Abstracts

Rationale: Epileptogenesis in severe Temporal Lobe Epilepsy (TLE) is linked to cell death of hilar GABAergic interneurons and aberrant patterns of neurogenesis of granule cells (GCs) in dentate gyrus (DG). In the mouse pilocarpine model of TLE, a large number of newborn GCs migrate ectopically into the hilus, undergo mossy fiber sprouting, and retain hilar-projecting basal dendrites. These changes lead to the formation of recurrent excitatory synapses among the GCs, resulting in hyperexcitability in limbic circuitry and spontaneous seizures. In mice with pilocarpine-induced TLE, we demonstrated previously that DG transplants of GABAergic interneuron progenitors derived from E13.5 Medial Ganglionic Eminence (MGE) synaptically integrate and suppress seizures (Henderson, Gupta et al, 2014). Our previous optogenetic interrogation of these transplants in hippocampal slices demonstrated that the transplanted interneurons formed robust functional inhibitory synapses onto the GCs in TLE mice. Here, we investigated whether MGE transplants in the DG of TLE mice selectively wire with populations of newborn GCs born after status epilepticus (SE).Methods: To identify GCs born during epileptogenesis, we made bilateral injections of a Moloney Murine Leukemia Virus (MoMLV) -based retrovirus into the subgranular zone of the DG one week after SE. MoMLV retroviral vectors require mitosis for cell transduction and have been previously used to label adult-born GCs (van Praag et al., 2002). We injected a mCherry-expressing retrovirus created using a pSie-based viral backbone with a ubiquitin promoter driving fluorophore expression followed by a woodchuck post-transcriptional regulatory element (Luikart et al., 2011). One week following retrovirus injections, we transplanted 100,000 MGE cells harvested from VGAT-ChR2 embryos into the hilus of each DG in TLE mice. To determine whether transplanted interneurons selectively form synapses onto adult-born GCs, we optogenetically stimulated the ChR2-expressing MGE cell transplants in hippocampal slices while simultaneously making electrophysiological recordings from the retrovirally-labeled GCs.Results: Light-mediated activation of ChR2-expressing transplanted cells induced strong inhibitory postsynaptic currents in the retrovirally-labeled GCs, including populations of retrovirally-labeled hilar ectopic GCs. The patterns of transplant-derived synaptic innervation onto newborn GCs were further examined by high resolution confocal microscopic imaging and single cell reconstructions (N=8). Quantitative analyses revealed an average of 150 putative inhibitory synaptic sites from the MGE transplants. Most of these putative inhibitory synapses were located on the secondary and tertiary dendrites of the GCs.Conclusions: Taken together, these data indicate that MGE cell transplants target newborn GCs in epileptic mice, and thereby provide strong post-synaptic inhibition onto the populations of newborn GCs most likely to undergo ectopic migration and develop hyperexcitable circuitry.