Abstracts

Rationale: Severe mesial temporal lobe epilepsy (MTLE) is associated with hippocampal sclerosis, mossy fiber sprouting, and death of CA1 pyramidal neurons and inhibitory interneurons in both CA1 and the dentate gyrus of the hippocampus. The goals of stem cell-based therapies for MTLE are to replace lost cell types, suppress seizures, and reverse cognitive impairments. As a first step, we assessed the ability of fetal GABAergic precursor cell grafts to prevent mossy fiber sprouting and suppress spontaneous recurrent seizures (SRS) in an experimental model of MTLE. Methods: To allow for cell tracking post-transplantation, stem cell grafts were isolated from donor embryonic day 13.5 medial ganglionic eminence (MGE) and nucleofected with a plasmid encoding Red Fluorescence Protein (RFP) (Amaxa Nucleoporator). The fetal MGE progenitors were then cultured in media containing pan-caspase inhibitor and brain derived neurotrophic factor. Approximately 100,000 fetal MGE-derived progenitors were transplanted bilaterally into the hilus of adult male C57Bl/6 mice (Harlan) with SRS induced by systemic injections of pilocarpine (280 mg/kg, i.p.). Control mice with SRS received stereotaxic injections of media devoid of cells. The incidence of SRS was then monitored in both treatment groups for approximately 3 months by means of continuous video and electroencephalography (V-EEG) using the Stellate Harmonie E system. Results: Mice with fetal stem cell transplants showed significantly reduced seizure frequency (p = 0.008) 61-80 days after SE. To examine the possible mechanisms for the disease-modifying effect of the grafts, we characterized the locations, distributions, and neurochemical properties of the transplanted cells. The grafts contained a high proportion of GABAergic interneurons expressing the calcium binding protein, parvalbumin, or somatostatin. Extensive axonal projections from the grafts innervated the host hippocampus and dentate gyrus, suggesting that the grafted cells integrated into the dentate gyrus. To further investigate this possibility, the extent of granule cell inhibition was compared in living hippocampal slices from control mice and slices from mice with MGE-derived fetal progenitor cell transplants. Additional studies examined whether the extent of mossy fiber sprouting was reduced in mice exhibiting seizure suppression, compared with control mice with SRS and no attenuation of seizures. Conclusions: Taken together, the findings suggest that transplants of fetal GABAergic progenitors from the MGE suppress SRS in adult mice by increasing synaptic inhibition in the dentate gyrus.