We have previously shown that apolipoprotein E (Apoe) promotes the formation of amyloid in brain and that astrocyte-specific expression of APOE markedly affects the deposition of amyloid-beta peptides (Abeta) in a mouse model of Alzheimer disease. Given the capacity of astrocytes to degrade Abeta, we investigated the potential role of Apoe in this astrocyte-mediated degradation. In contrast to cultured adult wild-type mouse astrocytes, adult Apoe(-/-) astrocytes do not degrade Abeta present in Abeta plaque-bearing brain sections in vitro. Coincubation with antibodies to either Apoe or Abeta, or with RAP, an antagonist of the low-density lipoprotein receptor family, effectively blocks Abeta degradation by astrocytes. Phase-contrast and confocal microscopy show that Apoe(-/-) astrocytes do not respond to or internalize Abeta deposits to the same extent as do wild-type astrocytes. Thus, Apoe seems to be important in the degradation and clearance of deposited Abeta species by astrocytes, a process that may be impaired in Alzheimer disease.
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 A42 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.
Recent studies suggest that bone marrow-derived macrophages can effectively reduce -amyloid (A) deposition in brain. To further elucidate the mechanisms by which macrophages degrade A, we cultured murine macrophages on top of A plaque-bearing brain sections from transgenic mice expressing PDAPP [human amyloid precursor protein (APP) with the APP 717VϾF mutation driven by the platelet-derived growth factor promoter]. Using this ex vivo assay, we found that macrophages from wild-type mice very efficiently degrade both soluble and insoluble A in a time-dependent manner and markedly eliminate thioflavine-S positive amyloid deposits. Because macrophages express and secrete apolipoprotein E (apoE), we compared the efficiency of A degradation by macrophages prepared from apoE-deficient mice or mice expressing human apoE2, apoE3, or apoE4. Macrophages expressing apoE2 were more efficient at degrading A than apoE3-expressing, apoE4-expressing, or apoE-deficient macrophages. Moreover, macrophage-induced degradation of A was effectively blocked by an anti-apoE antibody and receptor-associated protein, an antagonist of the low-density lipoprotein (LDL) receptor family, suggesting involvement of LDL receptors. Measurement of matrix metalloproteinase-9 (MMP-9) activity in the media from human apoE-expressing macrophages cocultured with A-containing brain sections revealed greater levels of MMP-9 activity in apoE2-expressing than in either apoE3-or apoE4-expressing macrophages. Differences in MMP-9 activity appear to contribute to the isoform-specific differences in A degradation by macrophages. These apoE isoform-dependent effects of macrophages on A degradation suggest a novel "peripheral" mechanism for A clearance from brain that may also, in part, explain the isoformdependent effects of apoE in determining the genetic risk for Alzheimer's disease.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.