Apolipoprotein E: Isoform Specific Differences in Tertiary Structure and Interaction with Amyloid-[beta] in Human Alzheimer Brain

We applied a novel application of FLIM-FRET to in situ measurement and quantification of protein interactions to explore isoform specific differences in A[beta]-ApoE interaction and ApoE tertiary conformation in senile plaques in human Alzheimer brain. ApoE3 interacts more closely with A[beta] than ApoE4, but a greater proportion of A[beta] molecules within plaques are decorated with ApoE4 than ApoE3, lending strong support to the hypothesis that isoform specific differences in ApoE are linked with A[beta] deposition. We found an increased number of ApoE N-terminal fragments in ApoE4 plaques, consistent with the observation that ApoE4 is more easily cleaved than ApoE3. In addition, we measured a small but significant isoform specific difference in ApoE domain interaction. Based on our in situ data, supported by traditional biochemical data, we propose a pathway by which isoform specific conformational differences increase the level of cleavage at the hinge region of ApoE4, leading to a loss of ApoE function to mediate clearance of A[beta] and thereby increase the risk of AD for carriers of the APOE[epsilon]4 allele.