Human APOE4 increases microglia reactivity at Aβ plaques in a mouse model of Aβ deposition

Abstract

BackgroundHaving the apolipoprotein E4 (APOE-ϵ4) allele is the strongest genetic risk factor for the development of Alzheimer’s disease (AD). Accumulation of amyloid beta (Aβ) in the brain is influenced by APOE genotype. Transgenic mice co-expressing five familial AD mutations (5xFAD) in the presence of human APOE alleles (ϵ2, ϵ3 or ϵ4) exhibit APOE genotype-specific differences in early Aβ accumulation, suggesting an interaction between APOE and AD pathology. Whether APOE genotype affects Aβ-plaque-associated neuroinflammation remains unclear. In the current study, we address the role of APOE genotype on Aβ-associated microglial reactivity in the EFAD transgenic mouse model.MethodsWe analyzed Aβ-induced glial activation in the brains of 6-month-old EFAD transgenic mice (E2FAD, E3FAD and E4FAD). Region-specific morphological profiles of Aβ plaques in EFAD brain sections were compared using immunofluorescence staining. We then determined the degree of glial activation in sites of Aβ deposition while comparing levels of the inflammatory cytokine Interleukin-1β (IL-1β) by ELISA. Finally, we quantified parameters of Aβ-associated microglial reactivity using double-stained EFAD brain sections.ResultsCharacterization of Aβ plaques revealed there were larger and more intensely stained plaques in E4FAD mice relative to E2FAD and E3FAD mice. E4FAD mice also had a greater percentage of compact plaques in the subiculum than E3FAD mice. Reactive microglia and dystrophic astrocytes were prominent in EFAD brains, and primarily localized to two sites of significant Aβ deposition: the subiculum and deep layers of the cortex. Cortical levels of IL-1β were nearly twofold greater in E4FAD mice relative to E3FAD mice. To control for differences in levels of Aβ in the different EFAD mice, we analyzed the microglia within domains of specific Aβ deposits. Morphometric analyses revealed increased measures of microglial reactivity in E4FAD mice, including greater dystrophy, increased fluorescence intensity and a higher density of reactive cells surrounding cortical plaques, than in E3FAD mice.ConclusionsIn addition to altering morphological profiles of Aβ deposition, APOE genotype influences Aβ-induced glial activation in the adult EFAD cortex. These data support a role for APOE in modulating Aβ-induced neuroinflammatory responses in AD progression, and support the use of EFAD mice as a suitable model for mechanistic studies of Aβ-associated neuroinflammation.

Identification and modification of amyloid-independent phenotypes of APOE4 mice

“…In mouse models, APOE4 brains have increased susceptibility to diseases associated with inflammation (Farrer et al, 1997; Tu et al, 2009), or inflammation caused by LPS (Zhu et al, 2012) or Aβ (Rodriguez et al, 2014). Treatment of a mouse model of amyloid with ibuprofen for 6 months led to decreased brain amyloid levels and glial activation (Lim et al, 2000).…”

Obesity and sex interact in the regulation of Alzheimer's disease

“…ApoE co-localizes with microglia around Aβ plaques (Liu et al, 2013), and apoE4 mice have greater microgliosis and astrogliosis in response to Aβ than do apoE3 mice (Belinson and Michaelson, 2009). Additionally, mice with both human apoE4 and familial AD mutations have higher levels of pro-inflammatory cytokines and increased microglial reactivity surrounding Aβ plaques than do their apoE3 counterparts (Rodriguez et al, 2014). Further, apoE4 macrophages are less effective at clearing Aβ (Zhao et al, 2009).…”

APOE Genotype Alters Immunoglobulin Subtypes in Knock-In Mice

“…However, the effects of APOE on AD pathogenesis may also be due in part to its role in regulation of inflammatory responses [10]. APOE knock-out [11] and APOE4 knock-in mice [12] have increased neuroinflammation in response to various agents [13–16], including the Aβ protein [17,18]. Microglia of APOE4 knock-in mice have a more active immune reaction in brain after infection compared to APOE3 mice, including increased proliferation and pro-inflammatory cytokine release [19,20].…”