Apolipoprotein E Regulates Injury-Induced Activation of Hippocampal Neural Stem and Progenitor Cells

Partial recovery from even severe traumatic brain injury (TBI) is ubiquitous and occurs largely through unknown mechanisms. Recent evidence suggests that hippocampal neural stem/progenitor cell (NSPC) activation and subsequent neurogenesis are responsible for at least some aspects of spontaneous recovery following TBI. Apolipoprotein E (ApoE) regulates postnatal neurogenesis in the hippocampus and is therefore a putative mediator of injury-induced neurogenesis. Further, ApoE isoforms in humans are associated with different cognitive outcomes following TBI. To investigate the role of ApoE in injury-induced neurogenesis, we exposed wild-type, ApoE-deficient, and human ApoE isoform-specific (ApoE3 and ApoE4) transgenic mice crossed with nestin-green fluorescent protein (GFP) reporter mice to controlled cortical impact (CCI) and assessed progenitor activation at 2 d post-injury using unbiased stereology. GFP+ progenitor cells were increased by approximately 120% in the ipsilateral hippocampus in injured wild-type mice, compared with sham mice (p<0.01). Co-localization of GFP+ cells with bromodeoxyrudine (BrdU) to label dividing cells indicated increased proliferation of progenitors in the injured hippocampus (p<0.001). This proliferative injury response was absent in ApoE-deficient mice, as no increase in GFP+ cells was observed in the injured hippocampus, compared with sham mice, despite an overall increase in proliferation indicated by increased BrdU+ cells (86%; p<0.05). CCI-induced proliferation of GFP+ cells in both ApoE3 and ApoE4 mice but the overall response was attenuated in ApoE4 mice due to fewer GFP+ cells at baseline. We demonstrate that ApoE is required for injury-induced proliferation of NSPCs after experimental TBI, and that this response is influenced by human APOE genotype.