles of Bcl-2 (caspase-3 suppressor) and Bax (caspase-3 accelerator) in the hypoxia-induced activation of caspase-3 (final triggering apoptosis factor) in the superior colliculus (SC) in a fetal rat (embryonic day 22), which was still connected with the anesthetized dam (urethane, 1.2-1.4 g/kg, i.p.) by the umbilical cord. Caspase-3 activity was measured with fluorescent caspase-3 substrate using an optical imaging system. Hypoxia was induced by the umbilical cord occlusion for 5 minutes. After the initial occlusion and reperfusion of umbilical blood flow, the 2nd-5th occlusions were repeated with reperfusion at 2-5 hours after the first occlusion. The Bcl-2-siRNA or Bax-siRNA was applied into cells of the SC by electropolation. Apoptosis of fetal SC was measured by using the anti-ssDNA antibody. Apoptosis appeared during the occlusion and at 1-5 hrs after the reperfusion of the umbilical cord. Caspase-3 activity was increased transiently during the occlusion and long-lasted largely after the reperfusion. A low dose of Bcl-2 inhibitor did not affect on caspase-3 activity during the occlusion and after the reperfusion. But, high dose of Bcl-2 inhibitor unexpectedly suppressed the caspase-3 activity. Bax inhibitor had no effect on caspase-3 activity. Further, changes in caspase-3 activity of the fetal rats were examined using Bcl-2siRNA and Bax-siRNA to inhibit Bcl-2 and Bax synthesis. Bcl-2-siRNA caused statistically insignificant increases in caspase-3 activity after the 2nd and 3rd reperfusions, whereas Bax-siRNA induced significant decreases. The results suggest that Bcl-2 and Bax may act on an apoptosis cascade in the immature brain during severe hypoxia.