Age-associated mitochondrial DNA deletions are not evident chronically after experimental brain injury in the rat

The enduring cognitive and sensorimotor deficits that result from traumatic brain injury (TBI) are associated with metabolic stress and free radical cascades, which establish conditions that may promote mitochondrial DNA (mtDNA) deletion and oxidation, often observed as a consequence of normal aging. Without substantial mtDNA repair mechanisms, permanent alterations to essential mitochondrial enzymes could perpetuate post-injury pathologic cascades. To determine whether mitochondria from the injured cortex and hippocampus sustain mtDNA damage after TBI, we evaluated mtDNA deletion and oxidation following lateral fluid percussion TBI in the anesthetized adult Sprague-Dawley rat (4 months) compared with uninjured adult and aged rats (n = 4/group). The presence of the 4.8-KB common deletion in mtDNA was assessed by conventional PCR to generate products representing total, non-deleted wild-type, and deleted mtDNA in homogenized tissue and isolated mitochondria 3 and 14 days following TBI. Total and wild-type mtDNA amplification products were obtained from cortical and hippocampal tissue and mitochondria for all conditions. Although no mtDNA deletions were observed following experimental TBI, mtDNA deletion was detected in cortical tissue, but not isolated mitochondria, of naive, aged (24 months) Sprague-Dawley rats, suggesting that the isolation protocol may exclude mitochondria harboring mtDNA damage. Oxidative mtDNA damage in isolated mitochondria assayed by ELISA for 8-hydroxy-2'-deoxyguanosine (8-OHdG) from cortical (0.50 +/- 0.08 pg 8-OHdG/ micro g mitochondria) and hippocampal (0.35 +/- 0.02) regions were unaffected by TBI. However, mitochondrial protein yields from injured and aged brains were comparable and significantly lower than uninjured brain, suggesting that the underlying pathology between TBI and aging may be similar.