Abstracts

Rationale: We discovered a novel proinflammatory/proconvulsant pathway involving the release of the danger signal High Mobility Group Box 1 (HMGB1) from neurons and glia and its interaction with Toll-like receptor 4 (TLR4), a key receptor of innate immunity, overexpressed by neurons and astrocytes following proconvulsant challenges. We now explore the role in ictogenesis and epileptogenesis of RAGE, a receptor also stimulated by HMGB1 and S100β, a protein released by astrocytes. Methods: C57BL/6 adult male wild-type (WT) or RAGE-/- mice were injected unilaterally in the hippocampus either with 7 ng of kainic acid to induce acute recurrent seizures for about 90 min, or with 200 ng of kainate to induce status epilepticus (SE) followed by spontaneous recurrent (drug-resistant) epileptic activity (n=6-8 each exp group). We studied in WT and RAGE-/- mice ictogenesis and epilepsy development by EEG analysis, cognitive performance, neuropathology and neurogenesis. Results: RAGE-/- mice displayed an average 30% reduction in EEG seizure activity vs WT mice following intrahippocampal injection of 7 ng kainic acid. Intrahippocampal injection of 10 µg HMGB1 in WT mice increased the number and duration of kainate seizures by 2.5-fold on average, and reduced their onset time by 50%. These proconvulsant effects were reduced by 2-fold in RAGE-/- mice, except for seizures onset which remained anticipated as in WT mice. SE developed similarly in WT and RAGE-/- mice, as assessed by EEG analysis of frequency and total duration of spike activity. The time to onset of spontaneous epileptic activity following SE was similar in both mouse strains. However, the time spent in epileptic activity and the number of paroxysmal episodes were both significantly decreased (p<0.05) in RAGE-/- vs WT mice. Both strains of epileptic mice showed similar cognitive deficits in the novel object recognition test. Histological brain evaluation in epileptic RAGE-/- mice showed increased neurodegeneration specifically in CA1 pyramidal cells while cell loss was similar to WT in the other hippocampal regions. Granule cell dispersion and neurogenesis were similarly affected in both strains of epileptic mice. The number of S100β−positive astrocytes in the hippocampus was increased in epileptic WT mice, like in human mTLE tissue, but not in epileptic RAGE-/- mice. Immunohistochemical studies showed evidence of RAGE induction in both astrocytes and microvessels in human and experimental mTLE hippocampi. Conclusions: RAGE signaling is activated in human and experimental epileptic tissue. RAGE contributes to the mechanisms of ictogenesis, and mediates the proconvulsant effects of HMGB1 in concert with TLR4. RAGE activation concurs both to spontaneous epileptic activity and CA1 cell loss developing after SE. Our findings unveil a new molecular mechanism of ictogenesis and epileptogenesis, supporting its involvement in human epilepsy.