Abstracts

RATIONALE: The activating transcription factor 2 (ATF-2) protein, a constitutively expressed transcription factor, is essential for the normal development of mammalian brain. ATF-2 is activated by c-Jun N-terminal kinase (JNK) and modulates both the induction of the c-jun gene and the function of the c-Jun protein, a mediator of neuronal death and survival. Lithium-pilocarpine induced status epilepticus (LPSE) causes neuronal cell death that has features of both necrosis and apoptosis. Since LPSE-induced neuronal death is both selective and age-dependent, we investigated the expression of the c-Jun and ATF-2 protein in the immature and adult rat hippocampus to begin to understand their roles in LPSE-induced neuronal death.
METHODS: Lithium chloride (3 mEq/kg) followed 24 h later by pilocarpine (35 mg/kg) was administered intraperitoneally in the 10-day (P10), 21-day-old (P21) and adult Sprague-Dawley rats. Saline injected animals served as control. The expression of c-Jun and ATF-2 protein was assessed by immunohistochemistry (N=3, each group) and Western blot (N=3, each group) in the hippocampus isolated at various times (4, 24, 72 hours) after LPSE. Neuronal injury was assessed by silver stain and cresyl violet stain (N= 6, each group).
RESULTS: The early induction of c-Jun measured at 4 h after the onset of seizures was present in dentate gyrus, CA1, and CA3 of hippocampus in all age groups studied but c-Jun immunoreactivity was most robust in adults and weakest in P10. The late induction of c-Jun measured at 24 h was absent from the P10 and weakly present in CA1 and CA3 of the hippocampus of P21 and adult rats. The expression of ATF-2 at 4 h after seizure was slightly increased in dentate gyrus, CA1, and CA3 of hippocampus in all age groups. ATF-2 immunoreactivity of adult rats at 24 and 72 h was significantly decreased in CA3 and CA1 of hippocampus than that in P10 or P21 rats. Western blot revealed that c-Jun and ATF-2 immunoreactivity were both increased at 4 h after seizure and returned basal level by 24 h in all three age groups. C-Jun protein expression was the highest in adult rats but ATF-2 protein expression was highest in P10 rats. The severity of neuronal injury by silver stain and cresyl violet stain at 72 h after the LPSE was in the following order: CA1[gt]CA3[gt]dentate gyrus; and adults[gt]P21[gt][gt]P10 (no damage).
CONCLUSIONS: C-Jun immunoreactivity increases but ATF-2 immunoreactivity decreases as the brain matures. P10 rats that had the highest ATF-2 and lowest c-Jun expression showed no neuronal damage, whereas the adult rats with the lowest ATF-2 and highest c-Jun expression suffered the most neuronal damage. Since c-Jun and ATF-2, both activated by JNK, are targets and competitors in the same signal-transduction cascade, one could speculate that ATF-2 may be competing with c-Jun for JNK phosphorylation. These results suggest a neuroprotective role of ATF-2 in this maturation-related evolution of neuronal cell death from status epilepticus.
[Supported by: NS37125 and Mayo Foundation]