Amyloid-beta oligomers: their production, toxicity and therapeutic inhibition

Despite extensive genetic and animal modelling data that support a central role for the amyloid beta-protein (A beta) in the genesis of Alzheimer's disease, the specific form(s) of A beta which causes injury to neurons in vivo has not been identified. In the present study, we examine the importance of soluble, pre-fibrillar assemblies of A beta as mediators of neurotoxicity. Specifically, we review the role of cell-derived SDS-stable oligomers, their blocking of hippocampal long-term potentiation in vivo and the finding that this blocking can be prevented by prior treatment of oligomer-producing cells with gamma-secretase inhibitors.

The amyloid hypothesis of Alzheimer's disease: progress and problems on the road to therapeutics

It has been more than 10 years since it was first proposed that the neurodegeneration in Alzheimer's disease (AD) may be caused by deposition of amyloid beta-peptide (Abeta) in plaques in brain tissue. According to the amyloid hypothesis, accumulation of Abeta in the brain is the primary influence driving AD pathogenesis. The rest of the disease process, including formation of neurofibrillary tangles containing tau protein, is proposed to result from an imbalance between Abeta production and Abeta clearance.

Intraneuronal Abeta42 accumulation in Down syndrome brain

Alzheimer's disease (AD) brains display A beta (Abeta) plaques, inflammatory changes and neurofibrillary tangles (NFTs). Converging evidence suggests a neuronal origin of Abeta. We performed a temporal study of intraneuronal Abeta accumulation in Down syndrome (DS) brains. Sections from temporal cortex of 70 DS cases aged 3 to 73 years were examined immunohistochemicallyf or immunoreactivity (IR) for the Abeta N-terminal, the Abeta40 C-terminus and the Abeta42 C-terminus. N-terminal antibodies did not detect intracellular Abeta. Abeta40 antibodies did not detect significant intracellular Abeta, but older cases showed Abeta40 IR in mature plaques. In contrast, Abeta42 antibodies revealed clear-cut intraneuronal IR. All Abeta42 antibodies tested showed strong intraneuronal Abeta42 IR in very young DS patients, especially in theyoungest cases studied (e.g., 3 or 4yr. old), but this IR declined as extracellular Abeta plaques gradually accumulated and matured. No inflammatory changes were associated with intraneuronal Abeta. We also studied the temporal development of gliosis and NFT formation, revealing that in DS temporal cortex, inflammation and NFT follow Abeta deposition. We conclude that Abeta42 accumulates intracellularly prior to extracellular Abeta deposition in Down syndrome, and that subsequent maturation of extracellular Abeta deposits elicits inflammatory responses andprecedes NFTs.

Proteolysis of chimeric beta-amyloid precursor proteins containing the Notch transmembrane domain yields amyloid beta-like peptides

gamma-Secretase is an unusual intramembranous protease that has been reported to cleave the beta-amyloid precursor protein (APP) near the middle of its transmembrane domain (TMD) but cleave Notch near the cytoplasmic end of its TMD. To ascertain whether the TMD sequence of the substrate determines where gamma-secretase cleaves and whether the region just before the TMD participates in recognition by the enzyme, we expressed chimeric human APP molecules containing either the TMD or pre-TMD regions of Notch or other transmembrane proteins. APP chimeras bearing either the Notch or the amyloid precursor-like protein-2 TMD released similar amounts of approximately 4-kDa amyloid beta-peptide (Abeta)-like peptides as did intact APP. Mass spectrometry revealed that the principal Abeta-like peptide ended at residue 40, indicating cleavage at the middle of the Notch TMD in the chimera. Generation of Abeta-like peptides was significantly decreased when the APP TMD was replaced by those of SREBP-1 or human epithelial growth factor receptor 3. Replacement of the APP pre-TMD region (Abeta 10-28) with that of SREBP-1 increased generation of Abeta-like peptides, while those of human epithelial growth factor receptor 3 or amyloid precursor-like protein-2 decreased it. We conclude that gamma-secretase can cleave near the middle of the Notch TMD, that Abeta-like peptides may arise during Notch processing, and that the pre-TMD sequence of the substrate influences recognition or binding by the enzyme.

Naturally occurring free D-aspartate is a nuclear component of cells in the mammalian hypothalamo-neurohypophyseal system

It is generally believed that only L-amino acids have a physiological role in species other than bacteria. Recently, the existence of some D-amino acids, particularly D-aspartate, in various organs of several higher animals has been reported. Here we demonstrate that naturally occurring free D-aspartate is localized subcellularly to the heterochromatin in the nucleoli (but not in either the dendrites or axonal terminals) of magnocellular neurosecretory neurons in the rat hypothalamus, and also of microglia and pericytes in the posterior pituitary. Our results imply that naturally occurring free D-aspartate might have a physiological role in nuclear function in mammals. The findings provide new insight for the biological function of D-stereoisomers of amino acids as well as the organization of the nucleus of at least some eukaryotic cells.

Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo

Although extensive data support a central pathogenic role for amyloid beta protein (Abeta) in Alzheimer's disease, the amyloid hypothesis remains controversial, in part because a specific neurotoxic species of Abeta and the nature of its effects on synaptic function have not been defined in vivo. Here we report that natural oligomers of human Abeta are formed soon after generation of the peptide within specific intracellular vesicles and are subsequently secreted from the cell. Cerebral microinjection of cell medium containing these oligomers and abundant Abeta monomers but no amyloid fibrils markedly inhibited hippocampal long-term potentiation (LTP) in rats in vivo. Immunodepletion from the medium of all Abeta species completely abrogated this effect. Pretreatment of the medium with insulin-degrading enzyme, which degrades Abeta monomers but not oligomers, did not prevent the inhibition of LTP. Therefore, Abeta oligomers, in the absence of monomers and amyloid fibrils, disrupted synaptic plasticity in vivo at concentrations found in human brain and cerebrospinal fluid. Finally, treatment of cells with gamma-secretase inhibitors prevented oligomer formation at doses that allowed appreciable monomer production, and such medium no longer disrupted LTP, indicating that synaptotoxic Abeta oligomers can be targeted therapeutically.

Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo

Although extensive data support a central pathogenic role for amyloid beta protein (Abeta) in Alzheimer's disease, the amyloid hypothesis remains controversial, in part because a specific neurotoxic species of Abeta and the nature of its effects on synaptic function have not been defined in vivo. Here we report that natural oligomers of human Abeta are formed soon after generation of the peptide within specific intracellular vesicles and are subsequently secreted from the cell. Cerebral microinjection of cell medium containing these oligomers and abundant Abeta monomers but no amyloid fibrils markedly inhibited hippocampal long-term potentiation (LTP) in rats in vivo. Immunodepletion from the medium of all Abeta species completely abrogated this effect. Pretreatment of the medium with insulin-degrading enzyme, which degrades Abeta monomers but not oligomers, did not prevent the inhibition of LTP. Therefore, Abeta oligomers, in the absence of monomers and amyloid fibrils, disrupted synaptic plasticity in vivo at concentrations found in human brain and cerebrospinal fluid. Finally, treatment of cells with gamma-secretase inhibitors prevented oligomer formation at doses that allowed appreciable monomer production, and such medium no longer disrupted LTP, indicating that synaptotoxic Abeta oligomers can be targeted therapeutically.

Activity-dependent isolation of the presenilin- gamma -secretase complex reveals nicastrin and a gamma substrate

Presenilin heterodimers apparently contain the active site of gamma-secretase, a polytopic aspartyl protease involved in the transmembrane processing of both the Notch receptor and the amyloid-beta precursor protein. Although critical to embryonic development and the pathogenesis of Alzheimer's disease, this protease is difficult to characterize, primarily because it is a multicomponent complex of integral membrane proteins. Here the functional gamma-secretase complex was isolated by using an immobilized active site-directed inhibitor of the protease. Presenilin heterodimers and nicastrin bound specifically to this inhibitor under conditions tightly correlating with protease activity, whereas several other presenilin-interacting proteins (beta-catenin, calsenilin, and presenilin-associated protein) did not bind. Moreover, anti-nicastrin antibodies immunoprecipitated gamma-secretase activity from detergent-solubilized microsomes. Unexpectedly, C83, the major endogenous amyloid-beta precursor protein substrate of gamma-secretase, was also quantitatively associated with the complex. These results provide direct biochemical evidence that nicastrin is a member of the active gamma-secretase complex, indicate that beta-catenin, calsenilin, and presenilin-associated protein are not required for gamma activity, and suggest an unprecedented mechanism of substrate-protease interaction.

Nasal vaccination with beta-amyloid peptide for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a severe neurodegenerative disease for which there is currently no effective prevention or treatment. The prediction that the number of U.S. patients with AD will triple to approximately 14 million over the next 50 years underscores the urgent need to explore novel therapeutic strategies for AD. The beta-amyloid protein (Abeta) accumulation and accompanying inflammation appear to play key roles in initiating the neuronal degeneration that underlies the signs and symptoms of AD. Interventions geared toward reducing Abeta accumulation and inflammatory responses should delay or prevent the onset of the clinical disease. Recently, several research groups, including ours, have shown that vaccination with Abeta results in a significant lowering of the Abeta burden in the brains of APP transgenic mice and, in some studies, improvement in their cognitive deficits. Our study described a novel approach, namely mucosal (intranasal) Abeta vaccination. Precisely how Abeta vaccination chronically lowers Abeta levels and reduces Abeta-associated pathology remains unclear. Here, we provide an overview of these studies, with particular emphasis on our work with intranasal Abeta vaccination. Examples of other intranasal vaccines and mucosal adjuvants are presented. Taken together, these data have implications for the future development of an intranasal Abeta vaccine for humans.

The intracellular domain of the beta-amyloid precursor protein is stabilized by Fe65 and translocates to the nucleus in a notch-like manner

The beta-amyloid precursor protein (APP) is a ubiquitous receptor-like molecule without a known function. However, the recent recognition that APP and Notch undergo highly similar proteolytic processing has suggested a potential signaling function for APP. After ligand binding, Notch is cleaved by the ADAM-17 metalloprotease followed by an intramembrane cleavage mediated by gamma-secretase. The gamma-secretase cut releases the Notch intracellular domain (NICD), which enters the nucleus and modulates transcription. Because APP is processed similarly by ADAM-17 and gamma-secretase, we reasoned that the APP intracellular domain (AICD) has a role analogous to the NICD. We therefore generated a plasmid encoding the AICD sequence and studied the subcellular localization of the expressed protein (C60). Our results demonstrate that the cytoplasmic domain of APP is a highly labile fragment that is stabilized by forming complexes with Fe65 and can then enter the nucleus in neurons and non-neural cells. These findings strongly support the hypothesis that APP signals in the nucleus in a manner analogous to the function of Notch.

Clearing the brain's amyloid cobwebs

Elevated cerebral levels of amyloid beta-protein occur universally in Alzheimer's disease, yet only a few patients show evidence of increased Abeta production. Therefore, defects in proteases that degrade Abeta could underlie some or many cases of familial and sporadic AD. This previously neglected topic has begun receiving serious attention. Understanding how proteolysis regulates Abeta levels in the cerebral cortex has implications for both the pathogenesis and the treatment of this protean disorder.

Presenilin, Notch, and the genesis and treatment of Alzheimer's disease

Elucidation of the proteolytic processing of the amyloid beta-protein precursor (APP) has revealed that one of the two proteases (gamma-secretase) that cleave APP to release amyloid beta-protein (Abeta) is likely to be presenilin. Presenilin also mediates the gamma-secretase-like cleavage of Notch receptors to enable signaling by their cytoplasmic domains. Therefore, APP and Notch may be the first identified substrates of a unique intramembranous aspartyl protease that has presenilin as its active-site component. In view of the evidence for a central role of cerebral build-up of Abeta in the pathogenesis of Alzheimer's disease, this disorder appears to have arisen in the human population as a late-life consequence of the conservation of a critical developmental pathway.

Immune hyporesponsiveness to amyloid beta-peptide in amyloid precursor protein transgenic mice: implications for the pathogenesis and treatment of Alzheimer's disease

Alzheimer's disease is a dementia that involves progressive deposition of amyloid beta-protein (Abeta) in brain regions important for memory and cognition, followed by secondary inflammation that contributes to the neuropathologic process. Immunization with Abeta can reduce cerebral Abeta burden and consequent neuropathologic changes in the brains of mice transgenic for the beta-amyloid precursor protein (APP). We found that transgenic expression of human APP in B6SJL mice, under the prion promoter, results in immune hyporesponsiveness to human Abeta, in terms of both antibody and cellular immune responses. The decreased antibody responses were related not to B cell tolerance but rather to the inability of Abeta-specific T cells to provide help for antibody production. The immune hyporesponsiveness could be overcome if T cell help was provided by coupling an Abeta B cell epitope to BSA. Our results suggest that expression of APP in transgenic mice is associated with an Abeta-specific impaired adaptive immune response that may contribute to the neuropathology. Moreover, humans with life-long elevation of brain and peripheral Abeta (e.g., patients with presenilin mutations or Down syndrome) could have reduced immune responses to Abeta vaccination.

alpha-Synuclein occurs in lipid-rich high molecular weight complexes, binds fatty acids, and shows homology to the fatty acid-binding proteins

alpha-Synuclein (alphaS) is a 140-residue neuronal protein that forms insoluble cytoplasmic aggregates in Parkinson's disease (PD) and several other neurodegenerative disorders. Two missense mutations (A53T and A30P) are linked to rare forms of familial PD. The normal function of alphaS is unknown, and cultured cell systems that model its modification from soluble monomers to aggregated forms have not been reported. Through a systematic centrifugal fractionation of mesencephalic neuronal cell lines and transgenic mouse brains expressing wild-type or A53T human alphaS, we observed unusual, previously unrecognized species of alphaS that migrate well above the 17-kDa monomeric form in denaturing gels. Incubation at 65 degrees C of high-speed cytosols from cells or brains revealed a modified alphaS species migrating at approximately 36 kDa and an extensive higher molecular mass alphaS-reactive smear. Extraction of the cytosols with chloroform/methanol or with a resin (Lipidex 1000) that binds fatty acids resulted in a similar pattern of higher molecular mass alphaS forms. On the basis of this effect of delipidation, we reexamined the primary structure of alphaS and detected a motif at the N and C termini that is homologous to a fatty acid-binding protein signature. In accord, we found that purified human alphaS binds oleic acid, with an apparent K(d) of 12.5 microM. We also observed an enhanced association of A53T alphaS with microsomal membranes in both mesencephalic cells and transgenic mouse brains. We conclude that alphaS has biochemical properties and a structural motif that suggest it is a novel member of the fatty acid-binding protein family and may thus transport fatty acids between the aqueous and membrane phospholipid compartments of the neuronal cytoplasm.

Ubiquitination of a new form of alpha-synuclein by parkin from human brain: implications for Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive accumulation in selected neurons of protein inclusions containing alpha-synuclein and ubiquitin. Rare inherited forms of PD are caused by autosomal dominant mutations in alpha-synuclein or by autosomal recessive mutations in parkin, an E3 ubiquitin ligase. We hypothesized that these two gene products interact functionally, namely, that parkin ubiquitinates alpha-synuclein normally and that this process is altered in autosomal recessive PD. We have now identified a protein complex in normal human brain that includes parkin as the E3 ubiquitin ligase, UbcH7 as its associated E2 ubiquitin conjugating enzyme, and a new 22-kilodalton glycosylated form of alpha-synuclein (alphaSp22) as its substrate. In contrast to normal parkin, mutant parkin associated with autosomal recessive PD failed to bind alphaSp22. In an in vitro ubiquitination assay, alphaSp22 was modified by normal but not mutant parkin into polyubiquitinated, high molecular weight species. Accordingly, alphaSp22 accumulated in a non-ubiquitinated form in parkin-deficient PD brains. We conclude that alphaSp22 is a substrate for parkin's ubiquitin ligase activity in normal human brain and that loss of parkin function causes pathological alphaSp22 accumulation. These findings demonstrate a critical biochemical reaction between the two PD-linked gene products and suggest that this reaction underlies the accumulation of ubiquitinated alpha-synuclein in conventional PD.

In vitro studies of amyloid beta-protein fibril assembly and toxicity provide clues to the aetiology of Flemish variant (Ala692-->Gly) Alzheimer's disease

In a Flemish kindred, an Ala(692)-->Gly amino acid substitution in the amyloid beta-protein precursor (AbetaPP) causes a form of early-onset Alzheimer's disease (AD) which displays prominent amyloid angiopathy and unusually large senile plaque cores. The mechanistic basis of this Flemish form of AD is unknown. Previous in vitro studies of amyloid beta-protein (Abeta) production in HEK-293 cells transfected with cDNA encoding Flemish AbetaPP have shown that full-length [Abeta(1-40)] and truncated [Abeta(5-40) and Abeta(11-40)] forms of Abeta are produced. In an effort to determine how these peptides might contribute to the pathogenesis of the Flemish disease, comparative biophysical and neurotoxicity studies were performed on wild-type and Flemish Abeta(1-40), Abeta(5-40) and Abeta(11-40). The results revealed that the Flemish amino acid substitution increased the solubility of each form of peptide, decreased the rate of formation of thioflavin-T-positive assemblies, and increased the SDS-stability of peptide oligomers. Although the kinetics of peptide assembly were altered by the Ala(21)-->Gly substitution, all three Flemish variants formed fibrils, as did the wild-type peptides. Importantly, toxicity studies using cultured primary rat cortical cells showed that the Flemish assemblies were as potent a neurotoxin as were the wild-type assemblies. Our results are consistent with a pathogenetic process in which conformational changes in Abeta induced by the Ala(21)-->Gly substitution would facilitate peptide adherence to the vascular endothelium, creating nidi for amyloid growth. Increased peptide solubility and assembly stability would favour formation of larger deposits and inhibit their elimination. In addition, increased concentrations of neurotoxic assemblies would accelerate neuronal injury and death.

Alzheimer's disease: genes, proteins, and therapy

Rapid progress in deciphering the biological mechanism of Alzheimer's disease (AD) has arisen from the application of molecular and cell biology to this complex disorder of the limbic and association cortices. In turn, new insights into fundamental aspects of protein biology have resulted from research on the disease. This beneficial interplay between basic and applied cell biology is well illustrated by advances in understanding the genotype-to-phenotype relationships of familial Alzheimer's disease. All four genes definitively linked to inherited forms of the disease to date have been shown to increase the production and/or deposition of amyloid beta-protein in the brain. In particular, evidence that the presenilin proteins, mutations in which cause the most aggressive form of inherited AD, lead to altered intramembranous cleavage of the beta-amyloid precursor protein by the protease called gamma-secretase has spurred progress toward novel therapeutics. The finding that presenilin itself may be the long-sought gamma-secretase, coupled with the recent identification of beta-secretase, has provided discrete biochemical targets for drug screening and development. Alternate and novel strategies for inhibiting the early mechanism of the disease are also emerging. The progress reviewed here, coupled with better ability to diagnose the disease early, bode well for the successful development of therapeutic and preventative drugs for this major public health problem.

Alzheimer's disease: genes, proteins, and therapy

Rapid progress in deciphering the biological mechanism of Alzheimer's disease (AD) has arisen from the application of molecular and cell biology to this complex disorder of the limbic and association cortices. In turn, new insights into fundamental aspects of protein biology have resulted from research on the disease. This beneficial interplay between basic and applied cell biology is well illustrated by advances in understanding the genotype-to-phenotype relationships of familial Alzheimer's disease. All four genes definitively linked to inherited forms of the disease to date have been shown to increase the production and/or deposition of amyloid beta-protein in the brain. In particular, evidence that the presenilin proteins, mutations in which cause the most aggressive form of inherited AD, lead to altered intramembranous cleavage of the beta-amyloid precursor protein by the protease called gamma-secretase has spurred progress toward novel therapeutics. The finding that presenilin itself may be the long-sought gamma-secretase, coupled with the recent identification of beta-secretase, has provided discrete biochemical targets for drug screening and development. Alternate and novel strategies for inhibiting the early mechanism of the disease are also emerging. The progress reviewed here, coupled with better ability to diagnose the disease early, bode well for the successful development of therapeutic and preventative drugs for this major public health problem.

Alzheimer's disease: genes, proteins, and therapy

Rapid progress in deciphering the biological mechanism of Alzheimer's disease (AD) has arisen from the application of molecular and cell biology to this complex disorder of the limbic and association cortices. In turn, new insights into fundamental aspects of protein biology have resulted from research on the disease. This beneficial interplay between basic and applied cell biology is well illustrated by advances in understanding the genotype-to-phenotype relationships of familial Alzheimer's disease. All four genes definitively linked to inherited forms of the disease to date have been shown to increase the production and/or deposition of amyloid beta-protein in the brain. In particular, evidence that the presenilin proteins, mutations in which cause the most aggressive form of inherited AD, lead to altered intramembranous cleavage of the beta-amyloid precursor protein by the protease called gamma-secretase has spurred progress toward novel therapeutics. The finding that presenilin itself may be the long-sought gamma-secretase, coupled with the recent identification of beta-secretase, has provided discrete biochemical targets for drug screening and development. Alternate and novel strategies for inhibiting the early mechanism of the disease are also emerging. The progress reviewed here, coupled with better ability to diagnose the disease early, bode well for the successful development of therapeutic and preventative drugs for this major public health problem.

Toward the characterization and identification of gamma-secretases using transition-state analogue inhibitors

The amyloid-beta protein (A beta), strongly implicated in the etiology of Alzheimer's disease (AD), is formed from the amyloid-beta precursor protein (APP) through sequential proteolysis by beta- and gamma-secretases. Cleavage by gamma-secretase takes place within the middle of the single transmembrane region of APP and results primarily in 40- and 42-amino acid A beta C-terminal variants, A beta 40 and A beta 42. The latter form of A beta is highly fibrillogenic, is invariably elevated in autosomal-dominant forms of AD, and is the major A beta component found presymptomatically in cerebral deposits. Thus, blocking production of A beta in general and A beta 42 in particular is considered an important therapeutic goal. We have developed transition-state analogue inhibitors of gamma-secretase as molecular probes for characterizing the active site of this enzyme, as pharmacological tools for understanding its role in biology, and as affinity labels toward its definitive identification. Specifically, we found that: (1) difluoro ketone and difluoro alcohol peptidomimetics are effective inhibitors of gamma-secretase activity in APP-transfected cells, strongly suggesting an aspartyl protease mechanism; (2) gamma-secretases that form A beta 40 and A beta 42 are pharmacologically distinct but are nevertheless closely similar; (3) large hydrophobic P1 substituents increase the inhibitory potency of these peptidomimetics, suggesting a large complementary S1 pocket for gamma-secretases; (4) A beta 42 production is increased several fold over control by these gamma-secretase inhibitors after replacement with inhibitor-free media; (5) a bromoacetamide derivative of one of these analogues continues to inhibit total A beta and A beta 42 production hours after replacement with compound-free media and should help identify the target(s) of these protease transition-state mimics.

Rapid Notch1 nuclear translocation after ligand binding depends on presenilin-associated gamma-secretase activity

Recent data suggest an intimate relationship between the familial Alzheimer disease gene presenilin 1 (PS1) and proteolytic processing of both the amyloid precursor protein (APP) and the important cell signaling molecule, Notch1. We now show, using mammalian cells transfected with full-length Notch1, that the C terminal domain of Notch1 rapidly translocates to the nucleus upon stimulation with the physiologic ligand Delta and initiates a CBF1-dependent signal transduction cascade. Using this assay, we demonstrate that the same aspartate mutations in PS1 that block APP processing also prevent Notch1 cleavage and translocation to the nucleus. Moreover, we show that two APP gamma-secretase inhibitors also diminish Notch1 nuclear translocation in a dose-dependent fashion. However, Notch1 signaling, assessed by measuring the activity of CBF1, a downstream gene, was reduced but not completely abolished in the presence of either aspartate mutations or gamma-secretase inhibitors. Our results support the hypothesis that similar PS1-related enzymatic activity is necessary for both APP and Notch1 processing, yet suggest that Notch signaling may remain relatively preserved with moderate levels of gamma-secretase inhibition.