Astrocyte-derived ApoE is Required for the Maturation of Injury-induced Hippocampal Neurons and Regulates Cognitive Recovery After Traumatic Brain Injury

Polymorphisms in the apolipoprotein E (ApoE) gene confer a major genetic risk for the development of late-onset Alzheimers disease (AD) and are predictive of outcome following traumatic brain injury (TBI). Alterations in adult hippocampal neurogenesis have long been associated with both the development of AD and recovery following TBI, and ApoE is known to play a role in this process. In order to determine how ApoE might influence hippocampal injury-induced neurogenesis, we developed a novel conditional system whereby functional ApoE from astrocytes was ablated just prior to injury. While successfully ablating 90% of astrocytic ApoE just prior to a closed cortical impact injury in mice, we observed an attenuation in the development of newly born neurons using a GFP-expressing retrovirus, but not in existing hippocampal neurons visualized with a Golgi stain. Intriguingly, animals with a "double-hit", i.e. injury and ApoE conditionally inactivated in astrocytes, demonstrated the most pronounced impairments in the hippocampal-dependent Morris water maze test, failing to exhibit spatial memory after both acquisition and reversal training trials. In comparison, conditional knockout mice without injury displayed impairments but only in the reversal phase of the test, suggesting accumulative effects of astrocytic ApoE deficiency and traumatic brain injury on AD-like phenotypes. Together, these findings demonstrate that astrocytic ApoE is required for functional injury-induced neurogenesis following traumatic brain injury. Significance StatementApoE has long been implicated in the development of Alzheimers disease and recovery from traumatic brain injury via unknown mechanisms. Using a novel conditional ablation model of mouse ApoE and subsequent tracing of individual hippocampal neurons, we demonstrate its requirement in injury-induced neurogenesis for proper dendritic arborization and cognitive function in hippocampal-dependent learning and memory tasks.