LRP and Alzheimer's disease

The low-density lipoprotein receptor (LDLR)-related protein, LRP, is a unique member of the LDLR family. Frequently referred to as a scavenger receptor, LRP is a large transmembrane endocytic receptor that can bind and internalize many functionally distinct ligands. Besides its role as a cargo-receptor, LRP has also been implicated in many signaling pathways. LRP knockout mice die at early embryonic age, which strongly suggests that LRP's functions are essential for normal development. Within the CNS, LRP is highly expressed in neuronal cell bodies and dendritic processes. In vitro, neurite outgrowth is stimulated by apolipoprotein E (apoE)-containing lipoprotein particles via binding to LRP. ApoE is the major cholesterol transporter in the brain and human carriers of one or two copies of the e4 allele of apoE are at a higher risk of developing Alzheimer's disease (AD). LRP also binds the amyloid precursor protein (APP) and its proteolytic fragment, the amyloid-beta peptide (Abeta), which are major players in the pathogenesis of AD. Finally, LRP has been linked to AD by genetic evidence. In this review we discuss the potential mechanisms by which LRP can affect APP and Abeta metabolism, and therefore contribute to the pathogenesis of AD.     

LRP-mediated clearance of Abeta is inhibited by KPI-containing isoforms of APP

The pathogenesis of Alzheimer's disease (AD) involves the abnormal accumulation and deposition of beta-amyloid in cerebral blood vessels and in the brain parenchyma. Critical in modulating beta-amyloid deposition in brain is the flux of Abeta across the blood brain barrier. The low-density lipoprotein receptor-related protein (LRP), is a large endocytic receptor that mediates the efflux of Abeta out of brain and into the periphery. The first step in the LRP-mediated clearance of Abeta involves the formation of a complex between Abeta and the LRP ligands apolipoprotein E (apoE) or alpha(2)-macroglobulin (alpha(2)M). The Abeta/chaperone complexes then bind to LRP via binding sites on apoE or alpha(2)M. The efflux of Abeta/chaperone complexes out of the neuropil and into the periphery may be attenuated by LRP-ligands that compete with apoE or alpha(2)M for LRP binding. LRP is also the cell surface receptor for Kunitz Protease Inhibitor (KPI) containing isoforms of Abeta's parent protein, the amyloid protein precursor (APP). Protein and mRNA levels of KPI-containing APP isoforms (APP-KPI) are elevated in AD brain and are associated with increased Abeta production. In this study we show that soluble non-amyloidogenic APP-KPI can also inhibit the uptake of Abeta/alpha(2)M in a cell culture model of LRP mediated Abeta clearance. Clearance of Abeta/apoE complexes was not inhibited by APP-KPI. Our findings are consistent with studies showing that apoE and alpha(2)M have discrete binding sites on LRP. Most significantly, our data suggests that the elevated levels of APP-KPI in AD brain may attenuate the clearance of Abeta, the proteins own amyloidogenic catabolic product.     

APP processing and the APP-KPI domain involvement in the amyloid cascade

Alternative APP mRNA splicing can generate isoforms of APP containing a Kunitz protease inhibitor (KPI) domain. KPI is one of the main serine protease inhibitors. Protein and mRNA KPI(+)APP levels are elevated in Alzheimer's disease (AD) brain and are associated with increased amyloid beta deposition. In the last years increasing evidence on multiple points in the amyloid cascade where KPI(+)APP is involved has been accumulated, admitting an outstanding position in the pathogenesis of AD to the KPI domain. This review focuses on the APP processing, the molecular activity of KPI and its physiological and pathological roles and the KPI involvement in the amyloid cascade through the nerve growth factor, the lipoprotein receptor-related protein, the tumor necrosis factor-alpha converting enzyme and the Notch1 protein.     

Cholesterol and apolipoprotein E in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia in North America and Europe. The incidence of the disease rises dramatically with age. AD is a complex multifactorial disorder that involves numerous susceptibility genes, but the exact pathogenesis and biochemical basis of AD is not well understood Cholesterol is receiving a great deal of attention as a potentially crucial factor in the etiology of AD. Almost all cholesterol in the brain is synthesized in the brain. Cholesterol exits the brain through the blood-brain barrier (BBB) in the form of apolipoprotein E (ApoE) or by first being converted to a more polar compound, 24(S)-hydroxycholesterol, which is elevated in individuals with AD. The key event leading to AD appears to be the formation and aggregation in the brain of amyloid beta (Abeta) peptide, a proteolytically derived product of amyloid precursor protein (APP). Cholesterol has been demonstrated to modulate processing of APP to Abeta. High levels of cholesterol are associated with increased risk of AD. Patients taking cholesterol-lowering statins have a lower prevalence of AD. ApoE, which transports cholesterol throughout the brain, exhibits an isoform-specific association with AD such that the E4 isoform, by unknown mechanisms, shifts the onset curve toward an earlier age.     

Functional role of the low-density lipoprotein receptor-related protein in Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder, characterized by neuronal loss, neurofibrillary tangle formation and the extracellular deposition of amyloid-beta (Abeta) plaques. The amyloid precursor protein (APP) and the enzymes responsible for Abeta generation seem to be the base elements triggering the destructive processes. Initially, the low-density lipoprotein receptor-related protein (LRP) was genetically linked to AD and later it emerged to impact on many fundamental events related to this disease. LRP is not only involved in Abeta clearance but is also the major receptor of several AD-associated ligands, e.g. apolipoprotein E and alpha2-macroglobulin. APP processing is mediated by LRP on many levels. Enhanced APP internalization through LRP decreases cell surface APP levels and thereby reduces APP shedding. As a consequence of increased APP internalization LRP enhances Abeta secretion. These effects could be attributed to the cytoplasmic tails of LRP and APP. The receptors bind via their NPXY motifs to the two PID domains of FE65 and form a tripartite complex. However, it appears that the second NPVY motif of LRP is the one responsible for the observed influence over APP metabolism. A more in-depth knowledge of the mechanisms regulating APP cleavage may offer additional targets for therapeutic intervention.     

Immunohistochemical localization of tissue factor pathway inhibitor-1 (TFPI-1), a Kunitz proteinase inhibitor, in Alzheimer''s disease

Senile plaques in Alzheimer's disease (AD) are composed principally of Aβ, a 4 kDa fragment of the amyloid precursor protein (APP). Longer forms of APP which contain a Kunitz proteinase inhibitor (KPI) domain are elevated in aged and in AD brains. Tissue factor pathway inhibitor-1 (TFPI) contains three tandem KPI domains and has been well characterized for its role as a natural anticoagulant in the extrinsic coagulation pathway. Functionally, the first two KPI domains of TFPI bind and inhibit the activity of factor Xa and VIIa respectively. In addition, TFPI and APP-KPI share a common clearance mechanism through the low density lipoprotein receptor-related protein (LRP). As part of an ongoing study of the role of KPI-containing proteins in AD, the current study examines TFPI localization in the brain. We report here that TFPI is immunohistochemically localized to microglia in both AD and non-AD individuals and is localized to some senile plaques in AD. Western blot analyses indicate that the amount of TFPI is elevated in frontal cortex samples from AD brains. We propose that TFPI may play a cell specific role in proteinase regulation in the brain.     

Alzheimer's disease pathogenesis and therapeutic interventions.

Alzheimer's disease (AD) is a neurodegenerative disorder of the central nervous system associated with progressive cognitive and memory loss. Molecular hallmarks of the disease are characterized by extracellular deposition of the amyloid beta peptide (Abeta) in senile plaques, the appearance of intracellular neurofibrillary tangles (NFT), cholinergic deficit, extensive neuronal loss and synaptic changes in the cerebral cortex and hippocampus and other areas of brain essential for cognitive and memory functions. Abeta deposition causes neuronal death via a number of possible mechanisms including oxidative stress, excitotoxicity, energy depletion, inflammation and apoptosis. Despite their multifactorial etiopathogenesis, genetics plays a primary role in progression of disease. To date genetic studies have revealed four genes that may be linked to autosomal dominant or familial early onset AD (FAD). These four genes include: amyloid precursor protein (APP), presenilin 1 (PS1), presenilin 2 (PS2) and apolipoprotein E (ApoE). Plaques are formed mostly from the deposition of Abeta, a peptide derived from APP. The main factors responsible for Abeta formation are mutation of APP or PS1 and PS2 genes or ApoE gene. All mutations associated with APP and PS proteins can lead to an increase in the production of Abeta peptides, specifically the more amyloidogenic form, Abeta42. In addition to genetic influences on amyloid plaque and intracellular tangle formation, environmental factors (e.g., cytokines, neurotoxins, etc.) may also play important role in the development and progression of AD. A direct understanding of the molecular mechanism of protein aggregation and its effects on neuronal cell death could open new therapeutic approaches. Some of the therapeutic approaches that have progressed to the clinical arena are the use of acetylcholinesterase inhibitors, nerve growth factors, nonsteroidal inflammatory drugs, estrogen and the compounds such as antioxidants, neuronal calcium channel blockers or antiapoptotic agents. Inhibition of secretase activity and blocking the formation of beta-amyloid oligomers and fibrils which may inhibit fibrilization and fibrilization-dependent neurotoxicity are the most promising therapeutic strategy against the accumulation of beta-amyloid fibrils associated with AD. Furthermore, development of immunotherapy could be an evolving promising therapeutic approach for the treatment of AD.     

A novel immunotherapy for Alzheimer's disease: antibodies against the beta-secretase cleavage site of APP

One of the main neuropathological lesions observed at brain autopsy of Alzheimer's disease (AD) patients are the extracellular senile plaques mainly composed of amyloid-beta (Abeta) peptides. Abeta is generated by proteolytic processing of amyloid precursor protein (APP) via beta and gamma-secretases. The beta-secretase APP cleaving enzyme 1 (BACE1) has become a target of intense research aimed at blocking the enzyme activity. Recent studies showed that BACE1 is involved in processing other non-APP substrates, and that other proteases are involved in APP processing. We have recently established a novel approach to inhibit Abeta production via antibodies against the beta-secretase cleavage site of APP. These antibodies bind wild type and Swedish mutated APP expressed in transgenic mice brain tissues. The isolated antibodies do not bind any form of Abeta peptides. Antibody up-take experiments, using Chinese hamster ovary cells expressing wild-type APP, suggest that antibody internalization and trafficking are mediated via the endocytic pathway. Administration of antibodies to the cells growing media resulted in a considerable decrease in intracellular Abeta levels, as well as in the levels of the corresponding C-terminal fragment (C99). The relevance of intra-neuronal accumulation of mainly Abeta42 as an early event in AD pathogenesis suggests that this approach may be applicable as a novel therapeutic strategy in AD treatment.     

Proteolytic processing of the Alzheimer's disease amyloid precursor protein in brain and platelets

Proteolytic processing of the amyloid precursor protein by beta -and gamma-secretases results in the production of Alzheimer's disease (AD) Abeta amyloid peptides. Modulation of secretase activity is being investigated as a potential therapeutic approach. Recent studies with human brain have revealed that the beta-secretase protein, BACE, is increased in cortex of AD patients. Analysis of betaCTF (or C99), the amyloid precursor protein (APP) product of BACE cleavage that is the direct precursor to Abeta, shows it is also elevated in AD, underlying the importance of beta-secretase cleavage in AD pathogenesis. The C-terminal product of gamma-secretase cleavage of APP, epsilonCTF (or AICD), is enriched in human brain cortical nuclear fractions, a subcellular distribution appropriate for a putative involvement of APP cytosolic domain in signal transduction. Analysis of AD cortex samples, particularly that of a carrier of a familial APP mutation, suggests that processing of APP transmembrane domain generates an alternative CTF product. All these particularities observed in the AD brain demonstrate that APP processing is altered in AD. The transgenic mouse model Tg2576 seems to be a promising laboratory tool to test potential modulators of Abeta formation. Indeed, C-terminal products of alpha-, beta-, and gamma-secretase cleavage are readily detectable in the brain of these transgenic mice. Finally, the finding of the same secretase products in platelets and neurons make platelets a potentially useful and easily accessible clinical tool to monitor effects of novel therapies based on inhibition of beta- or gamma-secretase.     

Nogo receptor interacts with brain APP and Abeta to reduce pathologic changes in Alzheimer's transgenic mice

Pathophysiologic hypotheses for Alzheimer's disease (AD) are centered on the role of the amyloid plaque Abeta peptide and the mechanism of its derivation from the amyloid precursor protein (APP). As part of the disease process, an aberrant axonal sprouting response is known to occur near Abeta deposits. A Nogo to Nogo-66 receptor (NgR) pathway contributes to determining the ability of adult CNS axons to extend after traumatic injuries. Here, we consider the potential role of NgR mechanisms in AD. Both Nogo and NgR are mislocalized in AD brain samples. APP physically associates with the NgR. Overexpression of NgR decreases Abeta production in neuroblastoma culture, and targeted disruption of NgR expression increases transgenic mouse brain Abeta levels, plaque deposition, and dystrophic neurites. Infusion of a soluble NgR fragment reduces Abeta levels, amyloid plaque deposits, and dystrophic neurites in a mouse transgenic AD model. Changes in NgR level produce parallel changes in secreted APP and AB, implicating NgR as a blocker of secretase processing of APP. The NgR provides a novel site for modifying the course of AD and highlights the role of axonal dysfunction in the disease.     

Amyloid precursor protein mRNA levels in Alzheimer's disease brain

Insoluble beta-amyloid deposits in Alzheimer's disease (AD) brain are proteolytically derived from the membrane bound amyloid precursor protein (APP). The APP gene is differentially spliced to produce isoforms that can be classified into those containing a Kunitz-type serine protease inhibitor domain (K(+), APP(751), APP(770), APRP(365) and APRP(563)), and those without (K(-), APP(695) and APP(714)). Given the hypothesis that Abeta is a result of aberrant catabolism of APP, differential expression of mRNA isoforms containing protease inhibitors might play an active role in the pathology of AD. We took 513 cerebral cortex samples from 90 AD and 81 control brains and quantified the mRNA isoforms of APP with TaqMan real-time RT-PCR. After adjustment for age at death, brain pH and gender we found a change in the ratio of KPI(+) to KPI(-) mRNA isoforms of APP. Three separate probes, designed to recognise only KPI(+) mRNA species, gave increases of between 28% and 50% in AD brains relative to controls (p=0.002). There was no change in the mRNA levels of KPI-(APP 695) (p=0.898). Therefore, whilst KPI-mRNA levels remained stable the KPI(+) species increased specifically in the AD brains.     

LRP and senile plaques in Alzheimer's disease: colocalization with apolipoprotein E and with activated astrocytes

The low density lipoprotein receptor-related protein (LRP) is a multifunctional receptor which is present on senile plaques in Alzheimer's disease (AD). It is suggested to play an important role in the balance between amyloid beta (Abeta) synthesis and clearance mechanisms. One of its ligands, apolipoprotein E (apoE), is also present on senile plaques and has been implicated as a risk factor for AD, potentially affecting the deposition, fibrillogenesis and clearance of Abeta. Using immunohistochemistry we show that LRP was present only on cored, apoE-containing senile plaques, in both PDAPP transgenic mice and human AD brains. We detected strong LRP staining in neurons and in reactive astrocytes, and immunostaining of membrane-bound LRP showed colocalization with fine astrocytic processes surrounding senile plaques. LRP was not present in plaques in young transgenic mice or in plaques of APOE-knockout mice. As LRP ligands associated with Abeta deposits in AD brain may play an important role in inducing levels of LRP in both neurons and astrocytes, our findings support the idea that apoE might be involved in upregulation of LRP (present in fine astrocytic processes) and act as a local scaffolding protein for LRP and Abeta. The upregulation of LRP would allow increased clearance of LRP ligands as well as clearance of Abeta/ApoE complexes.     

β-Site amyloid precursor protein–cleaving enzyme 1 activity is related to cerebrospinal fluid concentrations of sortilin-related receptor with A-type repeats,soluble amyloid precursor protein,and tau

Backgroundβ-Site amyloid precursor protein (APP)–cleaving enzyme 1 (BACE1) activity determines the rate of APP cleavage and is therefore the main driver of amyloid β production, which is a pathological hallmark of Alzheimer’s disease (AD).MethodsThe present study explored the correlation between BACE1 activity and cerebrospinal fluid (CSF) markers of APP metabolism and axonal degeneration in 63 patients with mild AD and 12 healthy control subjects.ResultsIn the AD group, positive correlations between BACE1 activity and soluble APP β, the APP sorting receptor sortilin-related receptor with A-type repeats (also known as SorLA or LR11), and tau were detected. BACE1 activity was not associated with amyloid β_1–42 or soluble APP α concentrations in the AD group, and no associations between BACE1 activity and any of the protein concentrations were found in the control group.ConclusionOur results confirm the relevance of BACE1 and sortilin-related receptor with A-type repeats within the amyloid cascade and also provide a further piece of evidence for the link between amyloid and tau pathology in AD.     

The beta-amyloid cascade hypothesis: a sequence of events leading to neurodegeneration in Alzheimer's disease

According to the beta-amyloid cascade hypothesis, the accumulation of beta-amyloid (Abeta) deposits as amyloid plaques in the patient's brain is the primary event in the pathogenesis of Alzheimer's disease (AD). Other neuropathological changes such as neurofibrillary tangles (NFTs), synaptic degeneration and neuronal cell loss are secondary and appear as a consequence of Abeta deposition. Abeta is generated during the proteolytic processing of the beta-amyloid precursor protein (APP). The endoproteolysis of APP is catalyzed by alpha-, beta-, and gamma-secretases. The alpha-secretase pathway releases non-amyloidogenic products: sAPPbeta, p3 and C83 peptides. In the beta-secretase pathway, apart from the sAPPalpha and C99 fragments also beta-amyloid peptides: Abeta40 and/or Abeta42 are generated. Abeta42 is neurotoxic and more hydrophobic than Abeta40, thus it has stronger tendency to oligomerize and aggregate. The imbalance between Abeta production and Abeta clearance is the basis for the formation of amyloid plaques. The majority of known APP and presenilin mutations responsible for familial early onset AD affect APP processing causing overproduction of Abeta, especially Abeta42. Both extracellular and intracellular accumulation of Abeta initiates a cascade of the following events leading to the neurodegeneration: synaptic and neuritic injury, microglial and astrocytic activation (inflammatory response), altered neuronal ionic homeostasis, oxidative damages, changes of kinases/phosphatases activities, formation of NFTs, and finally cell death. In this paper, we reviewed recent findings supporting the presented hypothesis.     

Lack of association between the levels of the low-density lipoprotein receptor-related protein (LRP) and either Alzheimer dementia or LRP exon 3 genotype

The low-density lipoprotein receptor-related protein (LRP), which interacts with the Alzheimer disease (AD) beta-amyloid precursor protein (APP), represents an important pathway in AD pathology. LRP-mediated receptor pathways appear to regulate both the production and the clearance of amyloid beta-protein (Abeta), a principal neuropathological product in AD. Several conflicting studies have examined levels of LRP in AD brains, as well as the relationship between the LRP exon 3 (C766T) polymorphism and LRP levels and/or disease susceptibility. In order to further investigate the role of LRP in AD, we examined well-characterized brain samples collected from subjects with varying degrees of cognitive impairment for LRP protein expression levels as well as for the presence of the LRP exon 3 polymorphism. We found no correlation between LRP levels and either presence of the disease or cognitive decline. In addition, we found no correlation between the LRP exon 3 polymorphism and either AD or LRP levels.     

Vicious cycles within the neuropathophysiologic mechanisms of Alzheimer's disease

Rigorous scientific research has identified multiple interactive mechanisms that parallel and are likely causative of the development of Alzheimer's disease (AD). Causative mechanisms include genomics, the creation of amyloid beta (Abeta), factors inhibiting the Abeta removal process, the transformation of Abeta to its toxic forms (various forms of Abeta aggregation), and lastly the oxidative, inflammatory, and other effects of toxic Abeta. Fibrillar beta-amyloid peptide, a major component of senile plaques in AD brain, is known to induce microglial-mediated neurotoxicity under certain conditions, but some recent studies support the notion that Abeta oligomers are the primary neurotoxins. Abeta-42 oligomers that are soluble and highly neurotoxic, referred to as Abeta-derived diffusible ligands (ADDLs), assemble under conditions that block fibril formation. These oligomers bind to dendrite surfaces in small clusters with ligand-like specificity and are capable of destroying hippocampal neurons at nanomolar concentrations. Evidence is presented that AD is triggered by these soluble, neurotoxic assemblies of Abeta rather than the late stage pathology landmarks of amyloid plaques and tangles. The premise is that AD symptoms stem from aberrant nerve cell signaling and synaptic failure rather than nerve cell death, which nevertheless follows and exacerbates the initial pathologies of AD. The defective clearance of amyloid leads to amyloid angiopathy that in turn perpetuates hypoperfusion that affects formation as well as absorption of CSF thereby altering clearance of amyloid and promoting vascular and parenchymal deposition[1]. Hypoperfusion, the defective clearance of amyloid, and resultant increase in amyloid deposition thus represent a vicious cycle. Chronic vascular hypoperfusion-induced mitochondrial failure results in oxidative damage, which drives caspase 3-mediated Abeta peptide secretion and enhances amyloidogenic APP processing. Intracellular Abeta accumulation in turn promotes a significant oxidative and inflammatory mechanism that generates a vicious cycle of Abeta generation and oxidation, each accelerating the other. Abeta activates astrocytes that add to the oxidative imbalance, upregulate the expression of APP via TGF-beta, and are capable of expressing BACE1. Each of these 3 actions accelerates the larger cycle of cholinergic neuron destruction. As oxidative stress induces lesions of cholinergic nuclei producing a reduction in cholinergic neurotransmission, a subsequent increase in cortical APP involving PKCepsilon leads to accelerated amyloidogenic APP metabolism. The linkage of cholinergic activation and APP metabolism completes an additional feedback loop wherein the damage wrought by Abeta accelerates further Abeta production. A comprehensive vision of the neuropathophysiologic mechanisms that result in AD reveals several vicious cycles within a larger vicious cycle, that is to say, a number of interactive systems that each, once set in motion, amplify their own processes, thus accelerating the development of AD.     

Low-Density Lipoprotein Receptor-Related Protein-1 (LRP1) C4408R Mutant Promotes Amyloid Precursor Protein (APP) α-Cleavage in Vitro

Previous studies have demonstrated that the low-density lipoprotein receptor-related protein-1 (LRP1) plays conflicting roles in Alzheimer’s disease (AD) pathogenesis, clearing β-amyloid (Aβ) from the brain while also enhancing APP endocytosis and resultant amyloidogenic processing. We have recently discovered that co-expression of mutant LRP1 C-terminal domain (LRP1-CT C4408R) with Swedish mutant amyloid precursor protein (APPswe) in Chinese hamster ovary (CHO) cells decreases Aβ production, while also increasing sAPPα and APP α-C-terminal fragment (α-CTF), compared with CHO cells expressing APPswe alone. Surprisingly, the location of this mutation on LRP1 corresponded with the α-secretase cleavage site of APP. Further experimentation confirmed that in CHO cells expressing APPswe or wild-type APP (APPwt), co-expression of LRP1-CT C4408R decreases Aβ and increases sAPPα and α-CTF compared with co-expression of wild-type LRP1-CT. In addition, LRP1-CT C4408R enhanced the unglycosylated form of LRP1-CT and reduced APP endocytosis as determined by flow cytometry. This finding identifies a point mutation in LRP1 which slows LRP1-CT-mediated APP endocytosis and amyloidogenic processing, while enhancing APP α-secretase cleavage, thus demonstrating a potential novel target for slowing AD pathogenesis.     

GM1 ganglioside regulates the proteolysis of amyloid precursor protein

Plaques containing amyloid beta-peptides (Abeta) are a major feature in Alzheimer's disease (AD), and GM1 ganglioside is an important component of cellular plasma membranes and especially enriched in lipid raft. GM1-bound Abeta (GM1/Abeta), found in brains exhibiting early pathological changes of AD including diffuse plaques, has been suggested to be involved in the initiation of amyloid fibril formation in vivo by acting as a seed. However, the role of GM1 in amyloid beta-protein precursor (APP) processing is not yet defined. In this study, we report that exogenous GM1 ganglioside promotes Abeta biogenesis and decreases sAPPalpha secretion in SH-SY5Y and COS7 cells stably transfected with human APP695 cDNA without affecting full-length APP and the sAPPbeta levels. We also observe that GM1 increases extracellular levels of Abeta in primary cultures of mixed rat cortical neurons transiently transfected with human APP695 cDNA. These findings suggest a regulatory role for GM1 in APP processing pathways.     

Co-expression of β-amyloid precursor protein (βAPP) and apolipoprotein E in cell culture: analysis of βAPP processing

Apolipoprotein E (ApoE) is the major genetic risk factor for Alzheimer's disease (AD). The ApoE4 allele is associated with earlier disease onset and greater cerebral deposition of the amyloid beta peptide (Aβ), the major constituent of senile (amyloid) plaques. The molecular mechanism underlying these effects of ApoE4 remains unclear; ApoE alleles could have different influences on Aβ production, extracellular aggregation, or clearance. Because the missense mutations on chromosomes 14 and 21 that cause familial forms of AD appear to lead to increased secretion of Aβ, it is important to determine whether ApoE4 has a similar effect. Here, we have examined the effects of all three ApoE alleles on the processing of βAPP and the secretion of Aβ in intact cells. We established neural (HS683 human glioma) and non-neural (Chinese hamster ovary) cell culture systems that constitutively secrete both ApoE and Aβ at concentrations like those in human cerebrospinal fluid. βAPP metabolites, generated in the presence of each ApoE allele, were analysed and quantified by two methods: immunoprecipitation and phosphorimaging, and ELISA. We detected no consistent allele-specific effects of ApoE on βAPP processing in either cell type. Our data suggest that the higher amyloid burden found in AD subjects expressing ApoE4 is not due to increased amyloidogenic processing of βAPP, in contrast to findings in AD linked to chromosome 14 or 21. These co-expressing cell lines will be useful in the further search for the effects of ApoE on Aβ aggregation or clearance under physiologically relevant conditions.     

Increased amyloid beta protein levels in children and adolescents with Down syndrome

BACKGROUND: Persons with Down syndrome (DS) (40 years and older) have neuropathological changes characteristic of Alzheimer disease (AD). Soluble forms of amyloid beta (Abeta) peptide generated from amyloid precursor protein (APP) end at C-terminal residues 40 and 42. The presence of the apolipoprotein E (ApoE) epsilon4 allele is a significant risk factor for the development of sporadic AD. Although preliminary studies have shown an association of plasma Abeta42 and ApoE epsilon4 allele in older persons with DS who have dementia, the relationship between plasma Abeta40 and Abeta42 levels and ApoE phenotypes in children with DS has not been examined. Inflammation might play a role in the growth of DS brains. Neopterin is an immune activation marker for the cell-mediated immune response. OBJECTIVE: To examine the levels of plasma Abeta40, Abeta42, and neopterin in children or adolescents with DS or controls. MATERIALS AND METHODS: Blood was collected from DS (N=35; 7+/-3.8 years old) and their siblings (N=34; 10+/-4.5). Plasma Abeta40 and Abeta42, and neopterin levels were quantitated by sandwich ELISA. RESULTS: Abeta40 and Abeta42 levels were higher in DS than controls. The ratio of Abeta42/Abeta40 was lower in DS than in controls. There were significant negative correlations between age and Abeta40 in DS and controls, and between age and Abeta42 levels in DS but not in controls. There was no association of Abeta40 or Abeta42 levels with Apo E in either group. Neopterin levels were higher in DS than controls, and the levels were not correlated with Abeta40 and Abeta42 levels in DS or controls. CONCLUSIONS: The over expression of APP gene in DS leads to increases in plasma Abeta40 and Abeta42 levels before plaque formation in DS brain. Higher neopterin concentrations in DS reflect inflammatory cell activation. Further studies are needed to determine whether DS children with lower plasma Abeta42/Abeta40 ratios are at increased risk of developing AD during aging than those with higher ratios.