Studies on ApoE in the neurobiology of Alzheimer’s disease

Abstract

AbstractBackgroundApoE4, the most important genetic risk factor for Alzheimer’s disease (AD), is mainly produced by astrocytes, to some extent by microglia, and under stress also by neurons. Under healthy conditions ApoE transports lipids from astrocytes to neurons, where it binds to receptors and is internalized. ApoE4 was reported to impair normal receptor recycling, modulates synaptic receptors and associates with enlarged endosomes in AD. ApoE4 target replacement (TR) mice in the absence of AD mutations show neuronal hyperactivity. However, precise mechanisms, cell types and targets of ApoE relating to AD remain unclear. Previous studies from our lab reported endosomal Aβ accumulation and Aβ‐induced endosome enlargement. Here we aim to elucidate the cellular mechanisms of different ApoE isoforms on neurobiological AD changes, including intraneuronal Aβ accumulation, endosomal alterations and synaptic dysfunction.MethodCortical and hippocampal mouse astrocytes and neurons are derived from ApoE KO, ApoE3‐TR and ApoE4‐TR mice. Human ApoE is obtained from various sources, including conditioned media of ApoE‐TR astrocytes or recombinant ApoE protein, among others. Wild‐type C57BL/6JRj, APP/PS1 and APP KO mouse neurons and mouse N2a cells are treated for different durations with ApoE3, ApoE4, ApoE KO or vehicle control, and analyzed by immunofluorescence, western blot and live cell calcium imaging.ResultAstrocytes from ApoE3‐TR and ApoE4‐TR mice secrete high levels of human ApoE. Recombinant ApoE3 and ApoE4 bind to neurites, seemingly preferentially to synaptic terminals of neurons. ApoE‐TR mouse neurons express endogenous human ApoE in vitro, potentially related to stress associated with culture conditions. Internalized recombinant and endogenous ApoE and internalized synthetic and endogenous Aβ show a mostly vesicular but non‐overlapping pattern of labeling.ConclusionEndogenous and internalized ApoE are present in a vesicular pattern, which however does not significantly overlap with internalized Aβ. In addition, particularly exogenous synthetic ApoE appears to preferentially bind to synapses. We hypothesize that determining the biology of ApoE within neurons, as well as its effects on endosomes, intraneuronal Aβ and synapses, can help to better understand the role of ApoE in neurons and can provide new insights on the role of ApoE in Alzheimer’s pathogenesis.