Apolipoprotein E4 Domain Interaction Mediates Detrimental Effects on Mitochondria and Is a Potential Therapeutic Target for Alzheimer Disease

Abstract

Apolipoprotein (apo) E4 is the major genetic risk factor for late-onset Alzheimer disease (AD). ApoE4 assumes a pathological conformation through an intramolecular interaction mediated by Arg-61 in the amino-terminal domain and Glu-255 in the carboxyl-terminal domain, referred to as apoE4 domain interaction. Because AD is associated with mitochondrial dysfunction, we examined the effect of apoE4 domain interaction on mitochondrial respiratory function. Steady-state amounts of mitochondrial respiratory complexes were examined in neurons cultured from brain cortices of neuron-specific enolase promoter-driven apoE3 (NSE-apoE3) or apoE4 (NSEapoE4) transgenic mice. All subunits of mitochondrial respiratory complexes assessed were significantly lower in NSEapoE4 neurons compared with NSE-apoE3 neurons. However, no significant differences in levels of mitochondrial complexes were detected between astrocytes expressing different apoE isoforms driven by the glial fibrillary acidic protein promoter, leading to our conclusion that the effect of apoE4 is neuron specific. In neuroblastoma Neuro-2A (N2A) cells, apoE4 expression reduced the levels of mitochondrial respiratory complexes I, IV, and V. Complex IV enzymatic activity was also decreased, lowering mitochondrial respiratory capacity. Mutant apoE4 (apoE4-Thr-61) lacking domain interaction did not induce mitochondrial dysfunction in N2A cells, indicating that the effect is specific to apoE4-expressing cells and dependent on domain interaction. Consistent with this finding, treatment of apoE4-expressing N2A cells with a small molecule that disrupts apoE4 domain interaction restored mitochondrial respiratory complex IV levels. These results suggest that pharmacological intervention with small molecules that disrupt apoE4 domain interaction is a potential therapeutic approach for apoE4-carrying AD subjects. Apolipoprotein (apo)2 E4 is the major genetic risk factor for late-onset Alzheimer disease (AD) (1-6). The protein sequence of apoE4 (299 amino acids) differs from the other two common isoforms (apoE2, apoE3) at residues 112 or 158; apoE3 has a cysteine at residue 112 and an arginine at residue 158, whereas apoE4 has arginines at both positions and apoE2 has cysteines (3, 4). These differences profoundly affect the tertiary protein structure (3-5). ApoE4 displays an intramolecular interaction between its amino-terminal and carboxylterminal domains (7,8), referred to as apoE4 domain interaction. The domain interaction is mediated by Arg-112 in apoE4, which induces a salt bridge between Arg-61 in the amino-terminal domain and Glu-255 in the carboxyl-terminal domain (7,8). This interaction is decreased in apoE2 and apoE3 due to the presence of Cys-112 (8). Substitution of a threonine for Arg-61 in apoE4 disrupts the domain interaction, converting apoE4 to an apoE3-like conformation (7-9).It is postulated that domain interaction is primarily responsible for the pathogenic effects of apoE4 in AD (3-5). Because AD is known to be associated with mitochondrial dysfunction (10 -...

Apolipoprotein E4 Impairs Macrophage Efferocytosis and Potentiates Apoptosis by Accelerating Endoplasmic Reticulum Stress

“…Specifically, apoE4-expressing astrocytes have elevated ER stress with increased expression of CHOP, ATF4, and XBP1 compared with apoE3-expressing astrocytes (45). In contrast, the apoE4-induced neuronal dysfunction is not related to ER stress but is a result of mitochondrial dysfunctions with reduced mitochondrial respiration activities (46). The ability of the ER chaperone tauroursodeoxycholic acid to improve efferocytosis efficiency and reduce apoptosis of APOE4 macrophages indicates that the apoE4-related macrophage dysfunction is due to its elevation of ER stress.…”

Towards Alzheimer's root cause: ECSIT as an integrating hub between oxidative stress, inflammation and mitochondrial dysfunction

“…2). Moreover, in AD affected neurons, APOE undergoes proteolysis to generate an N-terminal fragment that associates with NFT-like structures and a C-terminal truncation that binds to mitochondrial respiratory complexes, reducing their activities [58][59][60]. These results give insights into the association of APOE with cytoskeletal components in order to induce NFT-like inclusions containing phosphorylated tau and phosphorylated neurofilaments of high molecular weight in neurons [58].…”

Small Molecule Structure Correctors Abolish Detrimental Effects of Apolipoprotein E4 in Cultured Neurons

“…Mutant apoE4-R61T lacking domain interaction, like apoE3, binds preferentially to high density lipoproteins (12,13). In neurons, apoE4 reduces expression of the protein subunits of mitochondrial respiratory complexes, such as subunit 1 of complex IV (mtCOX1) and subunit ␣ of complex V, resulting in a reduction in mitochondrial respiratory function (16). Unlike apoE4, apoE3 and apoE4-R61T did not induce mitochondrial dysfunction (16), suggesting that domain interaction mediates apoE4-specific detrimental effects on mitochondria.…”

“…In neurons, apoE4 reduces expression of the protein subunits of mitochondrial respiratory complexes, such as subunit 1 of complex IV (mtCOX1) and subunit ␣ of complex V, resulting in a reduction in mitochondrial respiratory function (16). Unlike apoE4, apoE3 and apoE4-R61T did not induce mitochondrial dysfunction (16), suggesting that domain interaction mediates apoE4-specific detrimental effects on mitochondria. Similarly, apoE4 perturbs neuronal function by reducing mitochondrial motility, decreasing neurite outgrowth (17)(18)(19), and inhibiting synaptogenesis (20).…”