HIF-1 alpha-deficient mice have increased brain injury after neonatal hypoxia-ischemia

Evidence suggests that the activation of the transcription factor hypoxia-inducible factor 1 alpha (HIF-1 alpha) may promote cell survival in hypoxic or ischemic brain. To help understand the role of HIF-1 alpha in neonatal hypoxic-ischemic brain injury, mice with conditional neuron-specific inactivation of HIF-1 alpha underwent hypoxia-ischemia (HI). Mice heterozygous for Cre recombinase under the control of the calcium/calmodulin-dependent kinase II promoter were bred with homozygous 'floxed' HIF-1 alpha transgenic mice. The resulting litters produced mice with a forebrain predominant neuronal deletion of HIF-1 alpha (HIF-1 alpha(Delta)/(Delta)), as well as littermates without the deletion. In order to verify reduction of HIF-1 alpha at postnatal day 7, HIF-1 alpha(Delta)/(Delta) and wild-type mice were exposed to a hypoxic stimulus (8% oxygen) or room air for 1 h, followed by immediate collection of brain cortices for determination of HIF-1 alpha expression. Results of Western blotting of mouse cortices exposed to hypoxia stimulus or room air confirmed that HIF-1 alpha(Delta)/(Delta) cortex expressed a minimal amount of HIF-1 alpha protein compared to wild-type cortex with the same hypoxic stimulus. Subsequently, pups underwent the Vannucci procedure of HI at postnatal day 7: unilateral ligation of the right common carotid artery followed by 30 min of hypoxia (8% oxygen). Immunofluorescent staining of brains 24 h after HI confirmed a relative lack of HIF-1 alpha in the HIF-1 alpha(Delta)/(Delta) cortex compared to the wild type, and that HIF-1 alpha in the wild type is located in neurons. HIF-1 alpha expression was determined in mouse cortex 24 h after HI. Histological analysis for the degree of injury was performed 5 days after HI. HIF-1 alpha protein expression 24 h after HI showed a large increase of HIF-1 alpha in the hypoxic-ischemic cortex of the wild-type compared to the hypoxic only cortex. Histological analysis revealed that HI injury was increased in the neuronally deficient HIF-1 alpha(Delta)/(Delta) mouse brain (p < 0.05) and was more severe in the cortex. Genetic reduction of neuronal HIF-1 alpha results in a worsening of injury after neonatal HI, with a region-specific role for HIF-1 alpha in the setting of neonatal brain injury.