Lipofuscin autofluorescence confounds intracellular amyloid β detection in the aged mouse brain

Intracellular amyloid β (Aβ) accumulation is a contentious feature of Alzheimer's disease (AD), increasingly reported in young adults and aged animal models of AD. However, autofluorescent lipofuscin granules which consist of a mixture of highly oxidized lipids, misfolded proteins, and metals, accumulates with aging in neurons and microglia and renders difficult the interpretation of immunofluorescence-based studies. Here, we show that lipofuscin accumulation in aged wild-type (WT) mouse brains exhibits significant spectral overlap with commonly used antibodies for Aβ detection, leading to potential misinterpretation of intracellular Aβ signals. Through a combination of dye staining, immunohistochemistry (IHC), and confocal microscopy, we show that fluorescence signals resembling intracellular Aβ and commonly reported in aged animal models of AD, may reflect the presence of lipofuscin granules. Importantly, these signals persisted in control sections where primary Aβ antibodies were omitted, but disappeared following TrueBlack autofluorescence quencher. We also performed Aβ immunofluorescence staining using 5xFAD mice as model for AD, revealing that intracellular Aβ in these models can be diminished by TrueBlack treatment, thus confounding the interpretation of true intracellular Aβ signals. Our findings underscore the need for caution in interpreting intracellular Aβ signals in young adults and aged models of Aβ pathology inside neurons or microglia.