The interaction between apolipoprotein E and Alzheimer's amyloid beta-peptide is dependent on beta-peptide conformation

A non-toxic ligand for voxel-based MRI analysis of plaques in AD transgenic mice

Amyloid plaques are a characteristic feature in Alzheimer's disease (AD). A novel non-toxic contrast agent is presented, Gd-DTPA-K6Abeta1-30, which is homologous to Abeta, and allows plaque detection in vivo. microMRI was performed on AD model mice and controls prior to and following intracarotid injection with Gd-DTPA-K6Abeta1-30 in mannitol solution, to transiently open the blood-brain barrier. A gradient echo T2(*)-weighted sequence was used to provide 100 microm isotropic resolution with imaging times of 115 min. The scans were examined with voxel-based analysis (VBA) using statistical parametric mapping, for un-biased quantitative comparison of ligand-injected mice and controls. The results indicate that: (1) Gd-DTPA-K6Abeta1-30 is an effective, non-toxic, ligand for plaque detection when combined with VBA (p< or =0.01-0.001), comparing pre and post-ligand injection scans. (2) Large plaques can be detected without the use of a contrast agent and this detection co-localizes with iron deposition. (3) Smaller, earlier plaques require contrast ligand for MRI visualization. Our ligand when combined with VBA may be useful for following therapeutic approaches targeting amyloid in transgenic mouse models.

Time-dependent DNA condensation induced by amyloid beta-peptide

The major protein component of the amyloid deposition in Alzheimer's disease is a 39-43 residue peptide, amyloid beta (Abeta). Abeta is toxic to neurons, although the mechanism of neurodegeneration is uncertain. Evidence exists for non-B DNA conformation in the hippocampus of Alzheimer's disease brains, and Abeta was reportedly able to transform DNA conformation in vitro. In this study, we found that DNA conformation was altered in the presence of Abeta, and Abeta induced DNA condensation in a time-dependent manner. Furthermore, Abeta sheets, serving as condensation nuclei, were crucial for DNA condensation, and Cu(2+) and Zn(2+) ions inhibited Abeta sheet-induced DNA condensation. Our results suggest DNA condensation as a mechanism of Abeta toxicity.

Role of ApoE in conformation-prone diseases and atherosclerosis

Three isoforms of human plasma apolipoprotein E (apoE) are ligands to lipoprotein receptors and influence in different manner the synthesis and catabolism of pro-atherogenic triglyceride-rich lipoproteins. Among three isoforms, the apoE4 isoform is associated with increased frequency of atherosclerosis and Alzheimer's disease (AD). The conformational transitions of beta-amyloid (Abeta) influenced by apoE and serum amyloid P (SAP) component are key events in AD development, the accumulation of intermediate diffusible and soluble oligomers of Abeta being of particular significance. SAP and apoE, in a different manner for the three isoforms, serve as "pathological" chaperones during the aggregation of Abeta considered as a conformation-prone process. In turn, apoE consisting of two domains self-associates in solution and intermediate structures differently populated for the three isoforms exist. The different structures of the three isoforms determine their different distribution among various plasma lipoproteins. The structural and metabolic consideration of the common apoE pathway(s) in two pathologies assumes four molecular targets for AD correction: (i) inhibition of the accumulation of diffusible soluble Abeta oligomers; (ii) inhibition of apoE synthesis and secretion by astrocytes, in particular, under lipid-lowering therapy; (iii) inhibition of the binding of apoE and/or SAP to Abeta; (iv) stimulation of the expression of cholesterol transporter ABCA1.

Chronic thrombin exposure results in an increase in apolipoprotein-E levels

Studies have shown that individuals with both a history of traumatic brain injury and inheritance of apolipoprotein E-4 (ApoE4) allele are associated with a poor neurologic outcome and an increased risk for Alzheimer's disease. We assessed the hypothesis that thrombin released during brain injury causes an increase in apolipoprotein-E levels and such increase in the levels of apolipoprotein-E4 isoform may have amyloidogenic effects. Rats received either thrombin (100 nm, 0.25 microl/hr, 28 days) or vehicle via intracerebroventricular (i.c.v.) infusion. Thrombin treatment increased apolipoprotein-E levels in hippocampus as compared to vehicle treatment (P < 0.001). Infusion of human apolipoprotein-E4 (0.6 ng/hr, i.c.v., 56 days) into rats resulted in beta-amyloid deposition and increased the number of GFAP-positive astrocytes. ApoE4 infusion also resulted in significant spatial memory deficits. These findings suggest that thrombin released during brain injury may contribute to an increase in apolipoprotein-E levels. Such increase in Apolipoprotein-E4 isoform facilitates beta-amyloid deposition and cognitive deficits.

Kinetic analysis of the polymerization and depolymerization of β2-microglobulin-related amyloid fibrils in vitro

beta(2)-Microglobulin-related (Abeta(2)M) amyloidosis is a serious complication in patients on long-term dialysis, and partial unfolding of beta(2)-microglobulin (beta(2)-m) is believed to be prerequisite to its assembly into Abeta(2)M amyloid fibrils. Many kinds of amyloid-associated molecules (e.g., apolipoprotein E (apoE), glycosaminoglycans (GAGs), proteoglycans (PGs)) may contribute to the development of Abeta(2)M amyloidosis. The formation of Abeta(2)M amyloid fibrils in vitro was first observed at low pH (2.0-3.0). Very recently, low concentrations of 2,2,2-trifluoroethanol (TFE) and the sub-micellar concentration of sodium dodecyl sulfate, a model for anionic phospholipids, have been reported to cause the extension of Abeta(2)M amyloid fibrils at a neutral pH, inducing partial unfolding of beta(2)-m and stabilization of the fibrils. Moreover, apoE, GAGs and PGs were found to stabilize Abeta(2)M amyloid fibrils at a neutral pH, forming a stable complex with the fibrils. Some GAGs, especially heparin enhanced the fibril extension in the presence of TFE at a neutral pH. Some PGs, especially biglycan also induced the polymerization of acid-denatured beta(2)-m. These findings are consistent with the hypothesis that in vivo, specific molecules that affect the conformation and stability of beta(2)-m and amyloid fibrils will have significant effects on the deposition of Abeta(2)M amyloid fibrils.

Effect of d-amino acids at Asp23 and Ser26 residues on the conformational preference of Aβ20–29 peptides

The effects of d-amino acids at Asp(23) and Ser(26) residues on the conformational preference of beta-amyloid (Abeta) peptide fragment (Abeta(20-29)) have been studied using different spectroscopic techniques, namely vibrational circular dichroism (VCD), vibrational absorption, and electronic circular dichroism. To study the structure of the Abeta(20-29), [d-Asp(23)]Abeta(20-29), and [d-Ser(26)]Abeta(20-29) peptides under different conditions, the spectra were measured in 10mM acetate buffer (pH 3) and in 2,2,2-trifluoroethanol (TFE). The spectroscopic results indicated that at pH 3, Abeta(20-29) peptide takes random coil with beta-turn structure, while [d-Ser(26)]Abeta(20-29) peptide adopts significant amount of polyproline II (PPII) type structure along with beta-turn contribution and d-Asp-substituted peptide ([d-Asp(23)]Abeta(20-29)) adopts predominantly PPII type structure. The increased propensity for PPII conformation upon d-amino acid substitution, in acidic medium, has important biological implications. In TFE, Abeta(20-29), [d-Asp(23)]Abeta(20-29), and [d-Ser(26)]Abeta(20-29) peptides adopt 3(10)-helix, alpha-helix, and random coil with some beta-turn structures, respectively. The VCD data obtained for the Abeta peptide films suggested that the secondary structures for the peptide films are not the same as those for corresponding solution and are also different among the Abeta peptides studied here. This observation suggests that dehydration can have a significant influence on the structural preferences of these peptides.

A 13 kDa carboxy-terminal fragment of ApoE stabilizes Abeta hexamers

The pathological role of ApoE4 in Alzheimer's disease (AD) is not fully elucidated yet but there is strong evidence that ApoE is involved in Abeta deposition, which is an early hallmark of AD neuropathology. Overexpression of ApoE in neuroblastoma cells (Neuro2a) leads to the generation of an intracellular 13 kDa carboxy-terminal fragment of ApoE comparable to fragments seen in brains of AD patients. ApoE4 generates more of this fragment than ApoE2 and E3 suggesting a potential pathological role of these fragments in Alzheimer's disease. Analysis of this intracellular ApoE4 fragment by protease digest followed by MALDI-TOF mass spectrometry showed the proteolytic cleavage site close to residue 187 of ApoE. We have engineered and expressed the corresponding ApoE fragments in vitro. The recombinant 13 kDa carboxy-terminal fragment inhibited fibril formation of Abeta; this contrasts with the full-length ApoE and the corresponding amino-terminal ApoE fragment. Moreover, we show that the 13 kDa carboxy-terminal fragment of ApoE stabilizes the formation of Abeta hexamers. Complexes of Abeta with the 13 kDa carboxy-terminal ApoE fragment show toxicity in PC12 cells comparable to Abeta fibrils. These data suggest that cleavage of ApoE, leading to the generation of this fragment, contributes to the pathogenic effect of ApoE4 in AD.

apoE4 allele and the natural history of cardiovascular risk factors

The aims of the present study were to compare the longitudinal changes in traditional cardiovascular (CV) risk factors (blood pressure, BMI, total and HDL-cholesterol, triglycerides, and blood glucose) in men with and without the apolipoprotein (apo)E4 allele. Three hundred six men from the Baltimore Longitudinal Study of Aging, ranging in age from 20 to 92 yr, were studied. Repeated measurements of CV risk factors were performed over a median follow-up time of 7 yr (maximum 14.3 yr) for men. Longitudinal changes in these CV risk factors were analyzed by linear mixed-effects models. The prevalence of the apoE4 allele was 25.5%. apoE4 was independently associated with accelerated changes over time in fasting plasma glucose (+9.5% vs. no change in those without apoE4 in the 6th age-decade over 10 yr). No significant effect of apoE4 on longitudinal changes in total or HDL-cholesterol, triglycerides, or blood pressures was observed. In conclusion, apoE4 influences fasting plasma glucose and its changes over time. This could explain, in part, the increased CV risk associated with the apoE4 genotype observed in men.

Overexpression of monocyte chemotactic protein-1/CCL2 in beta-amyloid precursor protein transgenic mice show accelerated diffuse beta-amyloid deposition

Microglia accumulation at the site of amyloid plaques is a strong indication that microglia play a major role in Alzheimer's disease pathogenesis. However, how microglia affect amyloid-beta peptide (Abeta) deposition remains poorly understood. To address this question, we developed a novel bigenic mouse that overexpresses both amyloid precursor protein (APP) and monocyte chemotactic protein-1 (MCP-1; CCL2 in systematic nomenclature). CCL2 expression, driven by the glial fibrillary acidic protein promoter, induced mononuclear phagocyte (MP; monocyte-derived macrophage and microglial) accumulation in the brain. When APP/CCL2 transgenic mice were compared to APP mice, a fivefold increase in Abeta deposition was present despite increased MP accumulation around hippocampal and cortical amyloid plaques. Levels of full-length APP, its C-terminal fragment, and Abeta-degrading enzymes (insulin-degrading enzyme and neprilysin) in APP/CCL2 and APP mice were indistinguishable. Sodium dodecyl sulfate-insoluble Abeta (an indicator of fibrillar Abeta) was increased in APP/CCL2 mice at 5 months of age. Apolipoprotein E, which enhances Abeta deposition, was also increased (2.2-fold) in aged APP/CCL2 as compared to APP mice. We propose that although CCL2 stimulates MP accumulation, it increases Abeta deposition by reducing Abeta clearance through increased apolipoprotein E expression. Understanding the mechanisms underlying these events could be used to modulate microglial function in Alzheimer's disease and positively affect disease outcomes.

Molecular interactions in the formation and deposition of beta2-microglobulin-related amyloid fibrils

In beta2-microglobulin-related (A beta2M) amyloidosis, a serious complication in patients on long-term dialysis, partial unfolding of beta2-microglobulin (beta2-m) is believed to be prerequisite to its assembly into A beta2M amyloid fibrils. Many kinds of amyloid-associated molecules, (e.g., apolipoprotein E (apoE), glycosaminoglycans (GAGs), proteoglycans (PGs)) may contribute to the development of A beta2M amyloidosis. In 1990s, the formation of A beta2M amyloid fibrils in vitro was first observed at low pH (2.0-3.0). Very recently, low concentrations of 2,2,2-trifluoroethanol (TFE) and the sub-micellar concentration of sodium dodecyl sulfate, a model for anionic phospholipids, have been reported to cause the extension of A beta2M amyloid fibrils at a neutral pH, inducing partial unfolding of beta2-m and stabilization of the fibrils. Moreover, apoE, GAGs, and PGs were found to stabilize A beta2M amyloid fibrils at a neutral pH, forming a stable complex with the fibrils. Some GAGs, especially heparin, enhanced the fibril extension in the presence of TFE at a neutral pH. Some PGs, especially biglycan also induced the polymerization of acid-denatured beta2-m. These findings are consistent with the hypothesis that in vivo, specific molecules that affect the conformation and stability of beta2-m and amyloid fibrils will have significant effects on the deposition of A beta2M amyloid fibrils.

Eight-residue Abeta peptides inhibit the aggregation and enzymatic activity of Abeta42

Insoluble Abeta1-42 is the main component of the amyloid plaque. We have previously demonstrated that exposure to low pH can confer the molten globule state on soluble Abeta1-42 in vitro [Biochem. J. 361 (2000) 547] and unfolding experiments with guadinine hydrochloride (GdnHCl) have now confirmed this observation. The molten globule state of the protein has many biological properties and understanding the mechanisms of its formation is an important step in devising a therapeutic strategy for Alzheimer's disease (AD). We therefore investigated the ability of a series of synthetic eight-residue peptides derived from Abeta1-42 to inhibit the acid-induced aggregation of Abeta1-42 and identified the potent peptides to be Abeta15-22, Abeta16-23 and Abeta17-24. A1-antichymotrypsin, a member of the serine proteinase inhibitor (serpin) family is another major component of the amyloid plaque. In the present study, we investigated the proteolytic activity of Abeta1-42 against casein at different pHs. Chemical modification of amino acid residues in Abeta1-42 indicated that serine and histidine residues, but not aspartic acid, are necessary for enzymatic activity, suggesting that it is a serine proteinase. Amino acid substitution studies indicate that glutamic acids at positions 11 and 22 participate indirectly in proteolysis and we surmise that amino acid residues 29-42 are required to stabilize the conformer. A study of metal ions suggested that Cu2+ affected the enzymatic activity, but Zn2+ and Fe2+ did not. Interestingly, Abeta14-21 and Abeta15-22 were the only peptides that inhibited the proteolytic activity of Abeta42. Therefore, Abeta15-22 may control both aggregation of Abeta1-42 at acidic pH and its proteolytic activity at neutral pH. Consequently, we suggest that it may be of use in the therapy of Alzheimer's disease.

Fluorescence resonance energy transfer analysis of apolipoprotein E C-terminal domain and amyloid β peptide (1-42) interaction

The potential neurotoxicity of soluble forms of amyloid beta peptide (Abeta) as a key factor in early pathogenesis of Alzheimer's disease is being recognized. In addition, there is growing evidence of the essential role of apolipoprotein E (apoE) in amyloid formation, although molecular details of apoE/Abeta interaction are poorly understood. We employed apoE C-terminal (CT) domain comprising residues 201-299 to identify binding location of Abeta(1-42) by fluorescence resonance energy transfer (FRET) and quenching analyses. Native tryptophan (Trp) residues in the apoE CT domain served as FRET donor, whereas N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine (AEDANS) covalently attached to a unique cysteine residue substituted at position 4 of Abeta(1-42) (AEDANS-F4C-Abeta(1-42)) served as FRET acceptor. Fluorescence analysis verified that the oligomerization behavior of AEDANS-F4C-Abeta(1-42) was not abrogated by covalent attachment of AEDANS and that apoE CT domain/AEDANS-F4C-Abeta(1-42) association results in formation of a soluble complex. A large decrease in Trp fluorescence emission was noted in mixtures containing apoE CT domain and AEDANS-F4C-Abeta(1-42), accompanied by appearance of sensitized fluorescence emission of AEDANS as a result of intermolecular FRET. An average distance of separation of 22.6 Angstroms between donors and acceptor was calculated. Fluorescence quenching by potassium iodide (KI) did not reveal significant differences in apoE CT domain Trp microenvironment in the absence or the presence of Abeta(1-42). A twofold increase in quenching constant was noted for KI quenching of AEDANS fluorescence emission in the presence of apoE CT domain, indicative of alterations in Abeta conformation upon interaction with apoE CT domain. We propose intermolecular FRET analysis as a discriminating approach to examine apoE/Abeta interaction, a potentially critical factor in early events involved in amyloid formation.

Immunological and anti-chaperone therapeutic approaches for Alzheimer disease

Alzheimer disease (AD) is the most common cause of dementia. Currently available therapies only provide symptomatic relief. A number of therapeutic approaches are under development that aim to increase the clearance of brain Abeta peptides. These include immune mediated clearance of Abeta and the inhibition of the interaction between Abeta and its pathological chaperones. Both active and passive immunization has been shown to have robust effects in transgenic mouse models of AD on amyloid reduction and behavioral improvements. However, a human trial of active immunization has been associated with significant toxicity in a minority of patients. New generation vaccines are being developed which likely will reduce the potential for cell-mediated toxicity. In addition, the recent development of anti-chaperone therapy opens a new therapeutic avenue which is unlikely to be associated with toxicity.

A synthetic peptide blocking the apolipoprotein E/beta-amyloid binding mitigates beta-amyloid toxicity and fibril formation in vitro and reduces beta-amyloid plaques in transgenic mice

Alzheimer's disease (AD) is associated with accumulation of beta-amyloid (Abeta). A major genetic risk factor for sporadic AD is inheritance of the apolipoprotein (apo) E4 allele. ApoE can act as a pathological chaperone of Abeta, promoting its conformational transformation from soluble Abeta into toxic aggregates. We determined if blocking the apoE/Abeta interaction reduces Abeta load in transgenic (Tg) AD mice. The binding site of apoE on Abeta corresponds to residues 12 to 28. To block binding, we synthesized a peptide containing these residues, but substituted valine at position 18 to proline (Abeta12-28P). This changed the peptide's properties, making it non-fibrillogenic and non-toxic. Abeta12-28P competitively blocks binding of full-length Abeta to apoE (IC50 = 36.7 nmol). Furthermore, Abeta12-28P reduces Abeta fibrillogenesis in the presence of apoE, and Abeta/apoE toxicity in cell culture. Abeta12-28P is blood-brain barrier-permeable and in AD Tg mice inhibits Abeta deposition. Tg mice treated with Abeta12-28P for 1 month had a 63.3% reduction in Abeta load in the cortex (P = 0.0043) and a 59.5% (P = 0.0087) reduction in the hippocampus comparing to age-matched control Tg mice. Antibodies against Abeta were not detected in sera of treated mice; therefore the observed therapeutic effect of Abeta12-28P cannot be attributed to an antibody clearance response. Our experiments demonstrate that compounds blocking the interaction between Abeta and its pathological chaperones may be beneficial for treatment of beta-amyloid deposition in AD.

LRP/Amyloid β-Peptide Interaction Mediates Differential Brain Efflux of Aβ Isoforms

LRP (low-density lipoprotein receptor-related protein) is linked to Alzheimer's disease (AD). Here, we report amyloid beta-peptide Abeta40 binds to immobilized LRP clusters II and IV with high affinity (Kd = 0.6-1.2 nM) compared to Abeta42 and mutant Abeta, and LRP-mediated Abeta brain capillary binding, endocytosis, and transcytosis across the mouse blood-brain barrier are substantially reduced by the high beta sheet content in Abeta and deletion of the receptor-associated protein gene. Despite low Abeta production in the brain, transgenic mice expressing low LRP-clearance mutant Abeta develop robust Abeta cerebral accumulations much earlier than Tg-2576 Abeta-overproducing mice. While Abeta does not affect LRP internalization and synthesis, it promotes proteasome-dependent LRP degradation in endothelium at concentrations > 1 microM, consistent with reduced brain capillary LRP levels in Abeta-accumulating transgenic mice, AD, and patients with cerebrovascular beta-amyloidosis. Thus, low-affinity LRP/Abeta interaction and/or Abeta-induced LRP loss at the BBB mediate brain accumulation of neurotoxic Abeta.

Detection of Alzheimer's amyloid in transgenic mice using magnetic resonance microimaging

The presence of amyloid-beta (Abeta) plaques in the brain is a hallmark pathological feature of Alzheimer's disease (AD). Transgenic mice overexpressing mutant amyloid precursor protein (APP), or both mutant APP and presenilin-1 (APP/PS1), develop Abeta plaques similar to those in AD patients, and have been proposed as animal models in which to test experimental therapeutic approaches for the clearance of Abeta. However, at present there is no in vivo whole-brain imaging method to detect Abeta plaques in mice or men. A novel method is presented to detect Abeta plaques in the brains of transgenic mice by magnetic resonance microimaging (muMRI). This method uses Abeta1-40 peptide, known for its high binding affinity to Abeta, magnetically labeled with either gadolinium (Gd) or monocrystalline iron oxide nanoparticles (MION). Intraarterial injection of magnetically labeled Abeta1-40, with mannitol to transiently open the blood-brain barrier (BBB), enabled the detection of many Abeta plaques. Furthermore, the numerical density of Abeta plaques detected by muMRI and by immunohistochemistry showed excellent correlation. This approach provides an in vivo method to detect Abeta in AD transgenic mice, and suggests that diagnostic MRI methods to detect Abeta in AD patients may ultimately be feasible.

Apolipoprotein E enhances uptake of soluble but not aggregated amyloid-beta protein into synaptic terminals

The cellular mechanism by which apolipoprotein E (apoE) affects the pathogenesis of Alzheimer's disease (AD) is not understood. We have examined the effect of apolipoprotein E on the internalization of exogenous amyloid-beta 1-40 (Abeta40) into a rat brain crude synaptosomal preparation. Abeta40 peptide in soluble (within 1 h of dilution in buffer) or aggregated (aged 4 days before dilution in buffer) form was pre-incubated with lipidated apoE then added to synaptosomes; intraterminal amyloid-beta labeling was quantified using flow cytometry following immunolabeling with the anti-Abeta (10G4) antibody. The number of Abeta-positive synaptosomes was increased ( approximately 50%) by treatment with a soluble Abeta/apoE mixture compared with treatment with soluble Abeta40 alone. However, when the Abeta was aggregated, less sodium dodecyl sulfate (SDS)-stable Abeta/apoE complex was formed and the addition of apoE decreased the number of Abeta-positive terminals. The addition of the lipoprotein-receptor related protein (LRP) antagonist receptor-associated protein (RAP) inhibited the apoE-induced increase in synaptosomal Abeta, and controls treated with trypsin and heparinase confirm intraterminal localization of the majority of the soluble Abeta. The apoE-mediated increase in Abeta labeling was confirmed in intact cells by immunocytochemistry of dorsal root ganglion (DRG) neurons. These results suggest that complex formation with apoE enhances internalization of soluble Abeta uptake into terminals.

Co-localization of cholesterol, apolipoprotein E and fibrillar Abeta in amyloid plaques

Recent evidence strongly suggests a role for cholesterol and apolipoprotein E in the etiology of Alzheimer's disease. We have demonstrated the co-localization of cholesterol and apolipoprotein E with beta-amyloid immunoreactivity and thioflavin S immunofluorescence in AD type plaques of a transgenic mouse model. Cholesterol and apolipoprotein E co-localized to the core of thioflavin S-positive (fibrillar) plaques, but not thioflavin S-negative (diffuse) plaques from an early age. By 18 months of age, there was extensive coverage of fibrillar plaques immunopositive for apolipoprotein E and cholesterol oxidase. These findings support evidence that cholesterol and apolipoprotein E are involved in fibrillar plaque formation or maintenance, and suggest that cholesterol may impact amyloid formation extracellularly, as well as through an intracellular effect.

Apolipoprotein E affects amyloid formation but not amyloid growth in vitro: mechanistic implications for apoE4 enhanced amyloid burden and risk for Alzheimer's disease

The transition from the partially folded soluble Abeta monomer to insoluble Abeta amyloidfibrils is seminal to the formation and growth of amyloid plaques in Alzheimer's disease (AD). A detailed understanding of the role of AD risk factors in these processes is essential to understanding the physiochemical nature of this conformational rearrangement. The apolipoprotein E epsilon4 allele, a risk factor for AD, affects AD pathology by increasing amyloid burden relative to the much more common epsilon3 allele. In the present study, in vitro models were employed to probe the effect of these proteins on kinetically distinct steps in Abeta fibrillogenesis. Formation of Abeta amyloid was faster in the presence of apoE4 than apoE3, while growth of existing plaques was unaffected by either isoform. Further, experiments with Abeta stereoisomers establish that this effect of apoE3 is mediated through interaction with oligomeric fibrillogenic intermediates rather than through specific contacts with monomeric Abeta. Consistent with the altered pathology and enhanced risk for AD associated with inheritance of the epsilon4 allele, we conclude that APOE epsilon4 is a risk factor for AD not due to a pathological gain of function of apoE4 but to a loss of protective function of apoE3.

Melatonin reverses the profibrillogenic activity of apolipoprotein E4 on the Alzheimer amyloid Abeta peptide

Inheritance of apoE4 is a strong risk factor for the development of late-onset sporadic Alzheimer's disease (AD). Several lines of evidence suggest that apoE4 binds to the Alzheimer Abeta protein and, under certain experimental conditions, promotes formation of beta-sheet structures and amyloid fibrils. Deposition of amyloid fibrils is a critical step in the development of AD. We report here that addition of melatonin to Abeta in the presence of apoE resulted in a potent isoform-specific inhibition of fibril formation, the extent of which was far greater than that of the inhibition produced by melatonin alone. This effect was structure-dependent and unrelated to the antioxidant properties of melatonin, since it could be reproduced neither with the structurally related indole N-acetyl-5-hydroxytryptamine nor with the antioxidants ascorbate, alpha-tocophenol, and PBN. The enhanced inhibitory effects of melatonin and apoE were lost when bovine serum albumin was substituted for apoE. In addition, Abeta in combination with apoE was highly neurotoxic (apoE4 > apoE3) to neuronal cells in culture, and this activity was also prevented by melatonin. These findings suggest that reductions in brain melatonin, which occur during aging, may contribute to a proamyloidogenic microenvironment in the aging brain.

Role of apoe/Abeta interactions in the pathogenesis of Alzheimer's disease and cerebral amyloid angiopathy

The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The mechanism underlying this increased risk is not completely clear, yet mounting evidence supports the idea that the ability of apoE to interact with the amyloid-beta (Abeta) peptide and influence its conformation and clearance plays a major role. Evidence to support this concept comes from in vitro and in vivo studies of apoE/Abeta interactions and the effects of these interactions on Abeta conformation and cellular clearance. Recent studies on the effect of murine and human apoE in APP transgenic mice provide direct evidence that apoE is critically involved in the in vivo converstion of Abeta into forms which contain high 5-sheet content and associated cellular toxicity (neuritic plaques and CAA). These studies also suggest a role for human apoE in Abeta clearance in vivo.

Apolipoprotein E inhibits the depolymerization of beta 2-microglobulin-related amyloid fibrils at a neutral pH

beta 2-Microglobulin-related (A beta 2M) amyloidosis is a common and serious complication in patients on long-term hemodialysis, and beta 2-microglobulin (beta 2-m) is a major structural component of A beta 2M amyloid fibrils. Fluorescence spectroscopic analysis with thioflavin T and electron microscopic study revealed that A beta 2M amyloid fibrils readily depolymerize into monomeric beta 2-m at a neutral to basic pH. Circular dichroism analysis revealed that soon after the initiation of the depolymerization reaction at pH 7.5, the characteristic spectrum of beta 2-m in A beta 2M amyloid fibrils changes to resemble that of monomeric beta 2-m at pH 7.5. Apolipoprotein E (apoE), a representative amyloid-associated protein, formed a stable complex with A beta 2M amyloid fibrils and inhibited the depolymerization of A beta 2M amyloid fibrils dose-dependently in a range of 0--10 microM. These results showed that apoE could enhance the deposition of amyloid fibrils in vivo, possibly by binding directly to the surface of the fibrils and stabilizing the conformation of beta 2-m in the fibrils.

SDS-stable complex formation between native apolipoprotein E3 and beta-amyloid peptides

Extracellular senile plaques composed predominantly of fibrillar amyloid-beta (Abeta) are a major neuropathological feature of Alzheimer's disease (AD). Genetic evidence and in vivo studies suggest that apolipoprotein E (apoE) may contribute to amyloid clearance and/or deposition. In vitro studies demonstrate that native apoE2 and E3 form an SDS-stable complex with Abeta(1-40), while apoE4 forms little such complex. Our current work extends these observations by presenting evidence that apoE3 also binds to Abeta(1-42) and with less avidity to modified species of the peptide found in senile plaque cores. These modified peptides include a form that originates at residue 3-Glu as pyroglutamyl and another with isomerization at the 1-Asp and 7-Asp positions. In addition, we used binding reactions between apoE3 and various Abeta fragments, as well as binding reactions with apoE3 and Abeta(1-40) plus Abeta fragments as competitors, to identify the domain(s) of Abeta involved in the formation of an SDS-stable complex with apoE3. Residues 13-28 of Abeta appear to be necessary, while complex formation is further enhanced by the presence of residues at the C-terminus of the peptide. These results contribute to our understanding of the biochemical basis for the SDS-stable apoE3/Abeta complex and support the hypothesis that Abeta can be transported in vivo complexed with apoE. This complex may then be cleared from the interstitial space by apoE receptors in the brain or become part of an extracellular amyloid deposit.

Self-association of human apolipoprotein E3 and E4 in the presence and absence of phospholipid

Human apolipoprotein E (apoE) exists as three main isoforms, differing by single amino acid substitutions, with the apoE4 isoform strongly linked to the incidence of late onset Alzheimer's disease. We have expressed and purified apoE3 and apoE4 from Escherichia coli and compared their hydrodynamic properties by gel permeation liquid chromatography, capillary electrophoresis, circular dichroism, and sedimentation methods. Sedimentation velocity experiments, employing a new method for determining the size distribution of polydisperse macromolecules in solution (Schuck, P. (2000) Biophys. J. 78, 1606-1619), provide direct evidence for the heterogeneous solution structures of apoE3 and apoE4. In a lipid-free environment, apoE3 and apoE4 exist as a slow equilibrium mixture of monomer, tetramer, octamer, and a small proportion of higher oligomers. Both sedimentation velocity and equilibrium experiments indicate that apoE4 has a greater propensity to self-associate. We also demonstrate that apoE3 and apoE4 oligomers dissociate significantly in the presence of dihexanoylphosphatidylcholine micelles (20 mm) and to a lesser extent at submicellar concentrations (4 mm). The alpha-helical content for both isoforms was almost identical (50%) in the presence and absence of dihexanoylphosphatidylcholine. These results reveal that apoE oligomers undergo phospholipid-induced dissociation to folded monomers, suggesting the monomeric form prevails on the lipoprotein surface in vivo.

Apolipoprotein E and Alzheimer disease: an update on genetic and functional analyses

Exceptional advances have been made in understanding the genetics of how common polymorphisms of the apolipoprotein E gene influence the risk and age of onset of Alzheimer disease (AD). The major genetic susceptibility locus for the common forms of AD, there are 3 common alleles, designated epsilon2, epsilon3, and epsilon4. The inheritance of each dose of APOE4 increases the risk of disease and decreases the age of onset; conversely, the APOE2 allele appears to be protective, by lowering the risk of disease and increasing the age of onset. Testing for the APOE4 allele can be a clinically useful tool in the early diagnosis of cognitively impaired patients suspected of having AD. The APOE4 allele also negatively influences functional recovery following a variety of brain insults. What remains in the study of apolipoprotein E is an explanation of how minor changes in a protein can produce such striking differences in risk and age of onset. In vitro and animal model studies strongly suggest that brain apolipoprotein E is a multifunctional molecule, with potential roles in amyloid deposition and clearance, microtubule stability, intracellular signaling, immune modulation, glucose metabolism, oxidative stress, and other cellular processes. While the relevance of these proposed functions to the etiology of AD remains a mystery, these and other hypotheses will be tested as the field of apoE neurobiology grows, adding relevant new data to the functions of apoE in health and in the pathogenic mechanisms leading to AD.

Sodium dodecyl sulfate-resistant complexes of Alzheimer's amyloid beta-peptide with the N-terminal, receptor binding domain of apolipoprotein E

Immunocytochemical, biochemical, and molecular genetic studies indicate that apolipoprotein E (apoE) plays an important role in the process of amyloidogenesis-beta. However, there is still no clear translation of these data into the pathogenesis of amyloidosis-beta. Previous studies demonstrated sodium dodecyl sulfate (SDS)-resistant binding of apoE to the main component of Alzheimer's amyloid-A beta and modulation of A beta aggregation by apoE in vitro. To more closely characterize apoE-A beta interactions, we have studied the binding of thrombolytic fragments of apoE3 to A beta in vitro by using SDS-polyacrylamide gel electrophoresis and intrinsic fluorescence quenching. Here we demonstrate that SDS-resistant binding of A beta is mediated by the receptor-binding, N-terminal domain of apoE3. Under native conditions, both the N- and C-terminal domains of apoE3 bind A beta; however, the former does so with higher affinity. We propose that the modulation of A beta binding to the N-terminal domain of apoE is a potential therapeutic target for the treatment of amyloidosis-beta.

Apolipoprotein E includes a binding site which is recognized by several amyloidogenic polypeptides

Inheritance of the apolipoprotein E (apoE) epsilon 4 allele is a risk factor for late-onset Alzheimer's disease (AD). Biochemically apoE is present in AD plaques and neurofibrillary tangles of the AD brain. There is a high avidity and specific binding of apoE and the amyloid beta-peptide (A beta). In addition to AD apoE is also present in many other cerebral and systemic amyloidoses, Down's syndrome and prion diseases but the pathophysiological basis for its presence is still unknown. In the present study we have compared the interaction of apoE with A beta, the gelsolin-derived amyloid fragment AGel(183-210) and the amyloidogenic prion fragments PrP(109-122) and PrP(109-141). We show that, similar to A beta, also AGel and PrP fragments can form a complex with apoE, and that the interaction between apoE and the amyloidogenic protein fragments is mediated through the same binding site on apoE. We also show that apoE increases the thioflavin-T fluorescence of PrP and AGel and that apoE influences the content of beta-sheet conformation of these amyloidogenic fragments. Our results indicate that amyloids and amyloidogenic prion fragments share a similar structural motif, which is recognized by apoE, possibly through a single binding site, and that this motif is also responsible for the amyloidogenicity of these fragments.

125I-labeled ApoE binds competitively to beta(1-40) fibrils with pathological chaperone proteins

Radiolabeled Apolipoprotein E (Apo E) was used in a competitive binding filtration assay to amyloid fibrils preformed from beta(1-40) peptide as a probe of the binding sites for proteins either found in senile plaques in Alzheimer's Disease brain or reported to be associated with the soluble peptide. Apo E, Apo J, Apo A-I, Apo B, laminin, complement components C3 and C4, and alpha 1-antichymotrypsin all displayed sub-micromolar apparent affinities for the Apo E binding site on fibrils. Transthyretin, alpha 2-macroglobulin, amyloid P protein, heparan sulfate proteoglycan, complement component C1q, chondroitin sulfate A, and GM1 ganglioside were much less effective. The epsilon 2, epsilon 3, and epsilon 4 isoforms of Apo E showed different affinities for fibrils and lipidation of these lipoproteins made little difference. Other fibrillar beta-peptides also bound Apo E, with A beta 40-A beta 42 > A beta(12-28); A beta(25-35) = 0. A series of soluble beta-peptides and fragments failed to effect Apo E binding. Thus, both conformational and quaternary structural features are important in high affinity binding of Apo E to A beta 40 fibrils. Different amyloid plaque-associated molecules apparently associate with alternative primary and secondary structural features on fibrils.

Heparin specifically inhibits binding of apolipoprotein E to amyloid beta-peptide

Apolipoprotein E (apoE) binds to non-fibrillar amyloid beta-peptide with high affinity. We find here that heparin specifically inhibits apoE-amyloid beta-peptide (1-40) interaction. Low molecular weight heparins reduce the affinity of this interaction 3-fold as it was estimated by surface plasmon resonance. The binding is not affected by high salt concentration, which prevents heparin-induced changes of apoE conformation. We propose that rigid protein conformation, induced by high affinity heparin binding to apoE, is unfavorable for its interaction to amyloid beta-peptide. Using thioflavin T assay, we find that heparin promotes fibrillogenesis of amyloid beta-peptide whereas apoE abolishes this effect. The data suggests that the relationship between apoE and glycosaminoglycans may be important for amyloid beta-peptide fibril formation.

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.

Differential effects of apolipoprotein E isoforms on metal-induced aggregation of A beta using physiological concentrations

The epsilon 4 allele of apolipoprotein E (APOE) has been found to be a risk factor for late-onset Alzheimer's disease (AD). While the pathogenic mechanism of APOE in AD is not yet clear, APOE isoforms appear to differentially influence the aggregation of A beta, the principal component of Alzheimer-associated beta-amyloid deposits. To date, no data are available for the propensity of A beta to aggregate in the presence of APOE under conditions where these components are at physiological concentrations (in cerebrospinal fluid, APOE and A beta are approximately 100 nM and approximately 5 nM, respectively). We employed a novel in vitro filtration assay for detecting zinc(II)- and copper(II)-induced aggregation of A beta in solutions containing concentrations of the peptide that are similar to those reported for human cerebrospinal fluid. The potential for resolubilization with EDTA and the relative densities of zinc- and copper-induced A beta aggregates were also compared. Zinc-induced A beta aggregates were found to be denser and less easily resolubilized than copper-induced precipitates. Metal-induced aggregation of A beta was studied in the presence of purified apolipoprotein E2, apolipoprotein E3, and apolipoprotein E4 under conditions that approximate the physiological concentrations and ratios of these proteins. In the presence of all three APOE isoforms, zinc-induced aggregation of A beta was attenuated, while precipitation with copper was enhanced. Consistent with the increased risk for AD associated with the epsilon 4 allele of APOE, metal-induced aggregation of A beta was highest for both zinc and copper in the presence of apolipoprotein E4. Our data are consistent with a role for APOE as an in vivo molecular chaperone for A beta.

Beta-amyloid peptide interacts specifically with the carboxy-terminal domain of human apolipoprotein E: relevance to Alzheimer's disease

Growing evidence indicates the involvement of apolipoprotein E (apoE) in the development of late-onset and sporadic forms of Alzheimer's disease, although its exact role remains unclear. We previously demonstrated that beta-amyloid peptide (Abeta) displays membrane-destabilizing properties and that only apoE2 and E3 isoforms inhibit these properties. In this study, we clearly demonstrate that the carboxy-terminal lipid-binding domain of apoE (e.g., residues 200-299) is responsible for the Abeta-binding activity of apoE and that this interaction involves pairs of apoE amphipathic alpha-helices. We further demonstrate that Abeta is able to inhibit the association of the C-terminal domain of apoE with lipids due to the formation of Abeta/apoE complexes resistant to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. On the contrary, the amino-terminal receptor-binding domain of apoE (e.g., residues 129-169) is not able to form stable complexes with Abeta. These data extend our understanding of human apoE-dependent binding of Abeta by involving the C-terminal domain of apoE in the efficient formation of apoE/Abeta complex.

Opposite roles of apolipoprotein E in normal brains and in Alzheimer's disease

We have characterized the interaction between apolipoprotein E (apoE) and amyloid beta peptide (Abeta) in the soluble fraction of the cerebral cortex of Alzheimer's disease (AD) and control subjects. Western blot analysis with specific antibodies identified in both groups a complex composed of the full-length apoE and Abeta peptides ending at residues 40 and 42. The apoE-Abeta soluble aggregate is less stable in AD brains than in controls, when treated with the anionic detergent SDS. The complex is present in significantly higher quantity in control than in AD brains, whereas in the insoluble fraction an inverse correlation has previously been reported. Moreover, in the AD subjects the Abeta bound to apoE is more sensitive to protease digestion than is the unbound Abeta. Taken together, our results indicate that in normal brains apoE efficiently binds and sequesters Abeta, preventing its aggregation. In AD, the impaired apoE-Abeta binding leads to the critical accumulation of Abeta, facilitating plaque formation.

Alzheimer's beta-amyloid vasoactivity: identification of a novel beta-amyloid conformational intermediate

The beta-amyloid (A beta) peptide has previously been shown to enhance phenylephrine or endothelin-1 induced constriction of aortic rings in vitro. The characteristics of A beta vasoactivity (dose, fragment length, timing) suggest that the mechanism is distinct from A beta cytotoxicity. To identify which properties of A beta determine its biological activity on vessels, we investigated a number of A beta analogues and fragments, individually and in combination, including those that are known to be associated with Alzheimer's disease (A beta(1-42)) and hereditary cerebral hemorrhage with amyloidosis--Dutch type (A beta(22Q)(1-40)). The vasoactivity appears to be related to the conformation adopted by the peptide in solution. The beta-pleated sheet rich A beta(1-42) and A beta(22Q)(1-40) were each less vasoactive than the mainly random coil wild type A beta(1-40). However, the most vasoactive A beta peptides were combinations which contain mixtures of random coil and beta-sheet structure. The finding that peptides containing low or high levels of beta-pleated conformation are less vasoactive than those containing intermediate amounts of this structural motif allows us to propose the existence of a transitional form between random coil and beta-pleated that is the vasoactive species of A beta. This is the first time that A beta conformational intermediates have been identified and a biological activity associated with them.

Laminin inhibits A beta 40 fibril formation promoted by apolipoprotein E4 in vitro

The aggregation of soluble A beta into insoluble amyloid fibrils is believed to be an important step in the pathogenesis of Alzheimer's disease (AD) and the prevention of this process therefore seems to be a promising strategy for the treatment of AD. Both apolipoprotein E(apoE) and laminin are known to play important roles in the regeneration of the central nervous system and both are known to accumulate in the senile plaques of the AD brains. In the present study, we therefore investigated whether or not laminin has any effect on A beta 40 fibril formation promoted by apoE4 in vitro. A thioflavine-T fluorometric assay and electron microscopic observations using negative staining together demonstrated that laminin inhibits A beta 40 fibril formation in vitro while it also inhibits A beta 40 fibril formation promoted by apoE4. These results suggested that either laminin or its derivatives may thus be effective as therapeutic agents for AD.

Structural and kinetic features of amyloid beta-protein fibrillogenesis

Alzheimer's disease (AD) is an archetype of a class of diseases characterized by abnormal protein deposition. In each case, deposition manifests itself in the form of amyloid deposits composed of fibrils of otherwise normal, soluble proteins or peptides. An ever-increasing body of genetic, physiologic, and biochemical data supports the hypothesis that fibrillogenesis of the amyloid beta-protein is a seminal event in Alzheimer's disease. Inhibiting A beta fibrillogenesis is thus an important strategy for AD therapy. However, before this strategy can be implemented, a mechanistic understanding of the fibrillogenesis process must be achieved and appropriate steps selected as therapeutic targets. Following a brief introduction to AD, I review here the current state of knowledge of A beta fibrillogenesis. Special emphasis is placed on the morphologic, structural, and kinetic aspects of this complex process.

Effects of beta-amyloid peptides on the fluidity of membranes from frontal and parietal lobes of human brain. High potencies of A beta 1-42 and A beta 1-43

beta-amyloid peptide (A beta) and several A beta-fragments decrease the fluidity of human cortex membranes in a concentration dependent fashion. The effect of A beta on membrane fluidity increases with peptide length, is most pronounced for A beta 1-43 and can be seen at concentrations as low as 100 nmol/l. While the fragment A beta 25-35 is active, scrambled peptide (A beta 35-25) when investigated under similar conditions shows no effects on membrane fluidity. The effect of A beta peptides on fluidity of the phospholipid bilayer is more pronounced in the hydrocarbon core (labeled with the fluorescence probe 1,6-diphenylhexa-1,3,5-triene) than in the region of the hydrophilic heads (labeled with the fluorescence probe 1-[4'-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene). It is suggested that the effect of A beta on neuronal membranes is probably a major initial mechanism in a cascade of events finally leading to neurotoxicity and cell death in Alzheimer's disease.

Biology of A beta amyloid in Alzheimer's disease

The genetic associations with the pathological features of AD are diverse: A rapidly growing number of mutations in presenilin 1 and 2 on chromosomes 14 and 1, respectively, are found in many early-onset FAD patients (Lendon et al., 1997). In addition, beta PP mutations are found in a small percentage of early-onset FAD kindreds. The apoE4 allele on chromosome 19 is associated with the presence of the most common form of AD, sporadic AD (Wisniewski & Frangione, 1992; Namba et al., 1991). However, it is clear that other proteins are also involved in the pathogenesis of AD, since some early-onset FAD kindreds do not have linkage to PS1, PS2, apoE, or beta PP, while at least 50% of late-onset AD is unrelated to apoE. Other proteins which have been implicated in the formation of senile plaques, but so far are not known to have any genetic linkage to AD, include proteoglycans (Snow et al., 1987), apoA1 (Wisniewski et al., 1995a), alpha 1-antichymotrypsin (Abraham et al., 1988), HB-GAM (Wisniewski et al., 1996a), complement components (McGeer & Rogers, 1992), acetylcholinesterase (Friede, 1965), and NAC (Ueda et al., 1993). Which of these proteins will be the most important for the etiology of the most common form of AD, late-onset sporadic AD, remains an open question. Three of the genes which are now known to be linked to AD, including PS1, beta PP, and apoE, have been established immunohistochemically and biochemically to be components of senile plaques (see Fig. 1). This raises at least two possibilities: either each of these proteins is part of one pathway with A beta-related amyloid formation as a final causative pathogenic event or amyloid deposition in AD is a reactive process related to dysfunction of a number of different CNS proteins. Whether or not amyloid formation is directly causative in the pathogenesis of AD, current data suggest that new therapeutic approaches which may inhibit the aggregation and/or the conformational change of sA beta to A beta fibrils (Soto et al., 1996) have the greatest likelihood to make a significant impact on controlling amyloid accumulation in AD.

Detection of apolipoprotein E/dimeric soluble amyloid beta complexes in Alzheimer's disease brain supernatants

The inheritance of the apolipoprotein (apo) E4 allele is an important risk factor for late-onset Alzheimer's disease (AD). A major component of the Alzheimer's disease neuritic plaques is amyloid beta (A beta). We previously identified apoE/A beta complexes within neuritic plaques (1). It was not known if this interaction takes place before or after A beta peptides become incorporated into neuritic plaques. To address this question we sought evidence of apoE complexes with brain soluble A beta peptides in AD and control patients. In addition, numerous proteins have been shown to bind A beta peptides in vitro. It is not know if any of these bind brain sA beta in vivo. We found evidence for the presence of apoE/dimeric sA beta complexes in the AD brain and could not detect complexes with other A beta peptide binding proteins. The binding of sA beta to apoE may be one factor influencing its clearance from the brain and/or its conformational state.

Acetylcholinesterase promotes the aggregation of amyloid-beta-peptide fragments by forming a complex with the growing fibrils

Acetylcholinesterase (AChE), an enzyme involved in the hydrolysis of the neurotransmitter acetylcholine, consistently colocalizes with the amyloid deposits characteristic of Alzheimer's disease and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. In order to identify the structural domains of the amyloid-beta-peptide (Abeta) involved in the aggregation induced by AChE, we have studied the effect of this cholinergic enzyme on Abeta peptide fragments of different sizes. AChE enhanced the aggregation of the Abeta(12-28) and Abeta(25-35) peptides but not of the Abeta(1-16) fragment. The inductive effect of AChE on the aggregation of Abeta(12-28) was abolished by the presence of either Abeta(1-16) or Abeta(9-21). The effect of the enzyme was also analysed using two different mutant fragments, possessing a low and the other a high capacity for fibrillogenesis. The fragments used were Abeta(12-28)Val18-->Ala and Abeta(12-28)Glu22-->Gln, respectively. AChE was able to promote the aggregation of these fragments in a very specific way and both mutant peptides were able to form amyloid fibrils, as revealed by negative staining under the electron microscope. Binding assays indicated that AChE was bound to Abeta(12-28), as well as to the Abeta(1-16) peptide. AChE was seen to form strong complexes with the Abeta(12-28) fibrils as such complexes stained positively for both thioflavine-T and AChE activity, were resistant to high ionic strength treatment, and were partially sensitive to detergents, suggesting that hydrophobic interactions may play a role in the stabilization of the AChE-Abeta complex. Our results suggest that such amyloid-AChE complexes are formed when AChE interacts with the growing amyloid fibrils and accelerates the assembly of Abeta peptides. This is consistent with the fact that AChE is known to be present within Abeta deposits including the pre-amyloid diffuse and mature senile plaques found in Alzheimer's brain.

Amino-terminus truncated apolipoprotein E is the major species in amyloid deposits in Alzheimer's disease-affected brains: a possible role for apolipoprotein E in Alzheimer's disease

Amyloid deposits in Alzheimer's disease (AD) are composed of amyloid beta protein (A beta) and many other components called amyloid-associated proteins. Apolipoprotein E (apoE) is one of the most important amyloid-associated proteins. The role apoE plays in AD, however, is yet to be determined. In this study, we present the biochemical and histochemical nature of apoE in AD-affected brains using four monoclonal antibodies (mAbs) against apoE and newly established antibodies against the amino-terminal (anti-apoE-N), and carboxyl-terminal regions (anti-apoE-C) of apoE. Competitive ELISA and Western-blot analysis combined with thrombolytic digestion of apoE indicated that our four mAbs recognized at least two different epitopes within a 22-kDa amino-terminal domain of apoE. Using these mAbs and an anti-A beta mAb, double immunostaining showed that the majority of amyloid deposits were stained by both anti-apoE and anti-A beta mAbs, but the minority of them were detected only by either anti-apoE or anti-A beta mAbs. Differences in staining properties between anti-apoE-N and anti-apoE-C were that anti-apoE-C recognized both amyloid deposits and astrocytes similar to anti-apoE mAbs, but anti-apoE-N strongly stained only astrocytes. Preliminary semi-quantitative determinations of apoE in CSF and brain homogenate showed that the amount of apoE increased in AD and Creutzfeldt-Jakob disease brains compared to normal samples. Our immunological data, using antibodies specific for the amino and carboxyl termini of apoE, suggest that apoE may, in some circumstances, initiate plaque formation, and that apoE in amyloid deposits has at least part of its amino termini cleaved out.

Apolipoprotein E forms stable complexes with recombinant Alzheimer's disease beta-amyloid precursor protein

Apolipoprotein E (apoE), a protein genetically linked to the incidence of Alzheimer's disease, forms SDS-stable complexes in vitro with beta-amyloid peptide (Abeta), the primary component of senile plaques. In the present study, we investigated whether apoE was able to bind full-length Abeta precursor protein (APP). Using a maltose-binding-protein-APP fusion protein and human very-low-density lipoprotein (VLDL), we detected an interaction of apoE with APP that was inhibited by Abeta or anti-apoE antibody. Saturation-binding experiments indicated a single binding equilibrium with an apparent 1:1 stoichiometry and a dissociation constant of 15 nM. An interaction was also observed using apoE from cerebrospinal fluid or delipidated VLDL, as well as recombinant apoE. APP.apoE complexes were SDS-stable, and their formation was not inhibited by reducing conditions; however, they were dissociated by SDS under reducing conditions. ApoE.APP complexes formed high-molecular-mass aggregates, and competition experiments suggested that amino acids 14-23 of Abeta are responsible for complex-formation. Finally, no differences were found when studying the interaction of APP with apoE3 or apoE4. Taken together, our results demonstrate that apoE may form stable complexes with the Abeta moiety of APP with characteristics similar to those of complexes formed with isolated Abeta, and suggest the intriguing possibility that apoE-APP interactions may be pathologically relevant in vivo.

Concentration-dependent inhibitory effects of apolipoprotein E on Alzheimer's beta-amyloid fibril formation in vitro

Recently, many research groups have examined the effect of apolipoprotein E (apoE) on beta-amyloid fibril (betaAf) formation in vitro. However, their data were somewhat controversial and no exact kinetic assessment of the role of apoE has thus far been available. We examined the effect of human apoE on betaAf formation in vitro, starting with various concentrations of freshly prepared beta-amyloid(1-40) (beta1-40) and using fluorescence spectroscopy with thioflavine T. When 50 microM of beta1-40 was incubated with a 1:1000 to 1:100 molar ratio of apoE, a dose-dependent inhibitory effect of apoE was observed. Both the nucleation and extension phases of betaAf formation in vitro were inhibited by apoE. On the other hand, when 300 microM of beta1-40 was incubated with a 1:100 molar ratio of apoE, the inhibitory effect of apoE was completely abolished. We then focused our study on the kinetics of the inhibitory effect of apoE on the extension phase of betaAf formation in vitro, utilizing the recently established first-order kinetic model of betaAf extension in vitro [Naiki, H., & Nakakuki, K. (1996) Lab. Invest. 74, 374-383]. The mathematical treatment of the data suggests that apoE inhibits the extension of betaAf in vitro, by making a complex with beta1-40, thus eliminating free beta1-40 from the reaction mixture. The equilibrium association constant with beta1-40 was practically the same among the three major recombinant apoE isoforms. These results indicate that the effects of apoE on betaAf formation in vitro is differential and could settle some of the controversy about beta-amyloid-apoE interaction in vitro.

Amyloid-beta peptide-receptor for advanced glycation endproduct interaction elicits neuronal expression of macrophage-colony stimulating factor: a proinflammatory pathway in Alzheimer disease

In Alzheimer disease (AD), neurons are thought to be subjected to the deleterious cytotoxic effects of activated microglia. We demonstrate that binding of amyloid-beta peptide (Abeta) to neuronal Receptor for Advanced Glycation Endproduct (RAGE), a cell surface receptor for Abeta, induces macrophage-colony stimulating factor (M-CSF) by an oxidant sensitive, nuclear factor kappaB-dependent pathway. AD brain shows increased neuronal expression of M-CSF in proximity to Abeta deposits, and in cerebrospinal fluid from AD patients there was approximately 5-fold increased M-CSF antigen (P < 0.01), compared with age-matched controls. M-CSF released by Abeta-stimulated neurons interacts with its cognate receptor, c-fms, on microglia, thereby triggering chemotaxis, cell proliferation, increased expression of the macrophage scavenger receptor and apolipoprotein E, and enhanced survival of microglia exposed to Abeta, consistent with pathologic findings in AD. These data delineate an inflammatory pathway triggered by engagement of Abeta on neuronal RAGE. We suggest that M-CSF, thus generated, contributes to the pathogenesis of AD, and that M-CSF in cerebrospinal fluid might provide a means for monitoring neuronal perturbation at an early stage in AD.

Alzheimer's soluble amyloid beta is a normal component of human urine

Soluble A beta (Sa beta) is normally present at a low concentration in human plasma and cerebrospinal fluid. Although the factors involved in the regulation of Sa beta plasma levels are still unknown, we have explored its excretion in the urine as one of the possible homeostatic mechanisms. The presence of Sa beta in the urine was investigated via immunoprecipitation experiments with anti-A beta antibodies followed by detection and identification by immunoblot, MALDI mass spectrometry and sequence analysis. Soluble A beta (4.3 kDa) immunoreactivity was present in the urine of normal donors, Down's syndrome individuals as well as in patients with renal disorders exhibiting glomerular or mixed proteinuria. Edman degradation of the immunoprecipitated material yielded the intact A beta N-terminus and mass spectra analysis indicated the existence of a major component at mlz 4327, corresponding to the molecular mass of A beta1-40. Semiquantitative data obtained from the immunoprecipitation experiments indicate that under normal conditions the daily excretion of intact Sa beta in the urine represents less than 1% of the circulating pool.

Apolipoprotein E and Alzheimer's disease: a review of recent studies

There are three isoforms of the 33-kDa protein apolipoprotein E (apoE), termed apoE2, apoE3, and apoE4, each encoded by distinct genes APOE2, APOE3 and APOE4, respectively. In 1993, the APOE genotype was identified as a risk factor for Alzheimer's disease (AD) and was subsequently acknowledged to account for approximately 60% of all cases. The influence of the APOE genotype in AD is clearly isoform dependent, APOE4 imparting susceptibility and APOE2 protection. Thus, patients homozygous for the E4 allele show a very strong likelihood of developing the disease by age 75, whereas patients carrying at least one E2 allele are unlikely to develop symptoms of AD by this age. A major issue in AD research is therefore to understand the functional differences between the ApoE isoforms, with the ultimate aim of designing the next generation of drugs to treat this disease. The purpose of the present article is to summarise some of this work. This review encompasses the rapidly developing molecular, cellular and behavioural research into ApoE, and attempts to highlight those findings we consider to be of particular significance.

Preferential deposition of amyloid beta protein (Abeta) in the form Abeta40 in Alzheimer's disease is associated with a gene dosage effect of the apolipoprotein E E4 allele

The effect of apolipoprotein E (ApoE) genotype on the deposition of amyloid beta protein (Abeta) was examined in 54 patients with Alzheimer's disease. No difference in the amount of Abeta deposited as Abeta42(43) was seen between genotype groups with no, one or two E4 alleles. However, the amount of Abeta40 deposited increased according to the copy number of E4 alleles with patients possessing one E4 allele containing more than twice, and those with two E4 alleles, four times, the amount of Abeta40 in their brains compared to patients without an E4 allele. The increase in total Abeta deposited within the tissue (i.e. Abeta40 plus Abeta42(43) loads) in the presence of an E4 allele is therefore due entirely to an enhanced deposition of Abeta40. These data are consistent with the suggestion that the presence of E4 within pre-existing Abeta42(43) containing plaques may lower the threshold to fibrilization of Abeta40 thereby promoting its subsequent deposition. Thus, although the total amount of Abeta initially deposited in the brain as Abeta42(43) is not affected by the binding of any one particular ApoE isoform this does influence the subsequent maturation of plaques with a greater proportion transforming into Abeta40 containing cored plaques when the E4 isoform is present.