HumanAPOEIsoform-Dependent Effects on Brain β-Amyloid Levels in PDAPP Transgenic Mice

Abstract

To investigate the role of human apolipoprotein E (apoE) on A␤ deposition in vivo, we crossed PDAPP mice lacking mouse Apoe to targeted replacement mice expressing human apoE (PDAPP/TRE2, PDAPP/TRE3, or PDAPP/TRE4). We then measured the levels of apoE protein and A␤ peptides in plasma, CSF, and brain homogenates in these mice at different ages. We also quantified the amount of brain A␤ and amyloid burden in 18-month-old mice. In young PDAPP/TRE4 mice that were analyzed at an age before brain A␤ deposition, we observed a significant decrease in the levels of apoE in CSF and brain when compared with age-matched mice expressing either human E2 or E3. The brain levels of A␤42 in PDAPP/TRE4 mice were substantially elevated even at this very early time point. In older PDAPP/TRE4 mice, the levels of insoluble apoE protein increased in parallel to the dramatic rise in brain A␤ burden, and the majority of apoE was associated with A␤. In TRE4 only mice, we also observed a significant decrease in the level of apoE in brain homogenates. Since the relative level of apoE mRNA was equivalent in PDAPP/TRE and TRE only mice, it appears that post-translational mechanisms influence the levels of apoE protein in brain (E4 Ͻ E3 Ͻ Ͻ E2), resulting in early and dramatic apoE isoform-dependent effects on brain A␤ levels (E4 Ͼ Ͼ E3 Ͼ E2) that increase with age. Therapeutic strategies aimed at increasing the soluble levels of apoE protein, regardless of isoform, may effectively prevent and (or) treat Alzheimer's disease.

Levels of Soluble Apolipoprotein E/Amyloid-β (Aβ) Complex Are Reduced and Oligomeric Aβ Increased with APOE4 and Alzheimer Disease in a Transgenic Mouse Model and Human Samples*

“…In this study, soluble apoE4/A␤ complex levels were lower than apoE2/A␤ and apoE3/A␤ complex levels in EFAD mice. These data indicate an inverse association between apoE/A␤ and oA␤ levels and are consistent with previous publications that suggest that apoE/A␤ levels isoform-specifically modulate soluble A␤ (11,15). Synapse degeneration is considered a proximal cause of cognitive deficits in AD.…”

“…IHC analysis demonstrates that plaque deposition is greater with APOE4 compared with APOE3 in AD and nondemented controls (8,9) and that a higher proportion of A␤ within a plaque is associated with apoE4 than with apoE3 (10). Biochemical analysis confirms that the levels of apoE and A␤ are also higher with APOE4 compared with APOE3 in the insoluble extraction fraction from brains of FAD-Tg mice (11). Thus, APOE4 not only facilitates amyloid deposition but also forms a greater amount and/or more stable form of apoE4/amyloid than apoE3/amyloid.…”

“…Braak stages that rate the extent of neurofibrillary pathology into the neocortex and the NIAReagan Institute neuropathology classification, which includes counts of both neuritic senile plaques and neurofibrillary tangles and provides a likelihood staging of AD neuropathological diagnosis (47,48). For ELISA analysis, A␤42, total Tau (T-Tau), and phosphorylated Tau 181 (p-Tau-181) levels were measured using Innotest ELISA kits (Innogenetics, Gent, Belgium) according to the manufacturer's protocol; apoE levels were measured using ␣-apoE (Millipore) as capture and ␣-apoE (Meridian) as detection as described (11). oA␤ levels were measured using MOAB-2 capture (5) and biotinylated MOAB-2 as detection antibody as described previously (15).…”

Apolipoprotein E Promotes β-Amyloid Trafficking and Degradation by Modulating Microglial Cholesterol Levels

“…Recently, Bateman and co-workers demonstrated that A␤ clearance is impaired in human late onset AD patients (5). Importantly, the APOE4 allele is associated with decreased A␤ clearance (6,7). We have demonstrated that apoE plays a direct role in the normal, physiological clearance of A␤ from the brain.…”

Amyloid-β Pathology and APOE Genotype Modulate Retinoid X Receptor Agonist Activity in Vivo

“…Thus, EFAD mice are APOE-TR ϩ/ϩ /5ϫFAD ϩ/Ϫ . It should be noted, however, that the introduction of the h-APOE into FAD-Tg mouse models delays A␤ accumulation (34,50,51). Although the 5ϫFAD mice produce very high levels of primarily A␤42 and exhibit significant amyloid plaque deposition by 6 -8 weeks, introduction of APOE4 (E4FAD) delays A␤ accumulation ϳ4 months, and APOE3 (E3FAD) and APOE2 (E2FAD) delay deposition ϳ6 months (31,52).…”