Apolipoprotein E and low density lipoprotein receptor-related protein facilitate intraneuronal Aβ42 accumulation in amyloid model mice

The low density lipoprotein receptor-related protein (LRP) is highly expressed in the brain and has been shown to alter the metabolism of amyloid precursor protein and amyloid-β peptide (Aβ) in vitro. Previously we developed mice that overexpress a functional LRP minireceptor (mLRP2) in their brains and crossed them to the PDAPP mouse model of Alzheimer disease. Overexpression of mLRP2 in 22-month-old PDAPP mice with amyloid plaques increased a pool of carbonate-soluble Aβ in the brain and worsened memory-related behavior. In the current study, we examined the effects of mLRP2 overexpression on 3-month-old PDAPP mice that had not yet developed amyloid plaques. We found significantly higher levels of membrane-associated Aβ42 in the hippocampus of mice that overexpressed mLRP2. Using immunohistochemical methods, we observed significant intraneuronal Aβ42 in the hippocampus and frontal cortex of PDAPP mice, which frequently co-localized with the lysosomal marker LAMP-1. Interestingly, PDAPP mice lacking apolipoprotein E (apoE) had much less intraneuronal Aβ42. We also found that PC12 cells overexpressing mLRP2 cleared Aβ42 and Aβ40 more rapidly from media than PC12 cells transfected with the vector only. Preincubation of apoE3 or apoE4 with Aβ42 increased the rate of Aβ clearance, and this effect was partially blocked by receptor-associated protein. Our results support the hypothesis that LRP binds and endocytoses Aβ42 both directly and via apoE but that endocytosed Aβ42 is not completely degraded and accumulates in intraneuronal lysosomes.