TIME COURSES OF POST-INJURY MITOCHONDRIAL OXIDATIVE DAMAGE AND RESPIRATORY DYSFUNCTION AND NEURONAL CYTOSKELETAL DEGRADATION IN A RAT MODEL OF FOCAL TRAUMATIC BRAIN INJURY

摘要

Traumatic brain injury (TBI) results in rapid reactive oxygen species (ROS) production and oxidative damage to essential brain cellular components leading to neuronal dysfunction and cell death. It is increasingly appreciated that a major player in TBI-induced oxidative damage is the reactive nitrogen species (RNS) peroxynitrite (PN) which is produced in large part in injured brain mitochondria. Once formed, PN decomposes into highly reactive free radicals that trigger membrane lipid peroxidation (LP) of polyunsaturated fatty acids (e.g. arachidonic acid) and protein nitration (3-nitrotyrosine, 3-NT) in mitochondria and other cellular membranes causing various functional impairments to mitochondrial oxidative phosphorylation and calcium (Ca²⁺) buffering capacity. The LP also results in the formation of neurotoxic reactive aldehyde byproducts including 4-hydroxynonenal (4-HNE) and propenal (acrolein) which exacerbates ROS/RNS production and oxidative protein damage in the injured brain. Ultimately, this results in intracellular Ca²⁺ overload that activates proteolytic degradation of α-spectrin, a neuronal cytoskeletal protein. Therefore, the aim of this study was to establish the temporal evolution of mitochondrial dysfunction, oxidative damage and cytoskeletal degradation in the brain following a severe controlled cortical impact (CCI) TBI in young male adult rats. In mitochondria isolated from an 8mm diameter cortical punch including the 5 mm wide impact site and their respiratory function studied ex vivo, we observed an initial decrease in complex I and II mitochondrial bioenergetics within 3 hours (h). For complex I bioenergetics, this partially recovered by 12–16h, whereas for complex II respiration the recovery was complete by 12h. During the first 24h, there was no evidence of an injury-induced increase in LP or protein nitration in mitochondrial or cellular homogenates. However, beginning at 24h, there was a gradual secondary decline in complex I and II respiration that peaked at 72h. post-TBI that coincided with progressive peroxidation of mitochondrial and cellular lipids, protein nitration and protein modification by 4-HNE and acrolein. The oxidative damage and respiratory failure paralleled an increase in Ca²⁺-induced proteolytic degradation of the neuronal cytoskeletal protein α-spectrin indicating a failure of intracellular Ca²⁺ homeostasis. These findings of a surprisingly delayed peak in secondary injury, suggest that the therapeutic window and needed treatment duration for certain antioxidant treatment strategies following CCI-TBI in rodents may be longer than previously believed.