Induction and redistribution of XAF1, a new antagonist of XIAP in the rat brain after transient focal ischemia

Apoptotic cell death occurs in neurons after cerebral ischemia. To investigate the molecular basis of this mechanism of cell death, we explored the expression and localization of Smac/DIABLO, a newly identified mitochondrial apoptogenic molecule, and XAF1, a recently identified antagonist of XIAP anti-caspase activity in the rat brain following focal ischemia. Transient focal cerebral ischemia was produced for 90 min in rats. We observed changes in the expression of Smac, XAF1, and XIAP during reperfusion. The expression level of Smac/DIABLO was negligible under normal conditions and was moderately increased by 6-24 h reperfusion on both immunohistochemical and Western blotting levels. In opposition to the orthodox method of Western blotting employing electrophoretic analysis and homogenization, the immunohistochemical investigations of XIAP provided spatial information. Immunohistochemical analysis showed that the subcellular localization of XIAP became more extensive within cells during reperfusion, as compared with the normal state. Under normal conditions, XIAP was localized predominantly in the cytoplasm and the perinuclear region. However, at 6, 12, 24, and 48 h post-reperfusion, XIAP exhibited a diffuse distribution, including nuclear and cytoplasmic compartments. Interestingly, the expression of XAF1 exhibited significant changes during reperfusion. XAF1 expression was increased and there was a cellular redistribution with a nuclear localization in the post-ischemic phase by 6-24 h. XAF1 expression apparently enhances neuronal susceptibility to degeneration either by suppressing the ability of XIAP to complex with caspases or by sequestering XIAP in nuclear inclusions. These finding indicate that Smac/DIABLO, XAF1, and XIAP are implicated in the pathophysiological mechanisms of reperfusion injury.