Abstracts

Rationale: In order to identify molecular changes underlying epileptogenesis, we investigated time-dependent gene expression in the immature and mature hippocampus by capitalizing on age-dependent responses to kainic acid (KA). In adult rats, KA-induced status epilepticus(SE) causes hippocampal neurodegeneration, followed by a latency period between injury and recurrent seizures, and finally a chronic epileptic state. In contrast, neither cell death nor chronic spontaneous seizures occur in immature rats after KA. Using the P30 response as our seizure-induced epileptogenic model, we aimed to unveil the gene expression difference of an epileptogenic versus non-epileptogenic seizure. We hypothesize that these differentially expressed genes will reflect the changes occurring during the latency period. Methods: Oligonucleotide gene arrays were utilized to investigate transcriptional regulation at five time points following KA-induced SE at P15 and P30: 1, 6, 24, 72, and 240 h. Total RNA from hippocampi were isolated after KA-induced SE and microarray analysis was performed. Gene analysis methods used Mathematica and the AffyGO gene oncology database, and gene expression was quantified using fold change. Only genes showing a fold change of 2 (p<0.05) or greater were picked. Graphs were fitted using third order interpolation in Mathematica. Results: A total of 437 genes were significantly regulated in P15 or P30 hippocampi after KA-induced SE. Expression of genes peaking at earlier time points (1-24h) was associated with protein phosphorylation (n=95), transcription (n=57), and excitotoxicity (n=25); expression of genes peaking throughout the 10 days (1-240h) was associated with apoptosis (n=67), cell proliferation (n=46), cell cycle (n=32), inflammation (n=25), and glial response (n=7); expression of genes associated with synaptogenesis/synaptic response (n=71) and angiogenesis (n=12) peaked within the first 24 hours and again at 10 days. MAPK signaling, comprising 125 differentially expressed genes, was the largest functional group identified. In P30 animals, the ERK (n=52), p38 (n=35), and JNK (n= 16) signaling associated genes showed upregulation throughout the 10 days (1-240h), while in P15 animals , the ERK (n= 30), p38 (n=19), and JNK (n= 9) signaling associated genes showed upregulation only during the first 24 hours. Both age groups showed MAPK downregulation during the first 24 hours only. Conclusions: Genes associated with phosphorylation and transcription are regulated within hours while those associated with apoptosis, cell proliferation, and inflammation show delayed expression until days after intial epileptogenic insult. SE appears to cause time-dependent waves of molecular changes, predisposing the brain to spontaneous recurrent seizures. We found an overwhelming and sustained differential expression of genes related to MAPK (ERK, p38 and JNK) signaling pathway. MAPK may be the candidate signaling pathway underlying epileptogenesis as it is well-poised to translate acute transient stimuli such as seizures to enduring changes such as epilepsy via transcriptional regulation.