A detergent-insoluble membrane compartment contains A beta in vivo

Identification of novel γ-secretase-associated proteins in detergent-resistant membranes from brain

In Alzheimer disease, oligomeric amyloid β-peptide (Aβ) species lead to synapse loss and neuronal death. γ-Secretase, the transmembrane protease complex that mediates the final catalytic step that liberates Aβ from its precursor protein (APP), has a multitude of substrates, and therapeutics aimed at reducing Aβ production should ideally be specific for APP cleavage. It has been shown that APP can be processed in lipid rafts, and γ-secretase-associated proteins can affect Aβ production. Here, we use a biotinylated inhibitor for affinity purification of γ-secretase and associated proteins and mass spectrometry for identification of the purified proteins, and we identify novel γ-secretase-associated proteins in detergent-resistant membranes from brain. Furthermore, we show by small interfering RNA-mediated knockdown of gene expression that a subset of the γ-secretase-associated proteins, in particular voltage-dependent anion channel 1 (VDAC1) and contactin-associated protein 1 (CNTNAP1), reduced Aβ production (Aβ40 and Aβ42) by around 70%, whereas knockdown of presenilin 1, one of the essential γ-secretase complex components, reduced Aβ production by 50%. Importantly, these proteins had a less pronounced effect on Notch processing. We conclude that VDAC1 and CNTNAP1 associate with γ-secretase in detergent-resistant membranes and affect APP processing and suggest that molecules that interfere with this interaction could be of therapeutic use for Alzheimer disease.

MicroRNA-137/181c regulates serine palmitoyltransferase and in turn amyloid β, novel targets in sporadic Alzheimer's disease

The contribution of mutations in amyloid precursor protein (APP) and presenilin (PSEN) to familial Alzheimer's disease (AD) is well established. However, little is known about the molecular mechanisms leading to amyloid β (Aβ) generation in sporadic AD. Increased brain ceramide levels have been associated with sporadic AD, and are a suggested risk factor. Serine palmitoyltransferase (SPT) is the first rate-limiting enzyme in the de novo ceramide synthesis. However, the regulation of SPT is not yet understood. Evidence suggests that it may be posttranscriptionally regulated. Therefore, we investigated the role of miRNAs in the regulation of SPT and amyloid β (Aβ) generation. We show that SPT is upregulated in a subgroup of sporadic AD patient brains. This is further confirmed in mouse model studies of risk factors associated with AD. We identified that the loss of miR-137, -181c, -9, and 29a/b-1 increases SPT and in turn Aβ levels, and provides a mechanism for the elevated risk of AD associated with age, high-saturated-fat diet, and gender. Finally, these results suggest SPT and the respective miRNAs may be potential therapeutic targets for sporadic AD.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Suppression of amyloid beta A11 antibody immunoreactivity by vitamin C: possible role of heparan sulfate oligosaccharides derived from glypican-1 by ascorbate-induced, nitric oxide (NO)-catalyzed degradation

Amyloid β (Aβ) is generated from the copper- and heparan sulfate (HS)-binding amyloid precursor protein (APP) by proteolytic processing. APP supports S-nitrosylation of the HS proteoglycan glypican-1 (Gpc-1). In the presence of ascorbate, there is NO-catalyzed release of anhydromannose (anMan)-containing oligosaccharides from Gpc-1-nitrosothiol. We investigated whether these oligosaccharides interact with Aβ during APP processing and plaque formation. anMan immunoreactivity was detected in amyloid plaques of Alzheimer (AD) and APP transgenic (Tg2576) mouse brains by immunofluorescence microscopy. APP/APP degradation products detected by antibodies to the C terminus of APP, but not Aβ oligomers detected by the anti-Aβ A11 antibody, colocalized with anMan immunoreactivity in Tg2576 fibroblasts. A 50-55-kDa anionic, sodium dodecyl sulfate-stable, anMan- and Aβ-immunoreactive species was obtained from Tg2576 fibroblasts using immunoprecipitation with anti-APP (C terminus). anMan-containing HS oligo- and disaccharide preparations modulated or suppressed A11 immunoreactivity and oligomerization of Aβ42 peptide in an in vitro assay. A11 immunoreactivity increased in Tg2576 fibroblasts when Gpc-1 autoprocessing was inhibited by 3-β[2(diethylamino)ethoxy]androst-5-en-17-one (U18666A) and decreased when Gpc-1 autoprocessing was stimulated by ascorbate. Neither overexpression of Gpc-1 in Tg2576 fibroblasts nor addition of copper ion and NO donor to hippocampal slices from 3xTg-AD mice affected A11 immunoreactivity levels. However, A11 immunoreactivity was greatly suppressed by the subsequent addition of ascorbate. We speculate that temporary interaction between the Aβ domain and small, anMan-containing oligosaccharides may preclude formation of toxic Aβ oligomers. A portion of the oligosaccharides are co-secreted with the Aβ peptides and deposited in plaques. These results support the notion that an inadequate supply of vitamin C could contribute to late onset AD in humans.

Impairment of the activity of the plasma membrane Ca2+-ATPase in Alzheimer's disease

AD (Alzheimer's disease) is an age-associated neurodegenerative disorder where the accumulation of neurotoxic Aβ (amyloid β-peptide) in senile plaques is a typical feature. Recent studies point out a relationship between Aβ neurotoxicity and Ca2+ dyshomoeostasis, but the molecular mechanisms involved are still under discussion. The PMCAs (plasma membrane Ca2+-ATPases) are a multi-isoform family of proteins highly expressed in brain that is implicated in the maintenance of low intraneural Ca2+ concentration. Therefore the malfunction of this pump may also be responsible for Ca2+ homoeostasis failure in AD. We have found that the Ca2+-dependence of PMCA activity is affected in human brains diagnosed with AD, being related to the enrichment of Aβ. The peptide produces an inhibitory effect on the activity of PMCA which is isoform-specific, with the greatest inhibition of PMCA4. Besides, cholesterol blocked the inhibitory effect of Aβ, which is consistent with the lack of any Aβ effect on PMCA4 found in cholesterol-enriched lipid rafts isolated from pig brain. These observations suggest that PMCAs are a functional component of the machinery that leads to Ca2+ dysregulation in AD and propose cholesterol enrichment in rafts as a protector of the Aβ-mediated inhibition on PMCA.

Evidence from solid-state NMR for nonhelical conformations in the transmembrane domain of the amyloid precursor protein

The amyloid precursor protein (APP) is subject to proteolytic processing by γ-secretase within neuronal membranes, leading to Alzheimer's disease-associated β-amyloid peptide production by cleavage near the midpoint of the single transmembrane (TM) segment of APP. Conformational properties of the TM segment may affect its susceptibility to γ-secretase cleavage, but these properties have not been established definitively, especially in bilayer membranes with physiologically relevant lipid compositions. In this article, we report an investigation of the APP-TM conformation, using (13)C chemical shifts obtained with two-dimensional solid-state NMR spectroscopy as site-specific conformational probes. We find that the APP-TM conformation is not a simple α-helix, particularly at 37°C in multilamellar vesicles with compositions that mimic the composition of neuronal cell membranes. Instead, we observe a mixture of helical and nonhelical conformations at the N- and C-termini and in the vicinity of the γ-cleavage site. Conformational plasticity of the TM segment of APP may be an important factor in the γ-secretase cleavage mechanism.

β-amyloid triggers ALS-associated TDP-43 pathology in AD models

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease associated with loss of motor neurons in the brain and spinal cord. ALS is occasionally diagnosed with frontotemporal lobar dementia with ubiquitin-positive inclusions (FTLD-U). Alzheimer's disease (AD) is the most common type of age-associated dementia. Abnormal levels of aggregated Tar-DNA binding protein-43 (TDP-43) are detected in the majority of patients with ALS, FTLD and AD. We observed a significant increase (200%) in the levels of TDP-43 in cortical autopsies of late stage AD patients. Lentiviral expression of Aβ(1-42) in the rat motor cortex led to an increase in TDP-43 pathology, including up-regulation of the mature ~44kDa protein, identical to the pathological changes seen in AD. Furthermore, expression of Aβ(1-42) was associated with TDP-43 phosphorylation and accumulation in the cytosol. Clearance of Aβ with parkin prevented TDP-43 pathology. TDP-43 modifications were also observed in 3xTransgenic AD (3xTg-AD) compared to wild type mice, but these changes were attenuated in parkin-injected hippocampi, even in the presence of Tau pathology, suggesting that TDP-43 pathology is triggered by Aβ, independent of Tau. Increased levels of casein kinase (CK1 and CK2), which are associated with TDP-43 phosphorylation, were also observed in Aβ(1-42) expressing brains. These data indicate an overlap in TDP-43 pathology between AD and ALS-FTLD and suggest that Aβ triggers modifications of TDP-43.

Lipid Rafts: Linking Alzheimer's Amyloid-β Production, Aggregation, and Toxicity at Neuronal Membranes

Lipid rafts are membrane microdomains, enriched in cholesterol and sphingolipids, into which specific subsets of proteins and lipids partition, creating cell-signalling platforms that are vital for neuronal functions. Lipid rafts play at least three crucial roles in Alzheimer's Disease (AD), namely, in promoting the generation of the amyloid-β (Aβ) peptide, facilitating its aggregation upon neuronal membranes to form toxic oligomers and hosting specific neuronal receptors through which the AD-related neurotoxicity and memory impairments of the Aβ oligomers are transduced. Recent evidence suggests that Aβ oligomers may exert their deleterious effects through binding to, and causing the aberrant clustering of, lipid raft proteins including the cellular prion protein and glutamate receptors. The formation of these pathogenic lipid raft-based platforms may be critical for the toxic signalling mechanisms that underlie synaptic dysfunction and neuropathology in AD.

Neuropathology after active Abeta42 immunotherapy: implications for Alzheimer's disease pathogenesis

The amyloid cascade hypothesis of Alzheimer's disease (AD) is testable: it implies that interference with Abeta aggregation and plaque formation may be therapeutically useful. Abeta42 immunisation of amyloid precursor protein (APP) transgenic mice prevented plaque formation and caused removal of existing plaques. The first clinical studies of Abeta immunisation in AD patients (AN1792, Elan Pharmaceuticals) were halted when some patients suffered side effects. Since our confirmation that Abeta immunisation can prompt plaque removal in human AD, we have performed a clinical and neuropathological follow up of AD patients in the initial Elan Abeta immunisation trial. In immunised AD patients, we found: a lower Abeta load, with evidence that plaques had been removed; a reduced tau load in neuronal processes, but not in cell bodies; and no evidence of a beneficial effect on synapses. There were pathological "side effects" including: increased microglial activation; increased cerebral amyloid angiopathy; and there is some evidence for increased soluble/oligomeric Abeta. A pathophysiological mechanism involving effects on the cerebral vasculature is proposed for the clinical side effects observed with some active and passive vaccine protocols. Our current knowledge of the effects of Abeta immunotherapy is based on functional information from the early clinical trials and a few post mortem cases. Several further clinical studies are underway using a variety of protocols and important clinical, imaging and neuropathological data will become available in the near future. The information obtained will be important in helping to understand the pathogenesis not only of AD but also of other neurodegenerative disorders associated with protein aggregation.

Intracellular cholesterol homeostasis and amyloid precursor protein processing

Many preclinical and clinical studies have implied a role for cholesterol in the pathogenesis of Alzheimer's disease (AD). In this review we will discuss the movement of intracellular cholesterol and how normal distribution, transport, and export of cholesterol are vital for regulation of the AD related protein, Abeta. We focus on cholesterol distribution in the plasma membrane, transport through the endosomal/lysosomal system, control of cholesterol intracellular signaling at the endoplasmic reticulum and Golgi, the HMG-CoA reductase pathway and finally export of cholesterol from the cell.

Roles for dysfunctional sphingolipid metabolism in Alzheimer's disease neuropathogenesis

Sphingolipids in the membranes of neurons play important roles in signal transduction, either by modulating the localization and activation of membrane-associated receptors or by acting as precursors of bioactive lipid mediators. Activation of cytokine and neurotrophic factor receptors coupled to sphingomyelinases results in the generation of ceramides and gangliosides, which in turn, modify the structural and functional plasticity of neurons. In aging and neurodegenerative conditions such as Alzheimer's disease (AD), there are increased membrane-associated oxidative stress and excessive production and accumulation of ceramides. Studies of brain tissue samples from human subjects, and of experimental models of the diseases, suggest that perturbed sphingomyelin metabolism is a pivotal event in the dysfunction and degeneration of neurons that occurs in AD and HIV dementia. Dietary and pharmacological interventions that target sphingolipid metabolism should be pursued for the prevention and treatment of neurodegenerative disorders.

Mitochondria, cholesterol and amyloid beta peptide: a dangerous trio in Alzheimer disease

The molecular mechanisms of Alzheimer's disease (AD) are not fully understood. Extensive evidence from experimental models has involved the overgeneration and accumulation of toxic amyloid beta peptides (Abeta) in the onset and progression of the disease. The amyloidogenic processing of amyloid precursor protein into pathogenic Abeta fragments is thought to occur in specific domains of the plasma membrane and favored by cholesterol enrichment. Intracellular Abeta accumulation is known to induce oxidative stress, predominantly via mitochondria targeting of toxic Abeta. Recent evidence using mouse models of cholesterol loading has demonstrated that the specific mitochondrial cholesterol pool sensitizes neurons to Abeta-induced oxidant cell death and caspase-independent apoptosis due to selective mitochondrial GSH (mGSH) depletion induced by cholesterol-mediated perturbation of mitochondrial membrane dynamics. mGSH replenishment by permeable precursors such as glutathione ethyl ester protected against Abeta-mediated neurotoxicity and inflammation. Thus, these novel data expand the pathogenic role of cholesterol in AD indicating that in addition to fostering Abeta generation, mitochondrial cholesterol determines Abeta neurotoxicity via mGSH regulation.

Beta-amyloid1-42 gene transfer model exhibits intraneuronal amyloid, gliosis, tau phosphorylation, and neuronal loss

Alzheimer disease is characterized by extracellular beta-amyloid (Abeta) plaques and intracellular inclusions containing neurofibrillary tangles of phospho-Tau and intraneuronal Abeta associated with neuronal cell death. We generated a novel gene transfer animal model using lentiviral Abeta(1-42) that resulted in intracellular but not extracellular Abeta accumulations in the targeted rat primary motor cortex. Expression of intracellular Abeta(1-42) led to pathological changes seen in human Alzheimer disease brains, including cell death, inflammatory signs, activation of two Tau kinases, and Tau hyperphosphorylation. Promoting clearance of lentiviral Abeta(1-42) reversed these effects, demonstrating that intraneuronal Abeta(1-42) is a toxic peptide that lies upstream of Tau modification. These studies reveal the role of intracellular Abeta(1-42) in a novel gene transfer animal model, which is a useful tool to study intraneuronal Abeta(1-42)-induced pathology in the absence of extracellular plaques. Targeted delivery of Abeta will allow speedy delineation of pathological mechanisms associated with specific neurodegenerative lesions.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

The Relationship between Parkin and Protein Aggregation in Neurodegenerative Diseases

The most prominent changes in neurodegenerative diseases are protein accumulation and inclusion formation. Several neurodegenerative diseases, including Alzheimer's, the Synucleinopathies and Tauopathies share several overlapping clinical symptoms manifest in Parkinsonism, cognitive decline and dementia. As degeneration progresses in the disease process, clinical symptoms suggest convergent pathological pathways. Biochemically, protein cleavage, ubiquitination and phosphorylation seem to play fundamental roles in protein aggregation, inclusion formation and inflammatory responses. In the following we provide a synopsis of the current knowledge about protein accumulation and astrogliosis as a common denominator in neurodegenerative diseases, and we propose insights into protein degradation and anti-inflammation. We review the E3-ubiquitin ligase and other possible functions of parkin as a suppressant of inflammatory signs and a strategy to clear amyloid proteins in neurodegenerative diseases.

Receptor-associated protein interacts with amyloid-beta peptide and promotes its cellular uptake

Brain amyloid-beta (Abeta) peptide accumulation and aggregation are critical events in the pathogenesis of Alzheimer disease. Increasing evidence has demonstrated that LRP1 is involved in Alzheimer disease pathogenesis. The physiological ligands of LRP1, including apoE, play significant roles in the cellular clearance of Abeta. The receptor-associated protein (RAP) is a specialized chaperone for members of the low density lipoprotein receptor family. RAP shares structural and receptor-binding properties with apoE. Here, we show that RAP binds to both Abeta40 and Abeta42 in a concentration-dependent manner and forms complexes with them. Fluorescence-activated cell sorter analysis showed that RAP significantly enhances the cellular internalization of Abeta in different cell types, including brain vascular smooth muscle, neuroblastoma, glioblastoma, and Chinese hamster ovary cells. This effect of RAP was confirmed by fluorescence microscopy and enzyme-linked immunosorbent assay. RAP binds to both LRP1 and heparin; however, the ability of RAP to enhance Abeta cellular uptake was blocked by heparin and heparinase treatment but not by LRP1 deficiency. Furthermore, the effects of RAP were significantly decreased in heparan sulfate proteoglycan-deficient Chinese hamster ovary cells. Our findings reveal that RAP is a novel Abeta-binding protein that promotes cellular internalization of Abeta.

Intracellular amyloid induces impairments on electrophysiological properties of cultured human neurons

The role of intracellular amyloid beta (iAbeta) in Alzheimer's disease (AD) initiation and progression attracts more and more attention in recent years. To address whether iAbeta induces early alterations of electrophysiological properties in cultured human primary neurons, we delivered iAbeta with adeno-virus and measured the electrophysiological properties of infected neurons with whole-cell recordings. Our results show that iAbeta induces an increase in neuronal resting membrane potentials, a decrease in K(+) currents and a hyperpolarizing shift in voltage-dependent activation of K(+) currents. These results suggest the electrophysiological impairments induced by iAbeta may be responsible for its neuronal toxicity.

Mild cholesterol depletion reduces amyloid-beta production by impairing APP trafficking to the cell surface

It has been suggested that cellular cholesterol levels can modulate the metabolism of the amyloid precursor protein (APP) but the underlying mechanism remains controversial. In the current study, we investigate in detail the relationship between cholesterol reduction, APP processing and gamma-secretase function in cell culture studies. We found that mild membrane cholesterol reduction led to a decrease in Abeta(40) and Abeta(42) in different cell types. We did not detect changes in APP intracellular domain or Notch intracellular domain generation. Western blot analyses showed a cholesterol-dependent decrease in the APP C-terminal fragments and cell surface APP. Finally, we applied a fluorescence resonance energy transfer (FRET)-based technique to study APP-Presenilin 1 (PS1) interactions and lipid rafts in intact cells. Our data indicate that cholesterol depletion reduces association of APP into lipid rafts and disrupts APP-PS1 interaction. Taken together, our results suggest that mild membrane cholesterol reduction impacts the cleavage of APP upstream of gamma-secretase and appears to be mediated by changes in APP trafficking and partitioning into lipid rafts.

Parkin promotes intracellular Abeta1-42 clearance

Alzheimer's disease and Parkinson's disease are common neurodegenerative diseases that may share some underlying mechanisms of pathogenesis. Abeta(1-42) fragments are found intracellularly, and extracellularly as amyloid plaques, in Alzheimer's disease and in dementia with Lewy Bodies. Parkin is an E3-ubiquitin ligase involved in proteasomal degradation of intracellular proteins. Mutations in parkin, which result in loss of parkin function, lead to early onset Parkinsonism. Here we tested whether the ubiquitin ligase activity of parkin could lead to reduction in intracellular human Abeta(1-42). Lentiviral constructs encoding either human parkin or human Abeta(1-42) were used to infect M17 neuroblastoma cells. Parkin expression resulted in reduction of intracellular human Abeta(1-42) levels and protected against its toxicity in M17 cells. Co-injection of lentiviral constructs into control rat primary motor cortex demonstrated that parkin co-expression reduced human Abeta(1-42) levels and Abeta(1-42)-induced neuronal degeneration in vivo. Parkin increased proteasomal activity, and proteasomal inhibition blocked the effects of parkin on reducing Abeta(1-42) levels. Incubation of Abeta(1-42) cell lysates with ubiquitin, in the presence of parkin, demonstrated the generation of Abeta-ubiquitin complexes. These data indicate that parkin promotes ubiquitination and proteasomal degradation of intracellular Abeta(1-42) and demonstrate a protective effect in neurodegenerative diseases with Abeta deposits.

Mitochondrial cholesterol loading exacerbates amyloid beta peptide-induced inflammation and neurotoxicity

The role of cholesterol in Alzheimer's disease (AD) has been linked to the generation of toxic amyloid beta peptides (Abeta). Using genetic mouse models of cholesterol loading, we examined whether mitochondrial cholesterol regulates Abeta neurotoxicity and AD pathology. Isolated mitochondria from brain or cortical neurons of transgenic mice overexpressing SREBP-2 (sterol regulatory element binding protein 2) or NPC1 (Niemann-Pick type C1) knock-out mice exhibited mitochondrial cholesterol accumulation, mitochondrial glutathione (mGSH) depletion and increased susceptibility to Abeta1-42-induced oxidative stress and release of apoptogenic proteins. Similar findings were observed in pharmacologically GSH-restricted rat brain mitochondria, while selective mGSH depletion sensitized human neuronal and glial cell lines to Abeta1-42-mediated cell death. Intracerebroventricular human Abeta delivery colocalized with mitochondria resulting in oxidative stress, neuroinflammation and neuronal damage that were enhanced in Tg-SREBP-2 mice and prevented upon mGSH recovery by GSH ethyl ester coinfusion, with a similar protection observed by intraperitoneal administration of GSH ethyl ester. Finally, APP/PS1 (amyloid precursor protein/presenilin 1) mice, a transgenic AD mouse model, exhibited mitochondrial cholesterol loading and mGSH depletion. Thus, mitochondrial cholesterol accumulation emerges as a novel pathogenic factor in AD by modulating Abeta toxicity via mGSH regulation; strategies boosting the particular pool of mGSH may be of relevance to slow down disease progression.

Signal peptide peptidases: a family of intramembrane-cleaving proteases that cleave type 2 transmembrane proteins

Five genes encode the five human signal peptide peptidases (SPPs), which are intramembrane-cleaving aspartyl proteases (aspartyl I-CLiPs). SPPs have been conserved through evolution with family members found in higher eukaryotes, fungi, protozoa, arachea, and plants. SPPs are related to the presenilin family of aspartyl I-CLiPs but differ in several key aspects. Presenilins (PSENs) and SPPs both cleave the transmembrane region of membrane proteins; however, PSENs cleave type 1 membrane proteins whereas SPPs cleave type 2 membrane proteins. Though the overall homology between SPPs and PSENs is minimal, they are multipass membrane proteins that contain two conserved active site motifs YD and GxGD in adjacent membrane-spanning domains and a conserved PAL motif of unknown function near their COOH-termini. They differ in that the active site YD and GxGD containing transmembrane domains of SPPs are inverted relative to PSENs, thus, orienting the active site in a consistent topology relative to the substrate. At least two of the human SPPs (SPP and SPPL3) appear to function without additional cofactors, but PSENs function as a protease, called gamma-secretase, only when complexed with Nicastrin, APH-1 and Pen-2. The biological roles of SPP are largely unknown, and only a few endogenous substrates for SPPs have been identified. Nevertheless there is emerging evidence that SPP family members are highly druggable and may regulate both essential physiologic and pathophysiologic processes. Further study of the SPP family is needed in order to understand their biological roles and their potential as therapeutic targets.

Novel N-terminal cleavage of APP precludes Abeta generation in ACAT-defective AC29 cells

A common pathogenic event that occurs in all forms of Alzheimer's disease is the progressive accumulation of amyloid beta-peptide (Abeta) in brain regions responsible for higher cognitive functions. Inhibition of acyl-coenzyme A: cholesterol acyltransferase (ACAT), which generates intracellular cholesteryl esters from free cholesterol and fatty acids, reduces the biogenesis of the Abeta from the amyloid precursor protein (APP). Here we have used AC29 cells, defective in ACAT activity, to show that ACAT activity steers APP either toward or away from a novel proteolytic pathway that replaces both alpha and the amyloidogenic beta cleavages of APP. This alternative pathway involves a novel cleavage of APP holoprotein at Glu281, which correlates with reduced ACAT activity and Abeta generation in AC29 cells. This sterol-dependent cleavage of APP occurs in the endosomal compartment after internalization of cell surface APP. The resulting novel C-terminal fragment APP-C470 is destined to proteasomal degradation limiting the availability of APP for the Abeta generating system. The proportion of APP molecules that are directed to the novel cleavage pathway is regulated by the ratio of free cholesterol and cholesteryl esters in cells. These results suggest that subcellular cholesterol distribution may be an important regulator of the cellular fate of APP holoprotein and that there may exist several competing proteolytic systems responsible for APP processing within the endosomal compartment.

X11 proteins regulate the translocation of amyloid beta-protein precursor (APP) into detergent-resistant membrane and suppress the amyloidogenic cleavage of APP by beta-site-cleaving enzyme in brain

X11 and X11-like proteins (X11L) are neuronal adaptor proteins whose association to the cytoplasmic domain of amyloid beta-protein precursor (APP) suppresses the generation of amyloid beta-protein (Abeta) implicated in Alzheimer disease pathogenesis. The amyloidogenic, but not amyloidolytic, metabolism of APP was selectively increased in the brain of mutant mice lacking X11L (Sano, Y., Syuzo-Takabatake, A., Nakaya, T., Saito, Y., Tomita, S., Itohara, S., and Suzuki, T. (2006) J. Biol. Chem. 281, 37853-37860). To reveal the actual role of X11 proteins (X11s) in suppressing amyloidogenic cleavage of APP in vivo, we generated X11 and X11L double knock-out mice and analyzed the metabolism of APP. The mutant mice showed enhanced beta-site cleavage of APP along with increased accumulation of Abeta in brain and increased colocalization of APP with beta-site APP-cleaving enzyme (BACE). In the brains of mice deficient in both X11 and X11L, the apparent relative subcellular distributions of both mature APP and its beta-C-terminal fragment were shifted toward the detergent-resistant membrane (DRM) fraction, an organelle in which BACE is active and both X11s are not nearly found. These results indicate that X11s associate primarily with APP molecules that are outside of DRM, that the dissociation of APP-X11/X11L complexes leads to entry of APP into DRM, and that cleavage of uncomplexed APP by BACE within DRM is enhanced by X11s deficiency. Present results lead to an idea that the dysfunction of X11L in the interaction with APP may recruit more APP into DRM and increase the generation of Abeta even if BACE activity did not increase in brain.

A single mutation in the nonamyloidogenic region of islet amyloid polypeptide greatly reduces toxicity

Islet amyloid polypeptide (IAPP or amylin) is a 37-residue peptide secreted with insulin by beta-cells in the islets of Langerhans. The aggregation of the peptide into either amyloid fibers or small soluble oligomers has been implicated in the death of beta-cells during type 2 diabetes through disruption of the cellular membrane. The actual form of the peptide responsible for beta-cell death has been a subject of controversy. Previous research has indicated that the N-terminal region of the peptide (residues 1-19) is primarily responsible for the membrane-disrupting effect of the hIAPP peptide and induces membrane disruption to a similar extent as the full-length peptide without forming amyloid fibers when bound to the membrane. The rat version of the peptide, which is both noncytotoxic and nonamyloidogenic, differs from the human peptide by only one amino acid residue: Arg18 in the rat version while His18 in the human version. To elucidate the effect of this difference, we have measured in this study the effects of the rat and human versions of IAPP(1-19) on islet cells and model membranes. Fluorescence microscopy shows a rapid increase in intracellular calcium levels of islet cells after the addition of hIAPP(1-19), indicating disruption of the cellular membrane, while the rat version of the IAPP(1-19) peptide is significantly less effective. Circular dichroism experiments and dye leakage assays on model liposomes show that rIAPP(1-19) is deficient in binding to and disrupting lipid membranes at low but not at high peptide to lipid ratios, indicating that the ability of rIAPP(1-19) to form small aggregates necessary for membrane binding and disruption is significantly less than hIAPP(1-19). At pH 6.0, where H18 is likely to be protonated, hIAPP(1-19) resembles rIAPP(1-19) in its ability to cause membrane disruption. Differential scanning calorimetry suggests a different mode of binding to the membrane for rIAPP(1-19) compared to hIAPP(1-19). Human IAPP(1-19) has a minimal effect on the phase transition of lipid vesicles, suggesting a membrane orientation of the peptide in which the mobility of the acyl chains of the membrane is relatively unaffected. Rat IAPP(1-19), however, has a strong effect on the phase transition of lipid vesicles at low concentrations, suggesting that the peptide does not easily insert into the membrane after binding to the surface. Our results indicate that the modulation of the peptide orientation in the membrane by His18 plays a key role in the toxicity of nonamyloidogenic forms of hIAPP.

Overexpression of neprilysin reduces alzheimer amyloid-beta42 (Abeta42)-induced neuron loss and intraneuronal Abeta42 deposits but causes a reduction in cAMP-responsive element-binding protein-mediated transcription, age-dependent axon pathology, and premature death in Drosophila

The amyloid-beta42 (Abeta42) peptide has been suggested to play a causative role in Alzheimer disease (AD). Neprilysin (NEP) is one of the rate-limiting Abeta-degrading enzymes, and its enhancement ameliorates extracellular amyloid pathology, synaptic dysfunction, and memory defects in mouse models of Abeta amyloidosis. In addition to the extracellular Abeta, intraneuronal Abeta42 may contribute to AD pathogenesis. However, the protective effects of neuronal NEP expression on intraneuronal Abeta42 accumulation and neurodegeneration remain elusive. In contrast, sustained NEP activation may be detrimental because NEP can degrade many physiological peptides, but its consequences in the brain are not fully understood. Using transgenic Drosophila expressing human NEP and Abeta42, we demonstrated that NEP efficiently suppressed the formation of intraneuronal Abeta42 deposits and Abeta42-induced neuron loss. However, neuronal NEP overexpression reduced cAMP-responsive element-binding protein-mediated transcription, caused age-dependent axon degeneration, and shortened the life span of the flies. Interestingly, the mRNA levels of endogenous fly NEP genes and phosphoramidon-sensitive NEP activity declined during aging in fly brains, as observed in mammals. Taken together, these data suggest both the protective and detrimental effects of chronically high NEP activity in the brain. Down-regulation of NEP activity in aging brains may be an evolutionarily conserved phenomenon, which could predispose humans to developing late-onset AD.

Direct and potent regulation of gamma-secretase by its lipid microenvironment

gamma-Secretase is an unusual and ubiquitous aspartyl protease with an intramembrane catalytic site that cleaves many type-I integral membrane proteins, most notably APP and Notch. Several reports suggest that cleavage of APP to produce the Abeta peptide is regulated in part by lipids. As gamma-secretase is a multipass protein complex with 19 transmembrane domains, it is likely that the local lipid composition of the membrane can regulate gamma-activity. To determine the direct contribution of the lipid microenvironment to gamma-secretase activity, we purified the human protease from overexpressing mammalian cells, reconstituted it in vesicles of varying lipid composition, and examined the effects of individual phospholipids, sphingolipids, cholesterol, and complex lipid mixtures on substrate cleavage. A conventional gamma-activity assay was modified to include a detergent-removal step to facilitate proteoliposome formation, and this increased baseline activity over 2-fold. Proteoliposomes containing sphingolipids significantly increased gamma-secretase activity over a phosphatidylcholine-only baseline, whereas the addition of phosphatidylinositol significantly decreased activity. Addition of soluble cholesterol in the presence of phospholipids and sphingolipids robustly increased the cleavage of APP- and Notch-like substrates in a dose-dependent manner. Reconstitution of gamma-secretase in complex lipid mixtures revealed that a lipid raft-like composition supported the highest level of activity compared with other membrane compositions. Taken together, these results demonstrate that membrane lipid composition is a direct and potent modulator of gamma-secretase and that cholesterol, in particular, plays a major regulatory role.

Emerging and potential therapies for Alzheimer's disease

BACKGROUND

The amyloid beta (Abeta) peptide is critical to the development of Alzheimer's disease (AD), the major neurodegenerative disease of the elderly for which there is currently no cure.

OBJECTIVE

To review the literature on emerging treatments and potential therapeutic strategies for AD.

METHODS

Available published literature and information from pharmaceutical companies was utilised.

RESULTS/CONCLUSION

Several of the current treatments to combat AD are aimed at inhibiting the production, blocking the oligomerisation/aggregation or enhancing the degradation of Abeta. In our opinion, albeit based on limited available data, a future potential therapeutic strategy is to mimic the mechanism by which the normal cellular form of the prion protein inhibits the beta-secretase beta-site amyloid precursor protein cleaving enzyme-1 (BACE1), and hence the production of Abeta.

Role of ganglioside metabolism in the pathogenesis of Alzheimer's disease--a review

Gangliosides are expressed in the outer leaflet of the plasma membrane of the cells of all vertebrates and are particularly abundant in the nervous system. Ganglioside metabolism is closely associated with the pathology of Alzheimer's disease (AD). AD, the most common form of dementia, is a progressive degenerative disease of the brain characterized clinically by progressive loss of memory and cognitive function and eventually death. Neuropathologically, AD is characterized by amyloid deposits or "senile plaques," which consist mainly of aggregated variants of amyloid beta-protein (Abeta). Abeta undergoes a conformational transition from random coil to ordered structure rich in beta-sheets, especially after addition of lipid vesicles containing GM1 ganglioside. In AD brain, a complex of GM1 and Abeta, termed "GAbeta," has been found to accumulate. In recent years, Abeta and GM1 have been identified in microdomains or lipid rafts. The functional roles of these microdomains in cellular processes are now beginning to unfold. Several articles also have documented the involvement of these microdomains in the pathogenesis of certain neurodegenerative diseases, such as AD. A pivotal neuroprotective role of gangliosides has been reported in in vivo and in vitro models of neuronal injury, Parkinsonism, and related diseases. Here we describe the possible involvement of gangliosides in the development of AD and the therapeutic potentials of gangliosides in this disorder.

A protocol for immunoaffinity separation of the accumulated ubiquitin-protein conjugates solubilized with sodium dodecyl sulfate

Certain proteins insoluble in aqueous salt solutions are difficult to separate from impurities by immunoaffinity techniques, even when the proteins are solubilized with denaturants due to interference of the antigen-antibody reaction. Representative examples of such proteins are the ubiquitin-protein conjugates that accumulate in neuronal tissues of neurodegenerative diseases, the hallmark of such disorders. In this study, we developed a novel sample preparation method comprising two successive steps: Sodium dodecyl sulfate (SDS) removal from the SDS-containing extracts and renaturation of the denatured proteins. The application of this method was tested on ubiquitin-protein conjugates in the brains of Niemann-Pick type C disease mouse and in heat-shocked K562 erythroleukemia cells. The ubiquitin-protein conjugates in both cases are insoluble in Tris-buffered saline but soluble in 2% SDS. The SDS-solubilized fractions prepared from each of the samples were further pretreated by the method mentioned above, and the ubiquitin-protein conjugates were efficiently immunoprecipitated with the anti-ubiquitin antibody from them. This method was also applied successfully to the immunoprecipitation of flotillin-1, a lipid raft protein, from mouse brain extract prepared with 2% SDS. These results indicate that this simple protocol has potential applications for excellent immunoaffinity separation of the less-soluble proteins in diverse cells and tissues.

Abeta42 mutants with different aggregation profiles induce distinct pathologies in Drosophila

Aggregation of the amyloid-β-42 (Aβ42) peptide in the brain parenchyma is a pathological hallmark of Alzheimer's disease (AD), and the prevention of Aβ aggregation has been proposed as a therapeutic intervention in AD. However, recent reports indicate that Aβ can form several different prefibrillar and fibrillar aggregates and that each aggregate may confer different pathogenic effects, suggesting that manipulation of Aβ42 aggregation may not only quantitatively but also qualitatively modify brain pathology. Here, we compare the pathogenicity of human Aβ42 mutants with differing tendencies to aggregate. We examined the aggregation-prone, EOFAD-related Arctic mutation (Aβ42Arc) and an artificial mutation (Aβ42art) that is known to suppress aggregation and toxicity of Aβ42 in vitro. In the Drosophila brain, Aβ42Arc formed more oligomers and deposits than did wild type Aβ42, while Aβ42art formed fewer oligomers and deposits. The severity of locomotor dysfunction and premature death positively correlated with the aggregation tendencies of Aβ peptides. Surprisingly, however, Aβ42art caused earlier onset of memory defects than Aβ42. More remarkably, each Aβ induced qualitatively different pathologies. Aβ42Arc caused greater neuron loss than did Aβ42, while Aβ42art flies showed the strongest neurite degeneration. This pattern of degeneration coincides with the distribution of Thioflavin S-stained Aβ aggregates: Aβ42Arc formed large deposits in the cell body, Aβ42art accumulated preferentially in the neurites, while Aβ42 accumulated in both locations. Our results demonstrate that manipulation of the aggregation propensity of Aβ42 does not simply change the level of toxicity, but can also result in qualitative shifts in the pathology induced in vivo.

Ganglioside GM1 binding the N-terminus of amyloid precursor protein

Secreted amyloid precursor protein (APPs) plays a role in several neuronal functions, including the promotion of synaptogenesis, neurite outgrowth and neuroprotection. Previous study has demonstrated that ganglioside GM1 inhibits the secretion of APPs; however the underlying mechanism remains unknown. Here we reported that GM1 can bind cellular full length APP and APPs secreted from APP(695) stably-transfected SH-SY5Y cells. To characterize the GM1-APP interaction further, we expressed and purified recombinant fragments of the N-terminal APP. Immunoprecipitation experiments revealed that GM1 was able to bind the recombinant APP(18-81) fragment. Moreover, the synthetic peptide APP(52-81) could inhibit the binding. Therefore, the binding site for GM1 appears to be located within residues 52-81 of APP. Furthermore, we found that only GM1, but not GD1a, GT1b and ceramide, binds APP-N-terminus, indicating that the specific binding depends on the sugar moiety of GM1. Fluorescent studies revealed a decrease in the intrinsic fluorescence intensity of the APP(52-81) peptide in phosphatidylcholine (PC)/GM1 vesicles. By using FTIR techniques, we found that the major secondary structure of the APP(52-81) peptide was altered in PC/GM1 vesicles. Our results demonstrate that GM1 binds the N-terminus of APP and induces a conformational change. These findings suggest that secreted APP is decreased by membrane GM1 binding to its precursor protein and provide a possible molecular mechanism to explain the involvement of GM1 in APP proteolysis and pathogenesis of Alzheimer's disease.

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.

Alzheimer's disease: cholesterol, membrane rafts, isoprenoids and statins

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder and the most prevalent form of dementia worldwide. AD is characterized pathologically by amyloid-β plaques, neurofibrillary tangles and neuronal loss, and clinically by a progressive loss of cognitive abilities. At present, the fundamental molecular mechanisms underlying the disease are unclear and no treatment for AD is known. Epidemiological evidence continues to mount linking vascular diseases, such as hypertension and diabetes, and hypercholesterolaemia with an increased risk for developing AD. A growing amount of evidence suggests a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques in AD development. Cholesterol and statins clearly modulate β-amyloid precursor protein (βAPP) processing in cell culture and animal models. Statins not only reduce endogenous cholesterol synthesis but also exert other various pleiotrophic effects, such as the reduction in protein isoprenylation. Through these effects statins modulate a variety of cellular functions involving both cholesterol (and membrane rafts) and isoprenylation. Although clearly other factors, such as vascular inflammation, oxidative stress and genetic factors, are intimately linked with the progression of AD, this review focuses on the present research findings describing the effect of cholesterol, membrane rafts and isoprenylation in regulating βAPP processing and in particular γ-secretase complex assembly and function and AD progression, along with consideration for the potential role statins may play in modulating these events.

Role of lipid rafts in the processing of the pathogenic prion and Alzheimer's amyloid-beta proteins

The conformational conversion of the cellular form of the prion protein (PrP C) into the infectious form (PrP Sc) and the proteolytic processing of the amyloid-beta (Abeta) peptide are central pathogenetic events in the prion diseases and Alzheimer's disease, respectively. Cholesterol- and sphingolipid-rich lipid rafts have emerged as important sites for the conversion of PrP C into PrP Sc, and for the proteolytic production, degradation and aggregation of Abeta. Here, we discuss these findings and their implications for our understanding of these disease processes. In addition, the potential for rafts as sites for therapeutic intervention in prion diseases and Alzheimer's disease is considered.

Intracellular amyloid-beta in Alzheimer's disease

The primal role that the amyloid-beta (Abeta) peptide has in the development of Alzheimer's disease is now almost universally accepted. It is also well recognized that Abeta exists in multiple assembly states, which have different physiological or pathophysiological effects. Although the classical view is that Abeta is deposited extracellularly, emerging evidence from transgenic mice and human patients indicates that this peptide can also accumulate intraneuronally, which may contribute to disease progression.

Formation of Amyloids by Aβ-(1–42) on NGF-differentiated PC12 Cells: Roles of Gangliosides and Cholesterol

The conversion of soluble, non-toxic amyloid beta-protein (Abeta) to aggregated, toxic Abeta could be the key step in the development of Alzheimer's disease. Liposomal studies have proposed that Abeta-(1-40) preferentially recognizes a cholesterol-dependent cluster of gangliosides and a conformationally altered form of Abeta promotes the aggregation of the protein. Cell experiments using fluorescein-labeled Abeta-(1-40) supported this model. Here, the interaction of native Abeta-(1-42) with unfixed rat pheochromocytoma PC12 cells was visualized using the amyloid-specific dye Congo red. Abeta-(1-42) preferentially bound to ganglioside and cholesterol-rich domains of cell membranes and formed amyloids in a time-dependent manner. These observations corroborate the model involving ganglioside-mediated accumulation of Abeta. The NGF-induced differentiation of PC12 cells into neuron-like cells caused a marked increase in both gangliosides and cholesterol, and thereby greatly potentiated the accumulation and cytotoxicity of Abeta-(1-42). NGF-differentiated cells exposed to Abeta-(1-42) had degenerated neurites, in which ganglioside and cholesterol-rich domains were localized, preceding cell death. A reduction in the amount of cholesterol by the cholesterol synthesis inhibitor compactin almost nullified the formation of amyloids by Abeta-(1-42). Our system using NGF-differentiated PC12 cells and Congo red is useful for screening inhibitors of the formation of amyloids by and cytotoxicity of Abeta.

Amyloid beta as a regulator of lipid homeostasis

The beta-amyloid peptide (A beta) is widely considered to be the molecule that causes Alzheimer's disease (AD). Besides this pathological function of A beta, recently published data reveal that A beta also has an essential physiological role in lipid homeostasis. Cholesterol increases A beta production, and conversely A beta production causes a decrease in cholesterol synthesis. The latter appears to be mediated by the inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR), a key enzyme in cholesterol synthesis, in an action similar to that of statins. Moreover, A beta regulates sphingolipid metabolism by directly activating sphingomyelinases (SMases). This review summarizes the molecular basis for the known physiological functions of A beta and amyloid precursor protein (APP), the roles of A beta and APP in lipid homeostasis and the medical implications of addressing lipid homeostasis in respect to AD. This knowledge might provide new insights for current and future therapeutic approaches to AD.

Caveolin-1 and -2 and their relationship to cerebral amyloid angiopathy in Alzheimer's disease

Cerebral amyloid angiopathy (CAA) affects over 90% of patients with Alzheimer's disease (AD) and increases the risk of cerebral haemorrhage and infarction. Caveolae--cholesterol-enriched plasmalemmal microinvaginations--are implicated in the production of amyloid beta peptide (Abeta). Caveolin-1 (CAV-1) is essential for the formation of caveolae. Caveolin-2 (CAV-2) is expressed at the plasma membrane only when in a stable hetero-oligomeric complex with CAV-1. CAV-1 and CAV-2 are highly co-expressed by endothelium and smooth muscle. Recent studies suggest that down-regulation of CAV-1 causes a reduction in alpha-secretase activity and consequent accumulation of Abeta. We have used quantitative immunohistochemical techniques to assess the relationship between CAV-1 and CAV-2 with respect to Abeta accumulation in the cerebral vasculature in a series of post mortem brains. CAV-1 and CAV-2 were co-expressed within the tunica media and endothelium of cerebral blood vessels. There were regional differences in CAV-1 immunolabelling, which was significantly greater in the frontal cortex and white matter than in the parietal lobe (in both control and AD cases) or the temporal lobe (in AD alone). However, CAV-1 labelling in AD did not differ from that in controls in any of the three lobes examined. Assessment of CAV-1 labelling in relation to the severity of CAA showed CAV-1 to be significantly increased in the frontal white matter in a subgroup of AD cases with absent/mild CAA compared with controls with absent/mild CAA and to AD cases with moderate/severe CAA, but the latter groups did not show significant differences from one another. CAV-1 labelling did not vary with age, gender, APOE genotype, post mortem delay or brain weight. Only segments of blood vessels with particularly abundant Abeta and extensive loss of smooth muscle actin showed loss of CAV-1 and CAV-2 from the tunica media. Within these vessels endothelial CAV-1 was preserved and discontinuous CAV-2 labelling was noted along the outer aspect of the vessel wall. Our findings suggest that alterations in the expression of vascular CAV-1 and CAV-2 are unlikely to play a role in the development of CAA in AD.

Amyloid-beta aggregation

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease in the growing population of elderly people. A hallmark of AD is the accumulation of plaques in the brain of AD patients. The plaques predominantly consist of aggregates of amyloid-beta (Abeta), a peptide of 39-42 amino acids generated in vivo by specific, proteolytic cleavage of the amyloid precursor protein. There is a growing body of evidence that Abeta aggregates are ordered oligomers and the cause rather than a product of AD. The analysis of the assembly pathway of Abeta in vitro and biochemical characterization of Abeta deposits isolated from AD brains indicate that Abeta oligomerization occurs via distinct intermediates, including oligomers of 3-50 Abeta monomers, annular oligomers, protofibrils, fibrils and plaques. Of these, the most toxic species appear to be small Abeta oligomers. This article reviews the current knowledge of the mechanism of Abeta assembly in vivo and in vitro, as well as the influence of inherited amino acid replacements in Abeta and experimental conditions on Abeta aggregation. Challenges regarding the reproducible handling of the Abeta peptide for in vitro assembly studies are discussed.

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

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

Low‐density lipoprotein receptor‐related protein promotes amyloid precursor protein trafficking to lipid rafts in the endocytic pathway

The major defining pathological hallmark of Alzheimer's disease (AD) is the accumulation of amyloid beta protein (Abeta), a small peptide derived from beta- and gamma-secretase cleavages of the amyloid precursor protein (APP). Recent studies have shown that beta- and gamma-secretase activities of BACE1 and presenilin, respectively, are concentrated in intracellular lipid raft microdomains. However, the manner in which APP normally traffics to lipid rafts is unknown. In this study, using transient transfection and immuno-precipitation assays, we show that the cytoplasmic domain of low-density lipoprotein receptor-related protein (LRP) interacts with APP and increases Abeta secretion and APP beta-CTF (C-terminal fragment) generation by promoting BACE1-APP interaction. We also employed discontinuous sucrose density gradient ultracentrifugation to show that the LRP cytoplasmic domain-mediated effect was accompanied by greatly increased localization of APP and BACE1 to lipid raft membranes, where beta- and gamma-secretase activities are highly enriched. Moreover, we provide evidence that endogenous LRP is required for the normal delivery of APP to lipid rafts and Abeta generation primarily in the endocytic but not secretory pathway. These results may provide novel insights to block Abeta generation by targeting LRP-mediated delivery of APP to raft microdomains.

Cholesterol distribution, not total levels, correlate with altered amyloid precursor protein processing in statin-treated mice

There are now a number of studies that suggest that cholesterol might regulate the processing of the amyloid precursor protein to form the neurotoxic peptide Abeta. This research has opened the possibility that cholesterol-lowering drugs might be efficacious as anti-Abeta drugs for use in Alzheimer's disease. The use of HMG-CoA reductase inhibitors (commonly called statins) in vitro and in vivo has proven them to be Abeta-lowering agents, however, the mechanism of action of these drugs is not yet known. One possible mechanism is that they reduce Abeta levels indirectly by reducing cholesterol in the central nervous system (CNS). In this study, we administered three different statins (simvastatin, lovastatin, and atorvastatin) to nontransgenic mice. We found that all three compounds had similar effects on Abeta, reducing both Abeta40 and Abeta42. The statins decreased beta-cleaved C-terminal fragment (CTF) although having no effect on alpha-CTF levels. However, the drugs did not have a similar effect on cholesterol in the CNS. Only lovastatin significantly reduced total cholesterol in isolated plasma membranes. As cholesterol is not distributed evenly in the plasma membrane, we examined bilayer distribution of cholesterol and found that all three statins caused CNS cholesterol to translocate from the cytofacial leaflet to the exofacial leaflet. This data suggests that cholesterol distribution and not total cholesterol levels may be important to Abeta production in the CNS.