Impaired Expression of Neuroprotective Molecules in the HIF-1α Pathway following Traumatic Brain Injury in Aged Mice

Elderly traumatic brain injury (TBI) patients have higher rates of mortality and worse functional outcome than non-elderly TBI patients. The mechanisms involved in poor outcomes in the elderly are not well understood. Hypoxia-inducible factor-1 alpha (HIF-1 alpha) is a basic helix-loop-helix transcription factor that modulates expression of key genes involved in neuroprotection. In this study, we studied the expression of HIF-1 alpha and its target survival genes, heme oxygenase-1 (HO-1), vascular endothelial growth factor (VEGF), and erythropoietin (EPO) in the brains of adult versus aged mice following controlled cortical impact (CCI) injury. Adult (5-6 months) and aged (23-24 months) C57Bl/6 mice were injured using a CCI device. At 72 h post-injury, mice were sacrificed and the injured cortex was used for mRNA and protein analysis using real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting protocols. Following injury, HIF-1 alpha, HO-1, and VEGF showed upregulation in both the young and aged mice, but in the aged animals the increase in HIF-1 alpha and VEGF in response to injury was much lower than in the adult injured animals. EPO was upregulated in the adult injured brain, but not in the aged injured brain. These results support the hypothesis that reduced expression of genes in the HIF-1 alpha neuroprotective pathway in aging may contribute to poor prognosis in the elderly following TBI.