25-Hydroxycholesterol modulates tau-mediated neurodegeneration and microglial chemotaxis and phagocytosis.

Abstract

Immune activation is an important component of Alzheimer disease (AD) pathology, with microglia playing a critical role in driving ApoE-dependent tau-mediated neurodegeneration. The oxysterol 25-hydroxycholesterol (25-HC) is an established potent regulator of peripheral innate immune response, though less is known regarding its role in CNS neuroimmune pathobiology. The enzyme that synthesizes 25-HC, cholesterol 25-hydroxylase (CH25H), is exclusively expressed by microglia in the CNS and is significantly upregulated in human AD brain and in transgenic AD mouse models. We have recently reported pro-inflammatory effects of 25-HC in ApoE4-expressing mouse microglia, including increased microglial IL-1β secretion and inflammasome activation. In this study we explored the role of 25-HC in ApoE-dependent tau-mediated neurodegeneration. PS19 tauopathy mice previously crossed with ApoE targeted replacement (TR) mice were subsequently crossed with CH25H KO mice. Hippocampal (HC), entorhinal and pyriform cortical (EC/PC) and ventricular volumes were measured at 9 months of age. In vitro primary cell cultures containing neurons, mixed glia or microglia were utilized to evaluate effects of 25-HC or 7α,25-dihydroxycholesterol (7α,25-diHC), on neurotoxicity, microglial cytokine secretion, chemotaxis and phagocytic activity. We found that in female tau mice expressing human ApoE4, genetic deletion of CH25H was protective (increased EC/PC volumes and reduced ventricular volumes). Similar trends were observed in male mice though these were not statistically significant. In a neuronal-glial co-culture model of ApoE4-mediated neurotoxicity, 25-HC protected against neurite loss and neuronal death at low concentrations but was potently neurotoxic at higher concentrations. In primary microglial cultures, 25-HC modulated secretion of a number of chemokines and provoked a potent microglial chemotaxic response that was most robust in microglia derived from CH25H KO, ApoE4-TR or ApoE4-TR-CH25H KO mice. 7α,25-diHC also stimulated chemotaxis but less robustly than 25-HC. Assays testing the effect of 25-HC and 7α,25-diHC on phagocytosis in microglia were also performed, and these data will be presented. 25-HC appears to modulate tau-mediated neurodegeneration in an ApoE-dependent manner, especially in female mice, possibly through effects on critical microglial cellular functions. CH25H and 25-HC may be viable therapeutic targets for AD-related neuroimmune activation.