P4-371: Microglia and high mobility group box protein-1 in AD pathology

Abstract

One of the pathological features of AD is the extracellular deposition of fibrillar amyloid–β (Aβ) and its compaction into senile plaques that are associated with reactive microglia. Although the role of microglia in AD pathology has not been clarified, their involvement in Aβ clearance has been noted. On the other hand, high mobility group box protein–1 (HMGB1) is an abundant non–histone chromosomal protein. The involvement of HMGB1 in AD pathology is unknown. We investigated roles of microglia and HMGB1 in AD pathology using AD rodent models. Western blot analysis was performed to measure the expression levels of HMGB1 in normal and AD brains. Localizations of HMGB1 in brains of transgenic AD mouse models and Aβ1–42 (Aβ42)–injected and kainic acid (KA)–injected rat were investigated by immunohistochemistry. Laser confocal microscopic analysis and ELISA were applied to examine the effects of extracellular HMGB1 on Aβ phagocytosis of cultured rat microglia and Aβ clearance in Aβ42–injected rat brain, respectively. The effect of extracellular HMGB1 on Aβ neurotoxicity was evaluated using Aβ42–injected rat brain by immunohistochemistry. The binding ability to Aβ and effect on Aβ aggregation of HMGB1 were investigated by immunoprecipitation and western blot analysis. HMGB1 was diffusely associated with senile plaques and the total protein level significantly increased in AD brain. In Aβ42 and KA–injected rat hippocampi, diffuse HMGB1 immunoreactivity was observed around dying neurons. HMGB1 was also co–localized with Aβ in the Aβ42–injected rats but not in transgenic mice which show massive Aβ production without neuronal loss in their brains. In the in vitro study, HMGB1 inhibited microglial Aβ phagocytosis in rat microglial culture. In cell free system, HMGB1 bound to Aβ42 and stabilized Aβ42 oligomers which are thought to be more neurotoxic than their monomer or mature fibrils. Subsequently, co–injection of HMGB1 delayed the clearance of Aβ42 and accelerated neurodegeneration in Aβ42–injected rat brain. HMGB1 released from dying neurons may inhibit microglial Aβ42 clearance and enhance the neurotoxicity of Aβ42. Therefore, removal or suppression of HMGB1 may be another optimal target in the investigation of a therapeutic strategy for AD.