Introduction: Neurogenesis in the dentate gyrus of the hippocampus is conserved and highly regulated throughout the lifespan. It relies on neural stem cells – radial glia-like cells (RGL) – to proliferate or remain in quiescent state. Fatty acid oxidation (FAO) is important in the regulation of RGL activity. Specifically, FAO is prominent in the quiescent state, while de novo fatty acid synthesis is high during proliferation. Severe TBI is characterized by increased proliferation in the hippocampus, however, the extent of proliferation following mild TBI (mTBI) and the role of FAO were not delineated. We hypothesized that loss of FAO via genetic deletion of mitochondrial carnitine palmitoyl transferase 2 (Cpt2) will augment post-traumatic proliferation by activating RGL.
Methods: 9-10 weeks old Nestin-Cre/Cpt2 f/f (FAO knockout mice) were subjected to a concussion model of TBI – a traumatic impact on the intact skull over the midline suture at 1.5mm depth using a pneumatic CCI device. BrdU (50mg/kg ip x 3injections) was administered: 1) 24hr prior to surgery; 2) 1day, 3 days or 7 days after TBI and immunohistochemical analysis was performed 24hr, 3days and 28 days after TBI (n=5/group).
Results: Following TBI there was an increase in proliferation and no differences were observed between Nestin-Cre/Cpt2 f/f and Cpt2 f/f (control) mice, quantified by presence of Ki67(+) cells; Sox2 (+) cells and Ki67/Sox2(+) cells in DG. Following TBI, mice injected with BrdU 24hr prior to injury had comparable survival in both Nestin-Cre/Cpt2 f/f and Cpt2 f/f TBI, and most of these cells differentiated into astrocyte at 28days post-injury. Proliferation post-injury was assessed using BrdU injections at 1day, 3days and 7days postTBI and quantified by presence of BrdU(+)/Ki67(+). Prolifeation peaked at 3 days post TBI and was significantly increased in Nestin-Cre/Cpt2 f/f vs Cpt2 f/f injured mice. At 24hr after TBI, both Nestin-Cre/Cpt2 f/f and Cpt2 f/f groups had significantly lower number of quiescent cells quantified by Sox2(+)/GFAP(+) in the subgranular zone compared to sham-injured animals. There was also a significant decrease in the number of quiescent RGL in Nestin-Cre/Cpt2 f/f TBI animals compared to injured Cpt2 f/f.
Conclusion: Our current results suggest that mTBI results in increased proliferation in the hippocampus. Loss of brain FAO results in increased proliferation then injured wild type and demonstrate decreased number of quiescent cells after mTBI.
