Apolipoprotein E is a kinetic but not a thermodynamic inhibitor of amyloid formation: implications for the pathogenesis and treatment of Alzheimer disease.

Abstract

The apolipoprotein E4 (APOE4) allele is associated with an early age of onset of the nonfamilial form of Alzheimer disease (AD) and with increased 8 protein amyloid deposition in the brain. These two observations may both arise from an effect of the apoE family of proteins on the rate of in vivo amyloidogenesis. We report here that apoE3, the common apoE isoform, is an in vitro amyloid nucleation inhibitor at physiological concentrations. A significant delay in the onset of amyloid fibril formation by the ,3-amyloid protein of AD (181-40) was observed at a low apoE3 concentration (40 nM), corresponding to an apoE3/13 protein molar ratio of 1:1000. The inhibitory activity of a proteolytic fragment of apoE3, containing the N-terminal 191 amino acids, is comparable to the native protein, whereas the C-terminal fragment has no activity. ApoE4 is equipotent or slightly less potent than apoE3, which may be due to its inability to form a disulfide dimer, since the apoE3 dimer is a significantly more potent nucleation inhibitor than apoE4. Neither apoE3 nor apoE4 inhibits the seeded growth of amyloid or affects the solubility or structure of the amyloid fibrils, indicating that apoE is not a thermodynamic amyloid inhibitor. We propose that the linkage between the APOE4 allele and AD reflects the reduced ability of APOE4 homozygotes to suppress in vivo amyloid formation.Amyloid plaque is an ordered, fibrous protein aggregate that is characteristic of the Alzheimer disease (AD) brain. The acceleration of amyloid deposition may be a primary event in AD (1, 2). Thus, endogenous factors that affect the rate of amyloid formation could play a significant role in the pathogenesis of AD (3, 4). The apolipoprotein E genotype (APOE) has recently been recognized as a susceptibility factor for AD (5-8). Three isoforms of apoE occur in the human population. They differ by single amino acid changes. APOE3 is the most common allele (0.78), followed by APOE4 (0.14) and APOE2 (0.07) (9). ApoE4 and apoE3 differ with respect to lipoprotein binding (10), but the two proteins are indistinguishable with respect to low density lipoprotein receptor binding (11). ApoE3 and apoE4 are distinguished by a single point mutation in a sequence of 299 amino acids (apoE3, C112; apoE4, R112) (12).TheAPOE4 allele correlates with an earlier age of AD onset (5) and an increased amount of cerebral amyloid plaque (13) at the time of death relative to the common APOE3 allele. (/3,,). Once the nucleus has formed, association becomes favorable (Kg >> 1) and is not inhibited by apoE. Exogenous seed fibrils will bypass the lag time and result in rapid growth that is not inhibited by apoE (see Fig. 2).

Production and characterization of astrocyte-derived human apolipoprotein E isoforms from immortalized astrocytes and their interactions with amyloid-β

“…ApoE has been demonstrated to modulate Ah clearance and fibrillogenesis both in vitro (Beffert et al, 1999;Castano et al, 1995;Evans et al, 1994;Koistinaho et al, 2004;Ma et al, 1994;Yang et al, 1997Yang et al, , 1999 and in vivo (Bales et al, 1997;DeMattos et al, 2004;Holtzman et al, 2000;Shibata et al, 2000). In vivo, human apoE suppresses the onset of Ah deposition and subsequently results in an isoform-specific effect (E4 N E3 N E2) on the amount of Ah deposition, fibril formation, and neuritic plaque formation Holtzman et al, 2000).…”

“…To date, while the effects of apoE on Ah clearance have been seen in in vitro and in vivo studies, an isoform-specific effect of apoE has been difficult to identify (DeMattos et al, 2004;Koistinaho et al, 2004). Some in vitro studies do show differential effects of apoE isoforms on Ah fibrillogenesis (E4 N E3) with some studies demonstrating that apoE inhibits and others showing apoE enhances Ah fibril formation (Castano et al, 1995;Evans et al, 1994;Ma et al, 1994). Importantly, while lipidated apoE has been used in some Ah binding and association studies (LaDu et al, 1994(LaDu et al, , 1995Tokuda et al, 2000), lipidated apoE has not been extensively utilized in studies examining the effect of apoE on Ah clearance and fibrillogenesis.…”

Perlecan Binds to the β‐Amyloid Proteins (Aβ) of Alzheimer's Disease, Accelerates Aβ Fibril Formation, and Maintains Aβ Fibril Stability

“…formation at the time of analysis, (b) the concentration of A/I and amybid plaque component used, and (c) the time of analysis following incubation of A/I with a particular amybid plaque component. For example, a number of studies suggested that ApoE is a potent enhancer of A/I amyloid fibril formation (Soto et al, 1995, whereas others implicated ApoE as an A/I amyloid inhibitor (Evans et al, 1995;Ohman et a!., 1996). In some of these studies, it was clear that the ApoE isoform (e.g., ApoE3 versus ApoE4) was important for the observed enhancement or inhibitory effects.…”

Apolipoprotein E and Apolipoprotein E Receptors: Normal Biology and Roles in Alzheimer Disease

“…E3 . E2) (Ma et al 1994;Wisniewski et al 1994;Castano et al 1995), whereas others show that apoE inhibits fibrillization (Evans et al 1994;Wood et al 1996). Conflicting results between in vitro studies may be owing to the differences in apoE and Ab preparations or other factors.…”