Basic Science and Pathogenesis

Background: Alzheimer's disease (AD) is a progressive neurodegenerative disease and the most prevalent form of late-life dementia. The ε2 allele of the APOE gene encoding apolipoprotein E (APOE2) is associated with lower susceptibility to AD among the three genotypes (ε2, ε3, ε4), while APOE4 is the strongest genetic risk factor for late-onset AD. APOE plays a critical role in maintaining synaptic plasticity and neuronal function by controlling lipid homeostasis, with APOE2 having a superior function. Gene therapy that increases APOE2 levels in the brain has, therefore, emerged as a potential therapeutic strategy to treat AD.

Method: We conjugated PEGylated liposomes with transferrin and Penetratin, a cell-penetrating peptide, sufficiently deliver chitosan-APOE2 cDNA plasmid complex into amyloid model App^NL-G-F knockin mice at 12-month-old. Biochemical studies and brain transcriptome analysis were employed to investigate how brain-targeting liposome-based APOE2 gene delivery influences amyloid-β (Aβ)-related pathologies in App^NL-G-F knockin mice.

Result: One month after APOE2 gene therapy, there was a trend of reduced insoluble Aβ levels in the mouse cortices. Furthermore, in the App^NL-G-F knockin mice that received the APOE2 gene therapy, brain transcriptome analysis through RNA-sequencing identified the upregulation of genes/pathways related to neuronal development. This was supported by increases of Dlg4 and Syp mRNAs coding synaptic proteins in the experimental group. On the other hand, we found that APOE2 gene delivery increased soluble Aβ levels, including oligomers, as well as exacerbated neurite dystrophy and decreased synaptophysin.

Conclusion: Our results suggest that brain-targeting liposome-based APOE2 gene therapy is potentially beneficial for synaptic formation at the transcriptional level. Forced APOE2 expressions, however, may exacerbate Aβ toxicity by increasing the dissociation of Aβ oligomers from aggregates in the presence of considerable amyloid burden.