DNA fragmentation precedes aberrant expression of cell cycle-related protein in rat brain after MCA occlusion

Recent experiments suggest that apoptotic mechanisms are involved in neuronal cell death after ischemic injury. Although the exact mechanism that triggers activation of apoptotic machinery remains uncertain, in vitro studies revealed that forced expression of cell cycle-related proteins induced apoptosis. Thus, aberrant expression of such proteins might be related to ischemic neuronal death. In the present experiment, we investigated expression of cell cycle-related proteins, i.e., cyclin B1, cyclin D1, cdk4, and PCNA, in rat brain after transient MCA occlusion, and compared the temporal profile of the results with that of TUNEL study, which detects double strand breaks in DNA. There were no immunoreactivities for cyclin B1, cyclin D1, and PCNA in the brain with and without ischemia. As for cdk4, however, it became present at 1 and 3 days of reperfusion after 2 h of ischemia. On the other hand, TUNEL positive cells appeared as early as 3 h of reperfusion, which peaked at 1 and 3 days. These results indicate that aberrant expression of cdk4, but not cyclin B1, cyclin D1 or PCNA, actually takes place in the brain after MCA occlusion, but this is not the causative mechanism of apoptotic cell death in the brain with ischemia.