Temporal and Spatial Profile of Bid Cleavage after Experimental Traumatic Brain Injury

Apoptosis plays an essential role in the cascade of CNS cell degeneration after traumatic brain injury. However, the underlying mechanisms are poorly understood. The authors examined the temporal profile and cell subtype distribution of the proapoptotic protein Bid from 6 hours to 7 days after cortical impact injury in the rat. Increased protein levels of tBid were seen in the cortex ipsilateral to the injury site from 6 hours to 3 days after trauma. Immunohistologic examinations revealed expression of tBid in neurons, astrocytes, and oligodendrocytes from 6 hours to 3 days after impact injury, and concurrent assessment of DNA damage using TUNEL identified tBid-immunopositive cells with apoptoticlike morphology in the traumatized cortex. Moreover, Bid cleavage and activation of caspase-8 and caspase-9 occurred at similar time points and in similar brain regions (i.e., cortical layers 2 to 5) after impact injury. In contrast, there was no evidence of caspase-8 or caspase-9 processing or Bid cleavage in the ipsilateral hippocampus, contralateral cortex, and hippocampus up to 7 days after the injury. The results provide the first evidence of Bid cleavage in the traumatized cortex after experimental traumatic brain injury in vivo,and demonstrate that tBid is expressed in neurons and glial cells. Further, findings indicate that cleavage of Bid may be associated with the activation of the initiator caspase-8 and caspase-9. Finally, these data support the hypothesis that cleavage of Bid contributes to the apoptotic degeneration of different CNS cells in the injured cortex.