No abstract

Abstract. Alzheimer's disease (AD) is a neurodegenerative process involving amyloid-␤ (A␤) peptide deposition, neuroinflammation, and progressive memory loss. Here, we evaluated whether oral administration of retinoic acid receptor (RAR)␣,␤ agonist Am80 (tamibarotene) or specific retinoid X receptor (RXR) pan agonist HX630 or their combination could improve deficits in an AD model, 8.5-month-old amyloid-␤ protein precursor 23 (A␤PP23) mice. Co-administration of Am80 (0.5 mg/kg) and HX630 (5 mg/kg) for 17 days significantly improved memory deficits (Morris water maze) in A␤PP23 mice, whereas administration of either agent alone produced no effect. Only co-administration significantly reduced the level of insoluble A␤ peptide in the brain. These results thus indicate that effective memory improvement via reduction of insoluble A␤ peptide in 8.5-month-old A␤PP23 mice requires co-activation of RAR␣,␤ and RXRs. RAR␣-positive microglia accumulated A␤ plaques in the A␤PP23 mice. Rat primary microglia co-treated with Am80/HX630 showed increased degradation activity towards 125 I-labeled oligomeric A␤ 1-42 peptide in an insulin-degrading enzyme (IDE)-dependent manner. The co-administration increased mRNA for IDE and membrane-associated IDE protein in vivo, suggesting that IDE contributes to A␤ clearance in Am80/HX630-treated A␤PP23 mice. Am80/HX630 also increased IL-4R␣ expression in microglial MG5 cells. The improvement in memory of Am80/HX630-treated A␤PP23 mice was correlated with the levels and signaling of hippocampal interleukin-4 (IL-4). Therefore, Am80/HX630 may promote differentiation of IL-4-responsive M2-like microglia and increase their activity for clearance of oligomeric A␤ peptides by restoring impaired IL-4 signaling in A␤PP23 mice. Combination treatment with RAR and RXR agonists may be an effective approach for AD therapy.

The purpose of this study is to investigate whether a synthetic retinoid Am80 (tamibarotene) exhibits any improving effects on amyloid precursor protein (