AMYLOID-BETA DEGRADATION BY HUMAN ASTROCYTES IS IMPAIRED BY APOJ AND APOE

Abstract

Sporadic late onset Alzheimer's disease (AD) is associated with impaired clearance of amyloid-beta (A β) from the brain. Recently, our group showed by confocal microscopy and live cell imaging that astrocytes are capable of ingesting A β. Apolipoprotein (Apo)E and ApoJ genetic variants are risk factors to develop AD and both proteins co-localize with A β plaques. Moreover, we previously showed that ApoE and ApoJ reduced the uptake of A β oligomers by human astrocytes in vitro. Whether ApoJ and ApoE also alter the enzymatic degradation of A β in or surrounding human astrocytes remains unknown. In this study, we determined expression levels of two key enzymes in A β -degradation, neprilysin (NEP) and insulin degrading enzyme (IDE), in human astrocytes. In addition, we determined if A β oligomers and fibrils in the presence or absence of ApoJ and ApoE could alter the expression of these enzymes. To test this, we treated human astrocytes with increasing concentrations of A β oligomers and fibrils (0.1, 1 and 10 μ M) in either absence or presence of ApoJ (1 μ M) and ApoE (1 μ M) and determined NEP and IDE expression in cell lysates prepared after 18 hours. NEP and IDE expression were quantified by Western blot analysis. When exposed to A β oligomers and A β fibrils alone the expression of both NEP and IDE was dose dependently increased by approximately 50%. This effect was abrogated (20-50% reduction) when A β oligomers and fibrils, preincubated with either ApoE or ApoJ, were applied to the human astrocytes. Combined, these and our earlier results suggest that ApoJ and ApoE negatively affect A β clearance by astrocytes. Reducing the levels of ApoE and ApoJ in the brain could rescue A β clearance by human astrocytes.