Neurobiological role of Alzheimer's disease genetic risk factor ApoE on early synaptic changes in Alzheimer-like models

BACKGROUND: The presence of an apolipoprotein E4 (ApoE4) genotype is the major genetic risk factor for Alzheimer's disease (AD). Astrocytes are the main source of ApoE in the brain, however ApoE can also be produced by neurons and microglia. ApoE plays a role in many cell types and processes related to AD, however, it remains unclear which mechanism(s) and cellular source of ApoE are most critical for AD. One of the earliest changes in AD are early cellular changes such as endosomal and synaptic alterations. ApoE4 has been associated with impaired endosomal trafficking and dysregulated synaptic plasticity. Neuronal hyperexcitability has been reported in mice expressing human ApoE4, a dysregulation that is also seen in AD transgenic mice. Although ApoE seems to play a crucial role in neuronal changes linked to early AD, ApoE's synaptic localization and mechanisms remain poorly understood. In this study, the aim is to determine the role of ApoE, in particular ApoE4, on synaptic alterations in AD models. METHOD: Mouse neurons and astrocytes are derived from wild-type, ApoE knock-out (KO), humanized ApoE3 and ApoE4 knock-in mice. Astrocyte-conditioned medium from mouse astrocytes expressing human ApoE and recombinant ApoE are used as a source for human ApoE to treat ApoE KO, wild-type and AD transgenic APP/PS1 neurons. Additionally, ApoE KO and humanized ApoE neurons are treated with synthetic Aβ or vehicle control. Analysis is performed using immunofluorescence, confocal and live cell imaging. RESULT: Exogenously added and endogenously produced human ApoE is shown to be present at neurites and synaptic terminals of cultured neurons. Differences in neuronal activity are observed among different ApoE conditions using Ca2+ live cell microscopy, both in the presence and absence of elevated human Aβ. Added recombinant and endogenous ApoE appear to be present in the endosome-lysosome system of neurons. CONCLUSION: ApoE appears to localize at synapses and endosomes, sites associated with early cellular changes in AD, and seems to play a role in neuronal excitability. Determining the neurobiology of ApoE, in particular in connection with cellular sites vulnerable to early changes in AD, can contribute to a better understanding of the role of ApoE in AD.