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O4–01–06: Astrocytes have a reduced capacity to secrete apolipoprotein (APO) E4 leading to substantially reduced brain levels
Author(s) -
Riddell David R.,
Kirksey Yolanda,
Hu Yun,
Zheng Xu,
Pangalos Menelas N.,
Reinhart Peter H.
Publication year - 2006
Publication title -
alzheimer's and dementia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.713
H-Index - 118
eISSN - 1552-5279
pISSN - 1552-5260
DOI - 10.1016/j.jalz.2006.05.296
Subject(s) - apolipoprotein e , astrocyte , gene isoform , secretion , biology , endocrinology , allele , medicine , phenotype , cell culture , genetics , disease , gene , central nervous system
Inheritance of the apolipoprotein E4 allele increases the risk of developing Alzheimer’s disease, however, the molecular mechanisms underlying this association remain elusive. In order to study apoE4 in a physiologically relevant environment we sought to identify human astrocyoma cell lines that endogenously express apoE4. We apoE genotyped a range of human cell-lines and identified two astrocytomas that expressed the apoE4 isoform. In order to determine the relative amounts of apoE4 to total apoE secreted from these cell-lines we developed two specific immunoassays; an assay to measure total apoE irrespective of isoform and an apoE4 selective assay. Surprisingly, and in contrast to the expected equal contribution of each apoE isoform to the total apoE pool, we found that less than 20 % of the total apoE secreted into the media of apoE3/4 astrocytomas was attributed to apoE4. Next, we sought to determine whether the reduced level of apoE4 secretion from astrocytes translated into reduced absolute levels of apoE4 in the brain. We examined apoE protein levels in the brains of 12 week old humanized 3/3, 4/4 and 3/4 knock-in mice, where the apoE expression is under the control of endogenous promoters. Importantly, these apoE knock-in mice showed a genotype dependent decrease in cortical apoE levels; 3/3 3/4 4/4. Next, we sought to examine the relative contributions of apoE4 relative to apoE3 in the 3/4 mouse brains. Consistent with the in-vitro astrocyte data, apoE4 represented less than 20 % of the total cortical apoE. Moreover, the absolute amount of apoE3 per allele is similar between 3/3 and 3/4 mice, implying that the reduced levels of total apoE in 3/4 knock-in mice can be fully explained by the reduction in apoE4 levels. Taken together these data suggest that under physiological relevant conditions, astrocytes have a reduced capacity to secrete apoE4 that ultimately leads to substantially reduced brain apoE levels. Moreover, the genotype dependent decrease in cortical apoE levels observed in this study, mirror the relative risk of developing Alzheimer’s disease and suggest that low levels of total apoE exhibited by 4 carriers may directly contribute to the disease progression.