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

Abstract

We applied a novel application of FLIM-FRET to in situ measurement and quantification of protein interactions to explore isoform specific differences in Aβ-ApoE interaction and ApoE tertiary conformation in senile plaques in human Alzheimer brain. ApoE3 interacts more closely with Aβ than ApoE4, but a greater proportion of Aβ molecules within plaques are decorated with ApoE4 than ApoE3, lending strong support to the hypothesis that isoform specific differences in ApoE are linked with Aβ 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 Aβ and thereby increase the risk of AD for carriers of the APOEε4 allele.

APOE4-specific Changes in Aβ Accumulation in a New Transgenic Mouse Model of Alzheimer Disease

“…In E4FAD mice, total apoE levels in the brain are lower compared with E2FAD and E3FAD, as observed previously in human and apoE Tg mouse models (35)(36)(37)(38)(39)(40)(41). With TBS, TBSX, and FA extraction, apoE2 and apoE3 from the EFAD mouse tissue extract primarily to the TBSX fraction, consistent with the extraction of apoE-containing lipoproteins from plasma.…”

Molecular Basis for Increased Risk for Late-onset Alzheimer Disease Due to the Naturally Occurring L28P Mutation in Apolipoprotein E4

“…Although the mutant protein can still interact with lipids, both the kinetics and final products are altered. More importantly this single-amino acid mutation gives apoE4 the ability to promote the intracellular accumulation of A␤42, a function that has also been found previously in a heavily truncated apoE4 variant, apoE4[(⌬(166 -299)] (10), which has a similar molecular weight as carboxyl-terminal truncated apoE4 fragments found in the brain of AD patients (54,55). We have proposed previously an association between two molecular events that are considered to be early events in the pathogenesis of AD, namely the proteolysis of apoE4 and the intraneuronal accumulation of A␤42 that leads to persistent ROS formation and, therefore, increased oxidative stress, which is also an early event in AD (10,12,13,56).…”

Zinc affects the proteolytic stability of Apolipoprotein E in an isoform-dependent way

“…Interestingly, the levels of ApoE fragments showed a similar trend (ApoE2N ApoE3NApoE4) ( Fig. 6A and C) which is in contrast to previous findings showing that more ApoE4 fragments in AD brains compared with ApoE3 and ApoE2 (Jones et al, 2011). In summary, we found that the amount of both full-length and fragmental ApoE followed an order of ApoE2 NApoE3 N ApoE4, which was not affected by disease status.…”

“…Several mechanism relating ApoE isoforms to AD pathology have been reported, including both Aβ-dependent and -independent mechanisms (Liu et al, 2013). In addition, a number of studies have demonstrated genotype-dependent variability in ApoE concentrations, with the lowest levels in APOE4 carriers (Riddell et al, 2008;Ulrich et al, 2013;Jones et al, 2011;Rensen et al, 2000;Martinez-Morillo et al, 2014), which account for 65%-80% of all AD cases (Farrer et al, 1997). It is widely believed that not only the different isoforms per se may influence the risk of AD but the different ApoE concentrations may also be an AD risk-modulating factor (Huang and Mahley, 2014).…”