Inhibition of c-Jun N-terminal kinase increases apoE expression in vitro and in vivo

Apolipoprotein E (apoE), a ligand for the low-density lipoprotein receptor family, has been implicated in modulating glial inflammatory responses and the risk of neurodegeneration associated with Alzheimer’s disease. Glial cells activated by lipopolysaccharide (LPS) have decreased apoE levels and we recently showed that apoE itself can modulate the inflammatory response by reducing c-Jun N-terminal kinase (JNK) activation. Reduced JNK phosphorylation is vital to overcome the LPS-induced decrease in apoE expression, suggesting that JNK inhibition may be an effective way to increase apoE protein and protract its anti-inflammatory properties. This study investigates the impact of JNK inhibition on apoE production using two JNK inhibitors. Our work in primary glia and in vivo in mice injected with JNK inhibitor demonstrates that inhibition of JNK may be an effective way to increase apoE expression.     

Apolipoprotein E and lipoprotein lipase secreted from human monocyte-derived macrophages modulate very low density lipoprotein uptake

We investigated the roles of lipoprotein lipase and apolipoprotein E (apoE) secreted from human monocyte-derived macrophages in the uptake of very low density lipoproteins (VLDL). ApoCII-deficient VLDL were isolated from a patient with apoCII deficiency. The lipolytic conversion to higher density and the degradation of the apoCII-deficient VLDL by macrophages were very slight, whereas the addition of apoCII enhanced both their conversion and degradation. This suggests that the lipolysis and subsequent conversion of VLDL to lipoproteins of higher density are essential for the VLDL uptake by macrophages. VLDL incubated with macrophages obtained from subjects with E3/3 phenotype (E3/3-macrophages) showed a 17-fold greater affinity in inhibiting the binding of 2 micrograms/ml 125I-low density lipoprotein (LDL) to fibroblasts than native VLDL, whereas the incubation of VLDL with macrophages obtained from a subject with E2/2 phenotype (E2/2-macrophages) did not cause any increase in their affinity. Furthermore, 3 micrograms/ml 125I-VLDL obtained from a subject with E3/3 phenotype were degraded by E3/3-macrophages to a greater extent than by E2/2-macrophages (2-fold), indicating that VLDL uptake is influenced by the phenotype of apoE secreted by macrophages. From these results, we conclude that both lipolysis by lipoprotein lipase and incorporation of apoE secreted from macrophages alter the affinity of VLDL for the LDL receptors on the cells, resulting in facilitation of their receptor-mediated endocytosis.     

Apolipoprotein E and apolipoprotein E receptors modulate A beta-induced glial neuroinflammatory responses

Large numbers of activated glia are a common pathological feature of many neurodegenerative disorders, including Alzheimer's disease (AD). Several different stimuli, including lipopolysaccharide (LPS), dibutyryl (db)cAMP, and aged amyloid-β 1–42 (Aβ), can induce glial activation in vitro, as measured by morphological changes and the production of pro-inflammatory cytokines and oxidative stress molecules. Only Aβ-induced activation is attenuated by the addition of exogenous apolipoprotein E (apoE)-containing particles. In addition, only Aβ also induces an increase in the amount of endogenous apoE, the primary apolipoprotein expressed by astrocytes in the brain. The functional significance of the increase in apoE appears to be to limit the inflammatory response. Indeed, compared to wild type mice, glial cells cultured from apoE knockout mice exhibit an enhanced production of several pro-inflammatory markers in response to treatment with Aβ and other activating stimuli. The mechanism for both the Aβ-induced glial activation and the increase in apoE appears to involve apoE receptors, a variety of which are expressed by both neurons and glia. Experiments using receptor associated protein (RAP), an inhibitor of apoE receptors with a differential affinity for the low-density lipoprotein receptor (LDLR) and the LDLR-related protein (LRP), revealed that LRP mediates Aβ-induced glial activation, while LDLR mediates the Aβ-induced changes in apoE levels. In summary, both an apoE receptor agonist (apoE) and an antagonist (RAP) inhibit Aβ-induced glial cell activation. Thus, apoE receptors appear to translate the presence of extracellular Aβ into cellular responses, both initiating glial cell activation and limiting its scope by inducing apoE, an anti-inflammatory agent.     

Molecular size hyaluronan differently modulates toll-like receptor-4 in LPS-induced inflammation in mouse chondrocytes

Hyaluronan (HA) action depends upon its molecular size. Low molecular weight HA elicits pro-inflammatory responses by modulating the toll-like receptor-4 (TLR-4) or by activating the nuclear factor kappa B (NF-kB). In contrast, high molecular weight HA manifests an anti-inflammatory effect via CD receptors and by inhibiting NF-kB activation. Lipopolysaccharide (LPS) –mediated activation of TLR-4 complex induces the myeloid differentiation primary-response protein (MyD88) and the tumor necrosis factor receptor-associated factor-6 (TRAF-6), and ends with the liberation of NF-kB/Rel family members. The aim of this study was to investigate the influence of HA at different MWs (low, medium, high) on TLR-4 modulation in LPS-induced inflammatory response in mouse chondrocyte cultures.     

Apolipoprotein E: A potent inhibitor of endothelial and tumor cell proliferation

Recombinant human apolipoprotein E3 (apoE), purified from E. coli, inhibited the proliferation of several cell types, including endothelial cells and tumor cells in a dose- and time-dependent manner. ApoE inhibited both de novo DNA synthesis and proliferation as assessed by an increase in cell number. Maximal inhibition of cell growth by apoE was achieved under conditions where proliferation was dependent on heparin-binding growth factors. Thus, at low serum concentrations (0–2.5%) basic fibroblast growth factor (bFGF) stimulated the proliferation of bovine aortic endothelial (BAE) cells severalfold. The bFGF-dependent proliferation was dramatically inhibited by apoE with an IC₅₀ ≈ 50 nM. Under conditions where cell proliferation was mainly serum-dependent, apoE also suppressed growth but required higher concentrations to be effective (IC₅₀ ≈ 500 nM). ApoE also inhibited growth of bovine corneal endothelial cells, human melanoma cells, and human breast carcinoma cells. The IC₅₀ values obtained with these cells were generally 3–5 times higher than with BAE cells. Inhibition of cell proliferation by apoE was reversible and dependent on the time of apoE addition to the culture. In addition, apoE inhibited the chemotactic response of endothelial cells that were induced to migrate by a gradient of soluble bFGF. Inhibition of cell proliferation by apoE may be mediated both by competition for growth factor binding to proteoglycans and by an antiadhesive activity of apoE. The present results demonstrate that apoE is a potent inhibitor of proliferation of several cell types and suggest that apoE may be effective in modulating angiogenesis, tumor cell growth, and metastasis.     

Infection induces a positive acute phase apolipoprotein E response from a negative acute phase gene: role of hepatic LDL receptors

Apolipoprotein E (apoE) plays important roles in lipid homeostasis, anti-inflammation, and host defense. Since tissue apoE mRNA levels have been reported to decrease during inflammatory responses, we were surprised to find that plasma apoE levels were significantly elevated during septic infections in both humans and mice. This apparent paradox was also observed during lipopolysaccharide-induced acute inflammation in mice: plasma levels of apoE increased up to 4-fold despite sharply decreased apoE gene expression in the liver, macrophages, and extrahepatic tissues. We hypothesized that apoE levels were augmented by decreased plasma clearance. Our analysis revealed that apoE associated principally with HDL in mice and that apoE was cleared from the circulation principally via LDL receptors. The acute inflammatory response decreased LDL receptor expression in the liver and significantly reduced the rate of apoE clearance. In contrast, the same inflammatory stimuli increased LDL receptor expression in macrophages. Our results define a novel acute phase mechanism that increases circulating apoE levels as apoE production decreases. Diminished hepatic LDL receptor expression may thus cooperate with elevated LDL receptor expression in macrophages to facilitate the forward transport of apoE and its associated lipids to these key defense cells.     

Apolipoprotein E regulates the integrity of tight junctions in an isoform-dependent manner in an in vitro blood-brain barrier model

Apolipoprotein E (apoE) is a major apolipoprotein in the brain. The ε4 allele of apoE is a major risk factor for Alzheimer disease, and apoE deficiency in mice leads to blood-brain barrier (BBB) leakage. However, the effect of apoE isoforms on BBB properties are as yet unknown. Here, using an in vitro BBB model consisting of brain endothelial cells and pericytes prepared from wild-type (WT) mice, and primary astrocytes prepared from human apoE3- and apoE4-knock-in mice, we show that the barrier function of tight junctions (TJs) was impaired when the BBB was reconstituted with primary astrocytes from apoE4-knock-in mice (apoE4-BBB model). The phosphorylation of occludin at Thr residues and the activation of protein kinase C (PKC)η in mBECs were attenuated in the apoE4-BBB model compared with those in the apoE3-BBB model. The differential effects of apoE isoforms on the activation of PKCη, the phosphorylation of occludin at Thr residues, and TJ integrity were abolished following the treatment with an anti-low density lipoprotein receptor-related protein 1 (LRP1) antibody or a LRP1 antagonist receptor-associated protein. Consistent with the results of in vitro studies, BBB permeability was higher in apoE4-knock-in mice than in apoE3-knock-in mice. Our studies provide evidence that TJ integrity in BBB is regulated by apoE in an isoform-dependent manner.     

IL-1beta and IL-6 modulate apolipoprotein E gene expression in rat hepatocyte primary culture

Incubation of rat hepatocytes in primary culture with IL-1beta at a concentration of 2.5 units/ml resulted in an increase (+80%) in the amount of apoE mRNA without any effect upon apoE synthesis. IL-6 at a low concentration (10 units/ml) induced a decrease (-35%) in the amount of apoE mRNA, but increased apoE synthesis (+28%). No effect was observed with higher concentrations of IL-1beta (10 units/ml) or IL-6 (100 units/ml). These results suggest that inflammatory cytokines IL-1beta and IL-6 modulate the expression of apoE gene in cultured rat hepatocytes, at a concentration that does not induce the acute phase response.     

Impaired hepatic insulin signalling in PON2-deficient mice: a novel role for the PON2/apoE axis on the macrophage inflammatory response

Hepatic glucose metabolism is strongly influenced by oxidative stress and pro-inflammatory stimuli. PON2 (paraoxonase 2), an enzyme with undefined antioxidant properties, protects against atherosclerosis. PON2-deficient (PON2-def) mice have elevated hepatic oxidative stress coupled with an exacerbated inflammatory response from PON2-deficient macrophages. In the present paper, we demonstrate that PON2 deficiency is associated with inhibitory insulin-mediated phosphorylation of hepatic IRS-1 (insulin receptor substrate-1). Unexpectedly, we observed a marked improvement in the hepatic IRS-1 phosphorylation state in PON2-def/apoE (apolipoprotein E)(-/-) mice, relative to apoE(-/-) mice. Factors secreted from activated macrophage cultures derived from PON2-def and PON2-def/apoE(-/-) mice are sufficient to modulate insulin signalling in cultured hepatocytes in a manner similar to that observed in vivo. We show that the protective effect on insulin signalling in PON2-def/apoE(-/-) mice is directly associated with altered production of macrophage pro-inflammatory mediators, but not elevated intracellular oxidative stress levels. We further present evidence that modulation of the macrophage inflammatory response in PON2-def/apoE(-/-) mice is mediated by a shift in the balance of NO and ONOO(-) (peroxynitrite) formation. Our results demonstrate that PON2 plays an important role in hepatic insulin signalling and underscores the influence of macrophage-mediated inflammatory response on hepatic insulin sensitivity.     

Serum amyloid A (SAA)-induced remodeling of CSF-HDL

Inflammation is a risk factor for Alzheimer's disease. Serum amyloid A (SAA) is an acute phase protein that dissociates apolipoprotein AI (apoAI) from plasma HDL. In cerebrospinal fluid (CSF), the SAA concentration is much higher in subjects with Alzheimer's disease than in controls. CSF-HDL is rich in apoE, which plays an important role as a ligand for lipoprotein receptors in the central nervous system (CNS). To clarify whether SAA dissociates apoE from CSF-HDL, we added recombinant SAA to CSF and determined the apoE distribution in the CSF using native two-dimensional gel electrophoresis. We found that SAA dissociated apoE from CSF-HDL in a dose-dependent manner. This effect was more evident in apoE4 carriers than in apoE3 or apoE2 carriers. After a 24-h incubation at 37 degrees C, SAA continuously dissociated apoE from CSF-HDL. Amyloid beta (Abeta) fragments (1-42) were bound to large CSF-HDL but not to apoE dissociated by SAA. In conclusion, SAA dissociates apoE from CSF-HDL. We postulate that inflammation in the CNS may impair Abeta clearance due to the loss of apoE from CSF-HDL.     

Glycation of apolipoprotein E impairs its binding to heparin: identification of the major glycation site.

The increased glycation of plasma apolipoproteins represents a possible major factor for lipid disturbances and accelerated atherogenesis in diabetic patients. The glycation of apolipoprotein E (apoE), a key lipid-transport protein in plasma, was studied both in vivo and in vitro. ApoE was shown to be glycated in plasma very low density lipoproteins of both normal subjects and hyperglycemic, diabetic patients. However, diabetic patients with hyperglycemia showed a 2-3-fold increased level of apoE glycation. ApoE from diabetic plasma showed decreased binding to heparin compared to normal plasma apoE. The rate of Amadori product formation in apoE in vitro was similar to that for albumin and apolipoproteins A-I and A-II. The glycation of apoE in vitro significantly decreased its ability to bind to heparin, a critical process in the sequestration and uptake of apoE-containing lipoproteins by cells. Diethylenetriaminepentaacetic acid, a transition metal chelator, had no effect on the loss of apoE heparin-binding activity, suggesting that glycation rather than glycoxidation is responsible for this effect. In contrast, glycation had no effect on the interaction of apoE with amyloid beta-peptide. ApoE glycation was demonstrated to be isoform-specific. ApoE(2) showed a higher glycation rate and the following order was observed: apoE(2)>apoE(4)>apoE(3). The major glycated site of apoE was found to be Lys-75. These findings suggest that apoE is glycated in an isoform-specific manner and that the glycation, in turn, significantly decreases apoE heparin-binding activity. We propose that apoE glycation impairs lipoprotein-cell interactions, which are mediated via heparan sulfate proteoglycans and may result in the enhancement of lipid abnormalities in hyperglycemic, diabetic patients.     

Dietary vitamin C down-regulates inflammatory gene expression in apoE4 smokers

The deleterious impact of cigarette smoking on cardiovascular health may be in part attributable to a free radical mediated proinflammatory response in circulating monocytes. In the current investigation, the impact of vitamin C supplementation on monocyte gene expression was determined in apoE4 smokers versus non-smokers. A total of 10 smokers and 11 non-smokers consumed 60mg/day of vitamin C for four weeks and a fasting blood sample was taken at baseline and post-intervention for the determination of plasma vitamin C and monocyte gene expression profiles using cDNA array and real time PCR. In apoE4 smokers, supplementation resulted in a 43% increase in plasma vitamin C concentrations. Furthermore, a number of genes were differentially expressed more than 2-fold in response to treatment, including a downregulation of the proinflammatory mediators tumor necrosis factor (TNF) beta, TNF receptor, neurotrophin-3 growth factor receptor, and monocyte chemoattractant protein 1 receptor. The study has identified a number of molecular mechanisms underlying the benefit of vitamin C supplementation in smokers.     

Characterization of the Binding of Amyloid-β Peptide to Cell Culture-Derived Native Apolipoprotein E2, E3, and E4 Isoforms and to Isoforms from Human Plasma

Abstract: The ε4 allele of apolipoprotein E (apoE, protein; APOE, gene) is a major risk factor for Alzheimer's disease (AD). Genetically, the frequency of the ε4 allele is enriched in early-onset sporadic, late-onset familial, and common late-onset sporadic AD. ApoE is found in the extracellular amyloid-β (Aβ) deposits that are characteristic features of AD. In this study, we examined the interaction between Aβ and apoE isoforms. The apoE isoforms used in this study were either produced by stably transfected Chinese hamster ovary cells (CHO) or were from human plasma. We report that when similar concentrations of the apoE isoforms were used, native nonpurified apoE3 from recombinant CHO-derived sources bound Aβ, but apoE4 did not. In fact, in our system, binding of recombinant apoE4 to Aβ was never detectable, even after incubation for 4 days. Furthermore, using the same assay conditions, native apoE2, like apoE3, binds Aβ avidly. Furthermore, when human plasma apoE isoforms are tested in Aβ binding experiments, apoE3 bound Aβ more avidly than apoE4, and a major apoE/Aβ complex (the 40-kDa form) was observed with plasma apoE3 but not apoE4. These data extend our understanding of apoE isoform-dependent binding of Aβ by associating apoE2 with efficient apoE/Aβ complex formation and demonstrate that native apoE3 (whether recombinant or derived from human plasma) forms sodium dodecyl sulfate-stable apoE/Aβ complexes more readily than native apoE4. The different Aβ-binding properties of native apoE4 versus native apoE3 provide insight into the molecular mechanisms by which the APOE ε4 allele exerts its risk factor effects in AD.     

Low density receptor-related protein 1 (LRP1) promotes anti-inflammatory phenotype in murine macrophages

We have previously reported that apolipoprotein E (apoE), a protein component of very-low-density lipoproteins (VLDL) and high-density lipoproteins and a potent plasma-borne atheroprotective factor, exerts anti-inflammatory activity in macrophages by switching the activation profile from M1 (“classic”) to M2 (“alternative”) in a process involving signaling via low-density lipoprotein receptor (LDLR) family members including the VLDL receptor (VLDLR) or apoE receptor-2 (apoER2). The present study was undertaken to investigate whether LDLR-related protein 1 (LRP-1), another member of the LDLR family and a ubiquitously expressed multifunctional cell surface receptor, modulates M1→M2 conversion in murine macrophages. We investigate bone marrow or peritoneal macrophages isolated from wild-type C57/Bl6 mice or mice with conditional inactivation of the LRP-1 gene in the myeloid lineage for the expression of polarization markers. Our results suggest that the deficiency of LRP-1 down-regulates M2 marker expression in macrophages, while enhancing the macrophage response to M1 stimuli. To our knowledge, this is the first demonstration that LRP-1 affects macrophage polarization and promotes the development of an anti-inflammatory M2 functional phenotype.     

Apolipoprotein e4 impairs macrophage efferocytosis and potentiates apoptosis by accelerating endoplasmic reticulum stress

Apolipoprotein (apo) E4 is a major genetic risk factor for a wide spectrum of inflammatory metabolic diseases, including atherosclerosis, diabetes, and Alzheimer disease. This study compared diet-induced adipose tissue inflammation as well as functional properties of macrophages isolated from human APOE3 and APOE4 mice to identify the mechanism responsible for the association between apoE4 and inflammatory metabolic diseases. The initial study confirmed previous reports that APOE4 gene replacement mice were less sensitive than APOE3 mice to diet-induced body weight gain but exhibited hyperinsulinemia, and their adipose tissues were similarly inflamed as those in APOE3 mice. Peritoneal macrophages isolated from APOE4 mice were defective in efferocytosis compared with APOE3 macrophages. Increased cell death was also observed in APOE4 macrophages when stimulated with LPS or oxidized LDL. Western blot analysis of cell lysates revealed that APOE4 macrophages displayed elevated JNK phosphorylation indicative of cell stress even under basal culturing conditions. Significantly higher cell stress due mainly to potentiation of endoplasmic reticulum (ER) stress signaling was also observed in APOE4 macrophages after LPS and oxidized LDL activation. The defect in efferocytosis and elevated apoptosis sensitivity of APOE4 macrophages was ameliorated by treatment with the ER chaperone tauroursodeoxycholic acid. Taken together, these results showed that apoE4 expression causes macrophage dysfunction and promotes apoptosis via ER stress induction. The reduction of ER stress in macrophages may be a viable option to reduce inflammation and inflammation-related metabolic disorders associated with the apoE4 polymorphism.     

Sphingosylphosphorylcholine acts in an anti-inflammatory manner in renal mesangial cells by reducing interleukin-1beta-induced prostaglandin E2 formation

Sphingosylphosphorylcholine (SPC) is a bioactive lipid that binds to G protein-coupled-receptors and activates various signaling cascades. Here, we show that in renal mesangial cells, SPC not only activates various protein kinase cascades but also activates Smad proteins, which are classical members of the transforming growth factor-beta (TGFbeta) signaling pathway. Consequently, SPC is able to mimic TGFbeta-mediated cell responses, such as an anti-inflammatory and a profibrotic response. Interleukin-1beta-stimulated prostaglandin E(2) formation is dose-dependently suppressed by SPC, which is paralleled by reduced secretory phospholipase A(2) (sPLA(2)) protein expression and activity. This effect is due to a reduction of sPLA(2) mRNA expression caused by inhibited sPLA(2) promoter activity. Furthermore, SPC upregulates the profibrotic connective tissue growth factor (CTGF) protein and mRNA expression. Blocking TGFbeta signaling by a TGFbeta receptor kinase inhibitor causes an inhibition of SPC-stimulated Smad activation and reverses both the negative effect of SPC on sPLA(2) expression and the positive effect on CTGF expression. In summary, our data show that SPC, by mimicking TGFbeta, leads to a suppression of proinflammatory mediator production and stimulates a profibrotic cell response that is often the end point of an anti-inflammatory reaction. Thus, targeting SPC receptors may represent a novel therapeutic strategy to cope with inflammatory diseases.     

Interaction of the N-terminal domain of apolipoprotein E4 with heparin

Apolipoprotein E (apoE) is an important lipid-transport protein in human plasma and brain. It has three common isoforms (apoE2, apoE3, and apoE4). ApoE is a major genetic risk factor in heart disease and in neurodegenerative disease, including Alzheimer's disease. The interaction of apoE with heparan sulfate proteoglycans plays an important role in lipoprotein remnant uptake and likely in atherogenesis and Alzheimer's disease. Here we report our studies of the interaction of the N-terminal domain of apoE4 (residues 1-191), which contains the major heparin-binding site, with an enzymatically prepared heparin oligosaccharide. Identified by its high affinity for the N-terminal domain of apoE4, this oligosaccharide was determined to be an octasaccharide of the structure DeltaUAp2S(1-->[4)-alpha-D-GlcNpS6S(1-->4)-alpha-L-IdoAp2S(1-->](3)4)-alpha-D-GlcNpS6S by nuclear magnetic resonance spectroscopy, capillary electrophoresis, and polyacrylamide gel electrophoresis. Kinetic analysis of the interaction between the N-terminal apoE4 fragment and immobilized heparin by surface plasmon resonance yielded a K(d) of 150 nM. A similar binding constant (K(d) = 140 nM) was observed for the interaction between immobilized N-terminal apoE4 and the octasaccharide. Isothermal titration calorimetry revealed a K(d) of 75 nM for the interaction of the N-terminal apoE fragment and the octasaccharide with a binding stoichiometry of approximately 1:1. Using previous studies and molecular modeling, we propose a binding site for this octasaccharide in a basic residue-rich region of helix 4 of the N-terminal fragment. From the X-ray crystal structure of the N-terminal apoE4, we predicted that binding of the octasaccharide at this site would result in a change in intrinsic fluorescence. This prediction was confirmed experimentally by an observed increase in fluorescence intensity with octasaccharide binding corresponding to a K(d) of approximately 1 microM.     

Astroglial regulation of apolipoprotein E expression in neuronal cells. Implications for Alzheimer's disease

Although apolipoprotein (apo) E is synthesized in the brain primarily by astrocytes, neurons in the central nervous system express apoE, albeit at lower levels than astrocytes, in response to various physiological and pathological conditions, including excitotoxic stress. To investigate how apoE expression is regulated in neurons, we transfected Neuro-2a cells with a 17-kilobase human apoE genomic DNA construct encoding apoE3 or apoE4 along with upstream and downstream regulatory elements. The baseline expression of apoE was low. However, conditioned medium from an astrocytic cell line (C6) or from apoE-null mouse primary astrocytes increased the expression of both isoforms by 3-4-fold at the mRNA level and by 4-10-fold at the protein level. These findings suggest that astrocytes secrete a factor or factors that regulate apoE expression in neuronal cells. The increased expression of apoE was almost completely abolished by incubating neurons with U0126, an inhibitor of extracellular signal-regulated kinase (Erk), suggesting that the Erk pathway controls astroglial regulation of apoE expression in neuronal cells. Human neuronal precursor NT2/D1 cells expressed apoE constitutively; however, after treatment of these cells with retinoic acid to induce differentiation, apoE expression diminished. Cultured mouse primary cortical and hippocampal neurons also expressed low levels of apoE. Astrocyte-conditioned medium rapidly up-regulated apoE expression in fully differentiated NT2 neurons and in cultured mouse primary cortical and hippocampal neurons. Thus, neuronal expression of apoE is regulated by a diffusible factor or factors released from astrocytes, and this regulation depends on the activity of the Erk kinase pathway in neurons.     

Effects of human apolipoprotein E isoforms on the amyloid beta-protein concentration and lipid composition in brain low-density membrane domains

Apolipoprotein E4 (apoE4) encoded by epsilon 4 allele is a strong genetic risk factor for Alzheimer's disease (AD). ApoE4 carriers have accelerated amyloid beta-protein (A beta) deposition in their brains, which may account for their unusual susceptibility to AD. We hypothesized that the accelerated A beta deposition in the brain of apoE4 carriers is mediated through cholesterol-enriched low-density membrane (LDM) domains. Thus, the concentrations of A beta and various lipids in LDM domains were quantified in the brains of homozygous apoE3 and apoE4 knock-in (KI) mice, and in the brains of those mice bred with beta-amyloid precursor protein (APP) transgenic mice (Tg2576). The A beta 40 and A beta 42 concentrations and the A beta 42 proportions in LDM domains did not differ between apoE3 and apoE4 KI mice up to 18 months of age. The A beta 40 concentration in the LDM domains was slightly, but significantly higher in apoE3/APP mice than in apoE4/APP mice. The lipid composition of LDM domains was modulated in an apoE isoform-specific manner, but its significance for A beta deposition remains unknown. These data show that the apoE isoform-specific effects on the A beta concentration in LDM domains do not occur in KI mouse models.