BACKGROUND: Human apolipoprotein E (apoE) exists in three major isoforms: apoE2, 
apoE3 and apoE4. In the brain, apoE is produced mostly by astrocytes and 
transports cholesterol to neurons via apoE receptors. Among the gene alleles 
encoding the three isoforms, the APOE4 allele is the strongest genetic risk 
factor for late-onset Alzheimer's disease (AD), whereas APOE2 is protective. 
ApoE4 confers a gain of toxic function, a loss of neuroprotective function or a 
combination of both in AD pathogenesis. Given that therapeutic impacts of 
modulating apoE expression may be isoform-dependent, we sought to investigate 
the relationship between overexpressing apoE isoform and apoE-related functions 
in apoE-targeted replacement (TR) mice. Specifically, apoE isoform expression 
driven by the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter 
was built into an adeno-associated virus serotype 8 (AAV8) vector and injected 
into the ventricles of postnatal day 2 (P2) apoE3-TR or apoE4-TR mice. Upon 
confirmation of apoE isoform expression, effects on apoE lipidation and the 
levels of amyloid-β (Aβ) in the brain were assessed.
RESULTS: AAV8-GFAP-apoE isoforms were specifically expressed in astrocytes 
throughout all brain regions, which led to overall increased apoE levels in the 
brain. Viral mediated overexpression of apoE4 in the apoE4-TR background 
increased poorly-lipidated apoE lipoprotein particles and decreased 
apoE-associated cholesterol in apoE4-TR mice. Conversely, apoE2 overexpression 
in apoE4-TR mice enhanced apoE lipidation and associated cholesterol. 
Furthermore, overexpression of apoE4 elevated the levels of endogenous Aβ, 
whereas apoE2 overexpression trended to lower endogenous Aβ.
CONCLUSIONS: Overexpression of apoE isoforms induces differential effects in the 
apoE4-TR background: apoE4 decreases apoE lipidation and enhances Aβ 
accumulation, whereas apoE2 has the opposite effects. Our findings suggest that 
increasing apoE2 in APOE4 carriers is a beneficial strategy to treat AD, whereas 
increasing apoE4 in APOE4 carriers is likely harmful. We have also established 
novel methods to express apoE isoforms in mouse brain to study apoE-related 
pathways in AD and related dementia.