Abstracts

Rationale:
Traumatic brain injury (TBI) is a known risk factor for epilepsy and has been shown to be associated with altered neurogenesis in the adult neurogenic niche of the hippocampal dentate gyrus. However, how immature adult-born granule cell (abGC) generated after trauma impact the circuit and the timeline for their circuit effects are not known. Recently, we reported that suppressing neurogenesis after trauma altered dentate excitability as early as a week after injury (Neuberger et al., 2017). The current study was conducted in the lateral fluid percussion injury (FPI) model of concussive brain trauma in mice to identify the maturation and circuit effects of abGCs induced by brain injury and determine their role in posttraumatic neuropathology.
Method:
Wild-type or tamoxifen inducible Nestin-creERT2-ChR2-YFP mice on C57Bl6 background were subject to sham or moderate (1.5 atm) FPI at 8-10 weeks of age.  Cre-recombinase was induced by injecting Nestin-ChR2 mice with tamoxifen once daily for 3 days starting 2 hours after SHAM/FPI. A group of mice received BrdU (i. p) three injections at 2 hour intervals starting 2 hours after injury. Mice were sacrificed 4 hrs, 3 days and 7 days after SHAM/FPI followed by immunostaining and stereological quantification of Fluorojade-C (FJC) for cell death, doublecortin (DCX) to examine immature abGCs and BrdU/NeuN to assess maturation. Optically-evoked abGC mediated synaptic currents were recorded from unlabeled mature granule cells in acute hippocampal slices from Nestin-ChR2 mice prepared 1 week after SHAM/FPI. Cells were recoded in voltage clamp using a cesium-methansulfonate based internal solution and held at -70mV or 0mV to isolate excitatory and inhibitory currents respectively.  Circuit integration of abGCs were assessed in vivo by examining effect of optical activation of abGCs  on afferent evoked dentate local field potentials (LFP) one week after injury.
Results:
The consistent presence of  FJC in the dentate hilus and cell layer 4 hours of FPI and not in sham injured mice confirmed reliability of injury model. DCX labeling identified a significant increase in aberrant migration of immature abGCs to the outer 2/3rd’s of the granule cell layer three days post-injury (percentage ectopic cells: SHAM= 4.6 ±1.8, FPI=16.8 ±4.7, N=4 mice/group, p=.009 by t test).  The number of BrdU labeled cells as well as the percent of BrdU cells expressing NeuN, a marker for mature neurons, were increased one week after FPI.  Optically activating abGCs born after injury, evoked monosynaptic excitatory currents in 100% of mGCs in slices from FPI mice as early as one week post-injury and in none of the cells from SHAM controls.  Optically-evoked, abGC-mediated IPSCs were observed in 57 % of SHAM mGCs (4 of 7 cells in 3 mice) and in all mGCs in FPI mice (7 cells in 3 mice), and showed a significant increase in amplitude in recordings from FPI mice.  In vivo stimulus evoked LFPs revealed increase in  excitability and loss of paired-pulse facilitation 1 week after FPI. Optical activation of injury-induced abGCs selectively suppressed stimulus evoked pop-spike amplitude in FPI mice.
Conclusion:
Brain injury leads to accelerated neurochemical maturation and ectopic migration of adult born granule cells generated after trauma. The abGCs generated after injury integrate into the circuit and influence network activity within a week post-mitosis much earlier than expected based on the normal timeline for maturation and circuit integration of abGCs.
Funding:
:NIH/NINDS F31NS110220 to L.C, R01NS069861 and R01NS097750 to VS