TRANSIENT GLOBAL-ISCHEMIA TRIGGERS EXPRESSION OF THE DNA DAMAGE-INDUCIBLE GENE GADD45 IN THE RAT-BRAIN

Using in situ hybridization, Northern blot analysis, Western blot anal ysis, and immunocytochemistry, mRNA and protein expression of the nove l DNA damage-inducible gene GADD45 was examined in the rat brain at 0. 5, 2, 4, 8, 16, 24, 48, and 72 hours after 15 minutes of transient glo bal ischemia. Transient ischemia produced by the four-vessel occlusion method resulted in DNA double-strand breaks and delayed neuronal cell death in vulnerable neurons of the hippocampal CA1 sector, the hilus, dorsal caudate-putamen, and thalamus, as shown by in situ DNA nick en d-labeling and histologic staining. GADD45 mRNA was transiently increa sed in less-vulnerable regions such as the parietal cortex (up to 8 ho urs after ischemia) and dentate granule cells (up to 24 hours after is chemia) but was persistently increased in vulnerable neurons such as C A1 pyramidal neurons (up to 48 hours). GADD45 immunoreactivity was inc reased in both vulnerable and less-vulnerable regions at earlier reper fusion periods (4 to 16 hours), but thereafter immunoreactivity was de creased below control levels in most vulnerable regions before delayed cell death and DNA double-strand breaks. At 72 hours after transient ischemia, a moderate increase in GADD35 immunoreactivity was still det ectable in some CA3 neurons and in a few surviving neurons in the CA1 region. Double staining performed at 16 to 72 hours after ischemia rev ealed that GADD45 immunoreactivity was persistently increased in neuro ns that did not develop DNA damage. Because GADD45 protein may partici pate in the DNA excision repair process and because it has been shown that this protein is also overexpressed in neurons that survive focal ischemia and kainate-induced epileptic seizures, the results reported here support the hypothesis that GADD45 could have a protective role i n neuronal injury.