Abstracts

Synaptic reorganization after neural injury may cause recurrent excitatory networks and the development of spontaneous recurrent seizures. Axon guidance cues including the semaphorins may participate in the synaptic reorganization and epileptogenesis. It is unknown how upstream regulatory pathways influence semaphorin signaling and its subsequent physiologic effects on epileptogenesis. Neurotrophin signaling thru BDNF and NGF plays a role in the molecular mechanisms of epileptogenesis. Through the use of genetic murine models, we have identified the semaphorins as one possible subset of neurotrophin regulated genes that participate in epileptogenesis. FVB/NJ but not C57Bl/6J mice show kainic acid status epilepticus (KA-SE), increased cell death of CA3/Hilar neurons, decreased hippocampal gene expression of Semaphorin 3F ligand and receptor (Neuropilin 2) systems, axonal sprouting of hippocampal neurons, and epilepsy. Initial microarray experiments identified MAP kinase signaling pathways as the single regulated second messenger system which may modulate transcriptional regulatory complexes in hippocampal neurons of FVB/NJ mice 7 days after KA-SE. To evaluate the role of neurotrophin induced MAP kinase signaling on semaphorin/neuropilin gene expression, we have used adult murine hippocampal slice cultures from FVB/NJ or C57Bl/6J mice in the presence or absence of 100 ng/ml BDNF, NGF, or NT-3. NGF evokes a 5-10 fold (p[lt]0.0001) elevation of NPN2 gene expression in hippocampal cultures from FVB/NJ. Similar to kainic acid, both BDNF and NT-3 induce a 70-90% reduction (p[lt]0.001) of NPN2 gene expression in FVB/NJ cultures. None of three neurotrophins affected NPN2 gene expression in hippocampal neurons from C57Bl/6J cultures. In contrast, all three neurotrophins reduce neuropilin 1 gene expression by 80% (p[lt]0.02) irregardless of the strain origin of the hippocampal slice cultures. In vivo, disruption of NPN2 gene in FVB/NJ heterozygous NPN2 KO mice has so far prevented the development of spontaneous seizures three months post KA-SE. In contrast, their wild type FVB/NJ littermates have developed spontaneous seizures. In summary, we hypothesize that BDNF signaling thru MAP kinase pathways regulate semaphorin and neuropilin gene expression to promote synaptic reorganization and development of seizures in FVB/NJ mice after KA-SE. If confirmed, this work suggests that neurotrophin signaling may contribute to the distinct responses to neural injury in mouse strains from varied genetic backgrounds. (Supported by American Epilepsy Society [amp] Kentucky Spinal Cord/Head Injury Research Trust)