Temporal Profiles of the Subcellular Localization of Bim, a BH3-Only Protein, during Middle Cerebral Artery Occlusion in Mice

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Abstract
BH3-only proteins are a subfamily of proapoptotic Bcl-2 proteins that act upstream of the mitochondrially mediated cell death pathway, and their association with the pathogenesis of brain ischemia remains largely unknown. The authors explored the temporal profiles of the expression levels and subcellular localization of BH3-only proteins in permanent middle cerebral artery occlusion (MCAO) by Western blot analysis. They observed an increased mitochondrial distribution of Bim at 3 to 6 hours of MCAO that appeared unrelated to transcriptional upregulation, as assessed by semiquantitative reverse transcription—polymerase chain reaction. At 3 to 6 hours of MCAO, Bim immunoreactivity was enhanced in neurons and oligodendrocytes in the ischemic regions. The increased mitochondrial localization of Bim coincided with a marked cytochrome c release and preceded the peak of caspase-9 activation. The authors observed an association of Bim with the dynein intermediate chain, a major component of the dynein motor complex, in the brain using a coimmunoprecipitation assay. Cerebral ischemia induced a time-dependent significant decrease in dynein expression, which started at 3 hours of MCAO. The authors deduced that the liberation of Bim from the dynein motor complex is a likely mechanism for the increased mitochondrial localization of Bim. During MCAO, Bad did not show any change in phosphorylation state or subcellular localization.