Neural Repair by Enhancing Endogenous Hippocampal Neurogenesis Following Traumatic Brain Injury

Abstract

Traumatic brain injury (TBI) is a critical public health issue in the United States, affecting about 2.8 million people annually. Extensive cell death and neural degeneration directly and diffusively caused by the initial mechanical insult results in a wide range of neurological complications post-trauma. Learning and memory dysfunction is one of the most common complains. Hippocampal neuronal loss, together with other mechanisms, largely contributes to learning and memory impairment as well as other cognitive dysfunctions post-trauma. To date, no FDA-approved drug is available to target cell death or improve learning and memory following TBI. It is of great interest to develop alternative approaches targeting neural repair instead. Neural stem/progenitor cells (NSCs) in the adult hippocampus undergo life-long neurogenesis supporting learning and memory functions, thus hold great promise for post-traumatic neuronal replacement. The previous studies demonstrated that TBI transiently increase NSC proliferation. However, it is debated on whether TBI affects neurogenesis. The mechanism of TBI-enhanced NSC proliferation remains elusive. In the current studies, I have investigated post-traumatic neurogenesis after different injury severities, evaluated integration of post-injury born neurons, illustrated a molecular mechanism mediating TBI-enhanced NSC proliferation, proposed a de novo state of NSCs, and tested effects of a pharmacological approach on spatial learning and memory function recovery. My results demonstrated that post-traumatic neurogenesis is affected by injury severities, partially explained the pre-existing inconsistency among works from different groups. Post-injury born neurons integrate in neural network and receive local and distal inputs. TBI promotes functional recruitment of post-injury born neurons into neural circuits. Mechanistically, mechanistic target of rapamycin (mTOR) pathway is required primarily for TBI-enhanced NSC proliferation; NSCs feature a de novo alert state, in which NSCs are reversibly released from quiescence and primed for proliferation. Furthermore, my data demonstrated a beneficial role of ketamine in improving post-traumatic spatial learning possibly by activating mTOR signal in NSCs and/or promoting neuronal activity of post-injury born neurons. Together, my data support the feasibility of neurogenesis mediated neuronal replacement, provide a target for enhancing post-traumatic NSC proliferation and subsequent neurogenesis, and prove a potential pharmacological approach benefiting post-traumatic functional recovery in learning and memory.