Caspase-3 activation and caspase-like proteolytic activity in human perinatal hypoxic-ischemic brain injury

Human perinatal hypoxic-ischemic brain injury is an important cause of death and morbidity. One relatively common pattern of perinatal injury involves selective neuronal death in the ventral gray matter of the pons and in the subiculum of the hippocampal formation, classically termed 'pontosubicular neuronal necrosis' (PSN). The vulnerable neurons undergo karyorrhectic condensation of their nuclear chromatin and exhibit in situ end labeling for DNA fragmentation, leading to the recent reclassification of cell death in PSN as apoptotic. Caspase activation plays a central role in apoptosis and caspase-3 appears to be an especially important effector enzyme in neuronal apoptosis. In this study we performed immunohistochemistry on brain sections from six postmortem cases of PSN using two polyclonal antisera; CM1, a specific marker of caspase-3 activation, and fractin, which specifically recognizes a neoepitope at a caspase cleavage site in actin, and is therefore a marker of caspase-like proteolytic activity. Numerous CM1- and fractin-immunolabeled neurons were seen in the nuclei pontis and subiculum in each case, and the great majority showed karyorrhectic morphology. The immunostaining involved the nuclei and cytoplasm of these cells and the proximal portions at least of their neuritic processes. Some neurons exhibited a more extensive pattern of dendritic fractin labeling. Frequent CM1- and fractin-immunoreactive axonal segments were also seen. The identification of caspase-3 activation and caspase-like proteolytic activity in PSN cases in this study suggests that caspase inhibitors may potentially have a therapeutic neuroprotective role in human perinatal hypoxic-ischemic brain injury.