Oligomeric and fibrillar species of amyloid-beta peptides differentially affect neuronal viability

HIV-1 Tat inhibits neprilysin and elevates amyloid beta

OBJECTIVE

Aging is a risk factor for amyloid beta (Abeta) accumulation and dementia. Since highly active antiretroviral therapies have effectively lengthened the life expectancy of individuals infected with HIV-1, we investigated the affect of HIV-1 Tat, a viral transactivating transcription factor, on Abeta degradation in the brain by neprilysin (NEP), a neuronal endopeptidase.

DESIGN AND METHODS

Using neural cell membrane fractions from human brain aggregates, Tat inhibition of NEP activity was assessed in a fluorescence assay. Following treatment with Tat, conditioned medium of human brain aggregate cultures was assayed for Abeta1-40 by ELISA. We evaluated the potential consequence of Tat inhibition of NEP by immunostaining cortex sections from postmortem human brain for Abeta.

RESULTS

In an in vitro assay, Tat inhibited NEP activity by 80%. The cysteine-rich domain of Tat was essential for NEP inhibition. Recombinant Tat added directly to brain cultures, resulted in a 125% increase in soluble Abeta. Postmortem human brain sections from patients with HIV-1 infection (n = 14; 31-58 years old) had a significant increase in Abeta, compared to controls (n = 5; 30-52 years old). Correlative analysis identified a statistically significant relationship between Abeta load and duration of HIV-1 seropositive status.

CONCLUSION

We have shown that Tat, which is found in the brains of patients with HIV-1 infection, inhibits the Abeta-degrading enzyme, NEP. Abeta staining was significantly increased in human brain sections from individuals with HIV-1 infection compared to controls. These results have important implications for individuals living and aging with HIV-1 infection.

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

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

Enhanced susceptibility of S-100B transgenic mice to neuroinflammation and neuronal dysfunction induced by intracerebroventricular infusion of human β-amyloid

S-100B is an astrocyte-derived protein that is increased in focal areas of the brain most severely affected by neuropathological changes in Alzheimer's disease (AD). Cell-based and clinical studies have implicated S-100B in progression of a pathologic, glial-mediated pro-inflammatory state in the CNS. However, the relationship between S-100B levels and susceptibility to AD-relevant neuroinflammation and neuronal dysfunction in vivo has not been determined. To test the hypothesis that overexpression of S-100B increases vulnerability to beta-amyloid (Abeta)-induced damage, we used S-100B-overexpressing transgenic (Tg) and S-100B knockout (KO) mice in a mouse model that involves intracerebroventricular infusion of human oligomeric Abeta1-42. This model mimics many features of AD, including robust neuroinflammation, Abeta plaques, synaptic damage and neuronal loss in the hippocampus. S-100B Tg, KO, and wild-type (WT) mice were infused with Abeta for 28 days, sacrificed at 60 days, and hippocampal endpoints analyzed. We found that Tg mice showed increased vulnerability to Abeta-induced neuropathology relative to either WT or KO mice. Specifically, Tg mice exhibited enhanced glial activation and neuroinflammation, increased nitrotyrosine staining (a marker of glial-induced neuronal damage), and more pronounced loss of synaptic markers. Interestingly, Tg mice showed no significant differences in Abeta plaque burden compared with WT or KO mice, suggesting that, as in the human situation, the severity of neuronal dysfunction did not correlate with amyloid deposition. Our data are consistent with a model in which S-100B overexpression in AD enhances glial activation and leads to an augmented neuroinflammatory process that increases the severity of neuropathologic sequelae.

The ‘O-acyl isopeptide method’ for the synthesis of difficult sequence-containing peptides: application to the synthesis of Alzheimer's disease-related amyloid β peptide (Aβ) 1-42

An efficient 'O-acyl isopeptide method' for the synthesis of difficult sequence-containing peptides was applied successfully to the synthesis of amyloid beta peptide (Abeta) 1-42 via a water-soluble O-acyl isopeptide of Abeta1-42, i.e. '26-O-acyl isoAbeta1-42' (6). This paper describes the detailed synthesis of Abeta1-42 focusing on the importance of resin selection and the analysis of side reactions in the O-acyl isopeptide method. Protected '26-O-acyl isoAbeta1-42' peptide resin was synthesized using 2-chlorotrityl chloride resin with minimum side reactions in comparison with other resins and deprotected crude 26-O-acyl isoAbeta1-42 was easily purified by HPLC due to its relatively good purity and narrow elution with reasonable water solubility. This suggests that only one insertion of the isopeptide structure into the sequence of the 42-residue peptide can suppress the unfavourable nature of its difficult sequence. The migration of O-acyl isopeptide to intact Abeta1-42 under physiological conditions (pH 7.4) via O--N intramolecular acyl migration reaction was very rapid and no other by-product formation was observed while 6 was stable under storage conditions. These results concluded that our strategy not only overcomes the solubility problem in the synthesis of Abeta1-42 and can provide intact Abeta1-42 efficiently, but is also applicable in the synthesis of large difficult sequence-containing peptides at least up to 50 amino acids. This synthesis method would provide a biological evaluation system in Alzheimer's disease research, in which 26-O-acyl isoAbeta1-42 can be stored in a solubilized form before use and then rapidly produces intact Abeta1-42 in situ during biological experiments.

Intraneuronal Aβ, non-amyloid aggregates and neurodegeneration in a Drosophila model of Alzheimer’s disease

We have developed models of Alzheimer's disease in Drosophila melanogaster by expressing the Abeta peptides that accumulate in human disease. Expression of wild-type and Arctic mutant (Glu22Gly) Abeta(1-42) peptides in Drosophila neural tissue results in intracellular Abeta accumulation followed by non-amyloid aggregates that resemble diffuse plaques. These histological changes are associated with progressive locomotor deficits and vacuolation of the brain and premature death of the flies. The severity of the neurodegeneration is proportional to the propensity of the expressed Abeta peptide to form oligomers. The fly phenotype is rescued by treatment with Congo Red that reduces Abeta aggregation in vitro. Our model demonstrates that intracellular accumulation and non-amyloid aggregates of Abeta are sufficient to cause the neurodegeneration of Alzheimer's disease. Moreover it provides a platform to dissect the pathways of neurodegeneration in Alzheimer's disease and to develop novel therapeutic interventions.

Amyloid-β aggregates formed at polar-nonpolar interfaces differ from amyloid-β protofibrils produced in aqueous buffers

The deposition of aggregated amyloid-beta (Abeta) peptides in the brain as senile plaques is a pathological hallmark of Alzheimer's disease (AD). Several lines of evidence indicate that fibrillar and, in particular, soluble aggregates of these 40- and 42-residue peptides are important in the etiology of AD. Recent studies also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we review our recent reports that Abeta(1-40) in vitro can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta(1-40) in low ionic strength buffers. These aggregates were quite stable and disaggregated to only a limited extent on dilution. A second class of soluble Abeta aggregates was generated at polar-nonpolar interfaces. Aggregation in a two-phase system of buffer over chloroform occurred more rapidly than in buffer alone. In buffered 2% hexafluoroisopropanol (HFIP), microdroplets of HFIP were formed and the half-time for aggregation was less than 10 minutes. Like Abeta protofibrils, these interfacial aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. However, electron microscopy and atomic force microscopy revealed very different morphologies. The HFIP aggregates formed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP these aggregates initially were very unstable and disaggregated completely within 2 minutes. However, their stability increased as they progressed to fibers. It is important to determine whether similar interfacial Abeta aggregates are produced in vivo.

Beta-amyloid-derived pentapeptide RIIGLa inhibits Abeta(1-42) aggregation and toxicity

Pr-IIGL(a), a derivative of the tetrapeptide beta-amyloid 31-34 (Abeta(31-34)), exerts controversial effects: it is toxic in a neuroblastoma culture, but it protects glial cells from the cytotoxic action of Abeta(1-42). For an understanding of this phenomenon, a new pentapeptide, RIIGL(a) was synthetized, and both compounds were studied by different physicochemical and biological methods. Transmission electron microscopic (TEM) studies revealed that Pr-IIGL(a) forms fibrillar aggregates, whereas RIIGL(a) does not form fibrils. Congo red binding studies furnished the same results. Aggregated Pr-IIGL(a) acts as a cytotoxic agent in neuroblastoma cultures, but RIIGL(a) does not display inherent toxicity. RIIGL(a) co-incubated with Abeta(1-42) inhibits the formation of mature amyloid fibres (TEM studies) and reduces the cytotoxic effect of fibrillar Abeta(1-42). These results indicate that RIIGL(a) is an effective inhibitor of both the aggregation and the toxic effects of Abeta(1-42) and can serve as a lead compound for the design of novel neuroprotective peptidomimetics.

The lipophilic metal chelator DP-109 reduces amyloid pathology in brains of human beta-amyloid precursor protein transgenic mice

Metals such as zinc, copper and iron contribute to aggregation of amyloid-beta (Abeta) protein and deposition of amyloid plaques in Alzheimer's disease (AD). We examined whether the lipophilic metal chelator DP-109 inhibited these events in aged female hAbetaPP-transgenic Tg2576 mice. Daily gavage administration of DP-109 for 3 months markedly reduced the burden of amyloid plaques and the degree of cerebral amyloid angiopathy in brains, compared to animals receiving vehicle treatment. Moreover, DP-109 treatment appeared to facilitate the transition of Abeta from insoluble to soluble forms in the cerebrum. These results further support the hypothesis that endogenous metals are involved in the deposition of aggregated Abeta in brains of AD patients, and that metal chelators may be useful therapeutic agents in the treatment of AD.

Aminopyridazines inhibit beta-amyloid-induced glial activation and neuronal damage in vivo

The critical role of chronic inflammation in disease progression continues to be increasingly appreciated across multiple disease areas, especially in neurodegenerative disorders such as Alzheimer's disease. We report that late intervention with a recently discovered aminopyridazine suppressor of glial activation, developed to inhibit both oxidative and inflammatory cytokine pathways, attenuates human amyloid beta (Abeta)-induced glial activation in a murine model. Peripheral administration of the aminopyridazine MW01-070C, beginning 3 weeks after the start of intracerebroventricular infusion of human Abeta1-42, decreased the number of activated astrocytes and microglia and the levels of proinflammatory cytokines interleukin-1beta, tumor necrosis factor-alpha and S100B in the hippocampus. Inhibition of neuroinflammation correlated with a decreased neuron loss, restoration towards control levels of synaptic dysfunction biomarkers in the hippocampus, and diminished amyloid plaque deposition. The results from this in vivo chemical biology approach provide a proof of concept that targeting of key glia inflammatory cytokine pathways can suppress Abeta-induced neuroinflammation in vivo, with resultant attenuation of neuronal damage.

Biological tuning of synthetic tactics in solid-phase synthesis: application to A beta(1-42)

The beta-amyloid(1-42) sequence has long been recognized as a challenging target for solid-phase peptide synthesis. We found that the known disaggregating role of Met-35 sulfoxide could be capitalized during stepwise solid-phase assembly of the A beta(1-42) peptide chain to mitigate on-resin peptide chain aggregation, a presumed major source of synthetic difficulties. Furthermore, we demonstrate a hitherto-unreported on-resin reduction of the sulfoxide "aggregation protecting group" to allow for standard cleavage protocols, obviating a separate solution-phase sulfoxide reduction step.

Signal transduction during amyloid-β-peptide neurotoxicity: role in Alzheimer disease

Alzheimer's disease (AD) is a neurodegenerative disorder with progressive dementia accompanied by two main structural changes in the brain: intracellular protein deposits termed neurofibrillary tangles (NFT) and extracellular amyloid protein deposits surrounded by dystrophic neurites that constitutes the senile plaques. Currently, it is widely accepted that amyloid beta-peptide (A beta) metabolism disbalance is crucial for AD progression. A beta deposition may be enhanced by molecular chaperones, including metals like copper and proteins like acetylcholinesterase (AChE). At the neuronal level, several AD-related proteins interact with transducers of the Wnt/beta-catenin signaling pathway, including beta-catenin and glycogen synthase kinase 3 beta (GSK-3 beta) and both in vitro and in vivo studies suggest that Wnt/beta-catenin signaling is a target for A beta toxicity. Accordingly, activation of this signaling by lithium or Wnt ligands in AD-experimental animal models or in primary hippocampal neurons attenuate A beta neurotoxicity by recovering beta-catenin levels and Wnt-target gene expression of survival genes such as bcl-2. On the other hand, peroxisomal proliferator-activated receptor gamma (PPAR gamma) and muscarinic acetylcholine receptor (mAChR) agonists also activate Wnt/beta-catenin signaling and they have neuroprotective effects on hippocampal neurons. Our studies are consistent with the idea that a sustained loss of function of Wnt signaling components would trigger a series of events, determining the onset and development of AD and that modulation of this pathway through the activation of cross-talking signaling cascades should be considered as a possible therapeutic strategy for AD treatment.

Mechanisms of the inhibitory effects of amyloid β-protein on synaptic plasticity

Alzheimer's disease can be considered a protein misfolding disease. In particular, inappropriate processing of a proteolytic fragment of amyloid precursor protein, amyloid beta-protein (Abeta), in early stages of Alzheimer's disease may lead to stabilization of small oligomers that are highly mobile and have a potential to be extremely toxic assemblies. Recently, the importance of such soluble species of Abeta in triggering synaptic dysfunction, long before neuronal loss occurs, has become apparent. Animal models have revealed that plasticity of hippocampal excitatory synaptic transmission is relatively selectively vulnerable to Abeta both in vitro and in vivo. This review focuses on the mechanisms of Abeta inhibition of long-term potentiation at synapses in the rodent hippocampus from two complimentary perspectives. Firstly, we examine evidence that the synaptic activity of this peptide resides primarily in oligomeric rather than monomeric or fibrillar Abeta species. Secondly, the importance of different oxidative/nitrosative stress-linked cascades including JNK, p38 MAPK and NADPH oxidase/iNOS-generated reactive oxygen/nitrogen free radicals in mediating the inhibition of LTP by Abeta is emphasised. These mechanistic studies provide a plausible explanation for the sensitivity of hippocampus-dependent memory to impairment in the early preclinical stages of Alzheimer's disease.

Aggressive amyloidosis in mice expressing human amyloid peptides with the Arctic mutation

The Arctic mutation within the amyloid-beta (Abeta) peptide causes Alzheimer disease. In vitro, Arctic-mutant Abeta forms (proto)fibrils more effectively than wild-type Abeta. We generated transgenic mouse lines expressing Arctic-mutant human amyloid precursor proteins (hAPP). Amyloid plaques formed faster and were more extensive in Arctic mice than in hAPP mice expressing wild-type Abeta, even though Arctic mice had lower Abeta(1-42/1-40) ratios. Thus, the Arctic mutation is highly amyloidogenic in vivo.

Neurogenic effect of beta-amyloid peptide in the development of neural stem cells

The adult mammalian brain contains neural stem cells (NSCs) with self-renewal and multilineage potential in the hippocampus and subventricular zone. However, neurogenesis from these areas does not compensate for neuronal loss in age-related neurodegenerative disorders such as Alzheimer's disease (AD). To test whether an impairment of neurogenesis could contribute to the pathogenesis of AD, we examined the effects of amyloid-beta peptide (Abeta) on the survival and neuronal differentiation of cultured NSCs from striatum and hippocampus. We show that Abeta peptide does not impair the neurogenic rate in NSC progeny, but that it increases the total number of neurons in vitro in a dose-dependent manner. The neurogenic effect of Abeta peptide is not dependent on soluble factors released from the NSC progeny. Neurogenesis is induced by Abeta42 and not Abeta40 or Abeta 25-35, and the activity appears to be a property of Abeta oligomers and not fibrils. These results suggest that Abeta may have positive as well as deleterious actions, and that a knowledge of the mechanisms involved in the former could be valuable in exploiting the regenerative and plastic potential of the brain in preventing and treating Alzheimer's disease.

A dual role for apolipoprotein e in neuroinflammation: anti- and pro-inflammatory activity

Chronically activated glia associated with amyloid plaques might contribute to neuronal dysfunction in Alzheimer's disease (AD) through generation of neuroinflammatory molecules. Apolipoprotein E (apoE), also found associated with amyloid plaques, has been hypothesized to serve an anti-inflammatory role in the CNS through its ability to modulate beta-amyloid (Abeta)-induced glial activation. To further characterize the effect of apoE on inflammation, we examined the ability of exogenously added human apoE3 and apoE4 to modulate neuro inflammatory responses of cultured rat glia. Apolipoprotein E3 (apoE3) and apoE4 suppressed oligomeric Abeta-induced production of inducible nitric oxide synthase and cyclo-oxygenase-2, supporting an anti- inflammatory role for apoE. Exogenous apoE also inhibited Abeta-induced production of endogenous apoE. However, exogenous apoE in the absence of Abeta stimulated production of the pro-inflammatory cytokine interleukin-1beta in an isoform-dependent manner, with apoE4 inducing a significantly greater response than apoE3. These data support the idea that Abeta stimulation of glial apoE limits neuroinflammation but that overproduction of apoE by activated glia might exacerbate inflammation. In addition, the observation that apoE4 has more robust pro-inflammatory activity than apoE3 provides a mechanistic link between the APOE4 allele and AD, and suggests potential apoE-based therapeutic strategies.

Small assemblies of unmodified amyloid beta-protein are the proximate neurotoxin in Alzheimer's disease

Pioneering work in the 1950s by Christian Anfinsen on the folding of ribonuclease has shown that the primary structure of a protein "encodes" all of the information necessary for a nascent polypeptide to fold into its native, physiologically active, three-dimensional conformation (for his classic review, see [Science 181 (1973) 223]). In Alzheimer's disease (AD), the amyloid beta-protein (Abeta) appears to play a seminal role in neuronal injury and death. Recent data have suggested that the proximate effectors of neurotoxicity are oligomeric Abeta assemblies. A fundamental question, of relevance both to the development of therapeutic strategies for AD and to understanding basic laws of protein folding, is how Abeta assembly state correlates with biological activity. Evidence suggests, as argued by Anfinsen, that the formation of toxic Abeta structures is an intrinsic feature of the peptide's amino acid sequence-one requiring no post-translational modification or invocation of peptide-associated enzymatic activity.

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

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

Soluble Arctic amyloid beta protein inhibits hippocampal long-term potentiation in vivo

Mutations in the amyloid precursor protein that result in substitutions of glutamic acid at residue 22 of the amyloid beta protein (A beta) with glutamine (Q22, Dutch) or glycine (G22, Arctic) cause aggressive familial neurological diseases characterized by cerebrovascular haemorrhages or Alzheimer's-type dementia, respectively. The present study compared the ability of these peptides to block long-term potentiation (LTP) of glutamatergic transmission in the hippocampus in vivo. The effects of intracerebroventricular injection of wild-type, Q22 and G22 A beta(1-40) peptides were examined in the CA1 area of urethane-anaesthetized rats. Both mutant peptides were approximately 100-fold more potent than wild-type A beta at inhibiting LTP induced by high-frequency stimulation when solutions of A beta were freshly prepared. Fibrillar material, as determined by electron microscopy, was obvious in all these peptide solutions and exhibited appreciable Congo Red binding, particularly for A beta(1-40)G22 and A beta(1-40)Q22. A soluble fraction of A beta(1-40)G22, obtained following high-speed centrifugation, retained full activity of the peptide solution to inhibit LTP, providing strong evidence that in the case of the Arctic disease a soluble nonfibrillar form of A beta may represent the primary mediator of A beta-related cognitive deficits, particularly early in the disease. In contrast, nonfibrillar soluble A beta(1-40)Q22 supernatant solution was approximately 10-fold less potent at inhibiting LTP than A beta(1-40)G22, a finding consistent with fibrillar A beta contributing to the inhibition of LTP by the Dutch peptide.

Block of long-term potentiation by naturally secreted and synthetic amyloid beta-peptide in hippocampal slices is mediated via activation of the kinases c-Jun N-terminal kinase, cyclin-dependent kinase 5, and p38 mitogen-activated protein kinase as well as metabotropic glutamate receptor type 5

The mechanisms of action of human synthetic and naturally secreted cell-derived amyloid beta-peptide (Abeta)(1-42) on the induction of long-term potentiation (LTP) were investigated in the medial perforant path to dentate granule cell synapses in hippocampal slices. Synthetic and cell-derived Abeta strongly inhibited high-frequency stimulation (HFS)-induced LTP at peak HFS and 1 hr after HFS. Cell-derived Abeta was much more potent than synthetic Abeta at inhibiting LTP induction, with threshold concentrations of approximately 1 and 100-200 nm, respectively. The involvement of various kinases in Abeta-mediated inhibition of LTP induction was investigated by applying Abeta in the presence of inhibitors of these kinases. The c-Jun N-terminal kinase (JNK) inhibitor JNKI prevented the block of LTP induction by both synthetic and cell-derived Abeta. The block of LTP induced by synthetic Abeta was also prevented by the JNK inhibitor anthra[1,9-cd]pyrazol-6(2H)-one, the cyclin-dependent kinase 5 (Cdk5) inhibitors butyrolactone and roscovitine, and the p38 MAP kinase (MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole but not by the p42-p44 MAP kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene. The group I-group II metabotropic glutamate receptor (mGluR) antagonist 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid and the mGluR5 antagonist methyl-6-(phenylethynyl)pyridine prevented the block of LTP induction by Abeta. However, thealpha7 nicotinic ACh receptor antagonist methylcaconatine did not prevent the inhibition of LTP induction by Abeta. These studies provide evidence that the Abeta-mediated inhibition of LTP induction involves stimulation of the kinases JNK, Cdk5, and p38 MAPK after the activation of both the Abeta receptor(s) and mGluR5.

Transthyretin Aggregation under Partially Denaturing Conditions Is a Downhill Polymerization†

The deposition of fibrils and amorphous aggregates of transthyretin (TTR) in patient tissues is a hallmark of TTR amyloid disease, but the molecular details of amyloidogenesis are poorly understood. Tetramer dissociation is typically rate-limiting for TTR amyloid fibril formation, so we have used a monomeric variant of TTR (M-TTR) to study the mechanism of aggregation. Amyloid formation is often considered to be a nucleation-dependent process, where fibril growth requires the formation of an oligomeric nucleus that is the highest energy species on the pathway. According to this model, the rate of fibril formation should be accelerated by the addition of preformed aggregates or "seeds", which effectively bypasses the nucleation step. Herein, we demonstrate that M-TTR amyloidogenesis at low pH is a complex, multistep reaction whose kinetic behavior is incompatible with the expectations for a nucleation-dependent polymerization. M-TTR aggregation is not accelerated by seeding, and the dependence of the reaction timecourse is first-order on the M-TTR concentration, consistent either with a dimeric nucleus or with a nonnucleated process where each step is bimolecular and essentially irreversible. These studies suggest that amyloid formation by M-TTR under partially denaturing conditions is a downhill polymerization, in which the highest energy species is the native monomer. Our results emphasize the importance of therapeutic strategies that stabilize the TTR tetramer and may help to explain why more than eighty TTR variants are disease-associated. The differences between amyloid formation by M-TTR and other amyloidogenic peptides (such as amyloid beta-peptide and islet amyloid polypeptide) demonstrate that these polypeptides do not share a common aggregation mechanism, at least under the conditions examined thus far.

Rapid photochemical cross-linking--a new tool for studies of metastable, amyloidogenic protein assemblies

Amyloidoses comprise a class of diseases characterized pathologically by the presence of deposits of fibrillar, aberrantly folded proteins, known as amyloids. Historically, these deposits were considered the key factors causing disease. However, recent evidence suggests that soluble protein oligomers, which are precursors for amyloid fibrils, are the primary toxic effectors responsible for the disease process. Understanding the mechanism by which these oligomers exert their toxicity requires knowledge of the structure, kinetics, and thermodynamics of their formation and conversion into larger assemblies. Such studies have been difficult due to the metastable nature of the oligomers. For the amyloid beta-protein (Abeta), a consensus about the size and relative abundance of small oligomers has not been achieved. We describe here the application of the method Photoinduced Cross-Linking of Unmodified Proteins (PICUP) to the study of Abeta oligomerization. This approach distinguishes oligomerization patterns of amyloidogenic and nonamyloidogenic proteins, allows quantification of each component in oligomer mixtures, and provides a means of correlating primary structure modifications with assembly characteristics. PICUP thus is a powerful tool for the investigation of small, metastable protein oligomers. The method provides essential insights into the factors that control the assembly of pathogenic protein oligomers, facilitating efforts toward the development of therapeutic agents.

Importance of MAPK pathways for microglial pro-inflammatory cytokine IL-1 beta production

In Alzheimer's disease (AD), chronically activated glia contribute to neuronal dysfunction through production of neuroinflammatory molecules like interleukin (IL)-1beta. As a first step to address the signaling pathways important for pro-inflammatory cytokine induction, and whether different activators use distinct pathways, we tested the involvement of mitogen-activated protein kinase (MAPK) pathways in microglial IL-1beta production. Microglial cultures stimulated with lipopolysaccharide, S100B, or beta-amyloid showed rapid activation of three different MAPKs (p38, ERK1/2, and JNK) and a later increase in IL-1beta levels, consistent with a possible mechanistic relationship between MAPK and IL-1beta. To more directly test this possibility, we stimulated microglia in the presence of selective MAPK inhibitors, and found that inhibition of each of the three MAPK pathways inhibited IL-1beta production in a concentration-dependent manner. In addition, the relative importance of each MAPK to IL-1beta production depended on the activating stimulus. These data demonstrate that MAPK pathways are important for microglial IL-1beta production, and suggest that different glial activators use distinct sets of signaling pathways to induce the same disease-relevant end-point in microglia.

Environment- and mutation-dependent aggregation behavior of Alzheimer amyloid β-protein

The deposition of amyloid beta-protein in the brain is a fundamental process in the development of Alzheimerís disease; however, the mechanism underlying aggregation of amyloid beta-protein remains to be determined. Here, we report that a membrane-mimicking environment, generated in the presence of detergents or a ganglioside, is sufficient per se for amyloid fibril formation from soluble amyloid beta-protein. Furthermore, hereditary variants of amyloid beta-protein, which are caused by amyloid precursor protein gene mutations, including the Dutch (E693Q), Flemish (A692G) and Arctic (E693G) types, show mutually different aggregation behavior in these environments. Notably, the Arctic-type amyloid beta-protein, in contrast to the wild-type and other variant forms, shows a markedly rapid and higher level of amyloid fibril formation in the presence of sodium dodecyl sulfate or GM1 ganglioside. These results suggest that there are favorable local environments for fibrillogenesis of amyloid beta-protein.

A molecular switch in amyloid assembly: Met35 and amyloid beta-protein oligomerization

Aberrant protein oligomerization is an important pathogenetic process in vivo. In Alzheimer's disease (AD), the amyloid beta-protein (Abeta) forms neurotoxic oligomers. The predominant in vivo Abeta alloforms, Abeta40 and Abeta42, have distinct oligomerization pathways. Abeta42 monomers oligomerize into pentamer/hexamer units (paranuclei) which self-associate to form larger oligomers. Abeta40 does not form these paranuclei, a fact which may explain the particularly strong linkage of Abeta42 with AD. Here, we sought to determine the structural elements controlling paranucleus formation as a first step toward the development of strategies for treating AD. Because oxidation of Met(35) is associated with altered Abeta assembly, we examined the role of Met(35) in controlling Abeta oligomerization. Oxidation of Met(35) in Abeta42 blocked paranucleus formation and produced oligomers indistinguishable in size and morphology from those produced by Abeta40. Systematic structural alterations of the C(gamma)(35)-substituent group revealed that its electronic nature, rather than its size (van der Waals volume), was the factor controlling oligomerization pathway choice. Preventing assembly of toxic Abeta42 paranuclei through selective oxidation of Met(35) thus represents a potential therapeutic approach for AD.

Proposed pathogenetic cascade of inclusion-body myositis: importance of amyloid-beta, misfolded proteins, predisposing genes, and aging

PURPOSE OF REVIEW

Sporadic inclusion-body myositis, the most common muscle disease of older persons, is of unknown cause, and there is no successful treatment. Interest in sporadic inclusion-body myositis has been enhanced by the recent identification within the sporadic inclusion-body myositis muscle fibers of several abnormally accumulated proteins, which provides novel and important clues to the pathogenesis of sporadic inclusion-body myositis.

RECENT FINDINGS

This article summarizes the most recent findings leading to better understanding of the players in the pathogenetic cascade. It is suggested that lymphocytic inflammatory component is probably secondary, and it may contribute only slightly to muscle fiber damage in sporadic inclusion-body myositis. However, it is proposed that the identified abnormal accumulation, aggregation, and misfolding of proteins, combined with and perhaps provoked by an aging intracellular milieu, more essentially lead to the vacuolar degeneration and atrophy of the muscle fibers that are specific to sporadic inclusion-body myositis. Abnormal accumulations of the amyloid-beta precursor protein and of its proteolytic fragment, amyloid-beta, associated with the aging cellular muscle fiber environment, appear to be key pathogenic events.

SUMMARY

In conceptualizing a treatment for sporadic inclusion-body myositis, the accumulations of amyloid-beta42 and other unfolded proteins are now phenomena to be reckoned with. One would like to stop intracellular increase of the unfolded/misfolded proteins by reducing their formation and/or increasing their disposal. In addition, the identification of factors that would decrease intra-muscle fiber expressions of beta- and gamma-secretases might lead to decreased production of putatively myotoxic oligomeric amyloid-beta42. Better understanding of the mechanisms and consequences of genes that predispose to sporadic inclusion-body myositis, and of human muscle fiber aging, could also provide new avenues toward the therapy of sporadic inclusion-body myositis. How to therapeutically capitalize on the new findings is now the challenge.

Identification of Cathepsin B as a Mediator of Neuronal Death Induced by Aβ-activated Microglial Cells Using a Functional Genomics Approach

Alzheimer's disease is a progressive neurodegenerative disease characterized by senile plaques, neurofibrillary tangles, dystrophic neurites, and reactive glial cells. Activated microglia are found to be intimately associated with senile plaques and may play a central role in mediating chronic inflammatory conditions in Alzheimer's disease. Activation of cultured murine microglial BV2 cells by freshly sonicated Abeta42 results in the secretion of neurotoxic factors that kill primary cultured neurons. To understand molecular pathways underlying Abeta-induced microglial activation, we analyzed the expression levels of transcripts isolated from Abeta42-activated BV2 cells using high density filter arrays. The analysis of these arrays identified 554 genes that are transcriptionally up-regulated by Abeta42 in a statistically significant manner. Quantitative reverse transcription-PCR was used to confirm the regulation of a subset of genes, including cysteine proteases cathepsin B and cathepsin L, tissue inhibitor of matrix metalloproteinase 2, cytochrome c oxidase, and allograft inflammatory factor 1. Small interfering RNA-mediated silencing of the cathepsin B gene in Abeta-activated BV2 cells diminished the microglial activation-mediated neurotoxicity. Moreover, CA-074, a specific cathepsin B inhibitor, also abolished the neurotoxic effects caused by Abeta42-activated BV2 cells. Our results suggest an essential role for secreted cathepsin B in neuronal death mediated by Abeta-activated inflammatory response.

Self-assembly of Abeta(1-42) into globular neurotoxins

Amyloid beta 1-42 (Abeta(1-42)) is a self-associating peptide that becomes neurotoxic upon aggregation. Toxicity originally was attributed to the presence of large, readily formed Abeta fibrils, but a variety of other toxic species are now known. The current study shows that Abeta(1-42) can self-assemble into small, stable globular assemblies free of fibrils and protofibrils. Absence of large molecules was verified by atomic force microscopy (AFM) and nondenaturing gel electrophoresis. Denaturing electrophoresis revealed that the globular assemblies comprised oligomers ranging from trimers to 24mers. Oligomers prepared at 4 degrees C stayed fibril-free for days and remained so when shifted to 37 degrees C, although the spectrum of sizes shifted toward larger oligomers at the higher temperature. The soluble, globular Abeta(1-42) oligomers were toxic to PC12 cells, impairing reduction of MTT and interfering with ERK and Rac signal transduction. Occasionally, oligomers were neither toxic nor recognized by toxicity-neutralizing antibodies, suggesting that oligomers could assume alternative conformations. Tests for oligomerization-blocking activity were carried out by dot-blot immunoassays and showed that neuroprotective extracts of Ginkgo biloba could inhibit oligomer formation at very low doses. The observed neurotoxicity, structure, and stability of synthetic Abeta(1-42) globular assemblies support the hypothesis that Abeta(1-42) oligomers play a role in triggering nerve cell dysfunction and death in Alzheimer's disease.

Increased soluble amyloid-beta peptide and memory deficits in amyloid model mice overexpressing the low-density lipoprotein receptor-related protein

Amyloid-beta peptide (Abeta) is central to the pathogenesis of Alzheimer's disease, and the low-density lipoprotein receptor-related protein (LRP) has been shown to alter Abeta metabolism in vitro. Here, we show that overexpression of a functional LRP minireceptor in the brain of PDAPP mice results in age-dependent increase of soluble brain Abeta, with no changes in Abeta plaque burden. Importantly, soluble brain Abeta was found to be primarily in the form of monomers/dimers and to be highly correlated with deficits in spatial learning and memory. These results provide in vivo evidence that LRP may contribute to memory deficits typical of Alzheimer's disease by modulating the pool of small soluble forms of Abeta.

Residues 17-20 and 30-35 of beta-amyloid play critical roles in aggregation

We examined the effects of co-incubating nine different Abeta peptide fragments with full-length Abeta1-40 (Abeta40) on protein aggregation. Six fragments enhanced aggregation of Abeta40 (Abeta1-28, 12-28, 17-28, 10-20, 25-35 and 17-40), while three others did not (Abeta1-11, 1-16, and 20-29). All of the peptides that enhanced aggregation contained either residues 17-20 or 30-35, indicating the importance of these regions for promoting aggregation of full-length Abeta. Abeta25-35 in particular increased both the rate and extent of aggregation of Abeta40 considerably as indicated by fluorescence staining. Atomic force microscope imaging (AFM) indicates the increase in fluorescence staining with Abeta25-35 is primarily due to increased formation of oligomers and protofibrils rather than formation of large amyloid fibrils. AFM images of Abeta25-35 when incubated alone also indicate formation of aggregates and long thin filaments. The increase in formation of the small toxic oligomeric morphology of Abeta40, along with formation of Abeta25-35 oligomers and thin filaments, represent two different potential pathways for Abeta25-35 toxicity. The critical roles of residues 17-20 and 30-35 of Abeta provide further insight into mechanism that underlie the formation of toxic aggregates in Alzheimer Disease (AD) and suggest targets for the design of beta-sheet breakers to modulate the aggregation and inhibit toxicity of full-length Abeta.

O?N intramolecular acyl migration reaction in the development of prodrugs and the synthesis of difficult sequence-containing bioactive peptides

N-Ointramolecular acyl migration in Ser- or Thr-containing peptides is a well-known side reaction in peptide chemistry. It results in the mutual conversion of ester and amide bonds. Our medicinal chemistry study focused on the fact that the O-acyl product can be readily converted to the original N-acyl form under neutral or slightly basic conditions in an aqueous buffer and the liberated ionized amino group enhances the water solubility of O-acyl products. Because of this, we have developed a novel class of "O-N intramolecular acyl migration"-type water-soluble prodrugs of HIV-1 protease inhibitors. These prodrugs released the parent drugs via a simple chemical mechanism with no side reaction. In this study, we applied this strategy to important cancer chemotherapeutic agents, paclitaxel and its derivatives, to develop water-soluble taxoid prodrugs, and found that these prodrugs, 2'-O-isoform of taxoids, showed promising results with higher water solubility and proper kinetics in their parent drug formation by a simple pH-dependent chemical mechanism with O-N intramolecular acyl migration. These results suggest that this strategy would be useful in toxicology and medical economics. After the successful application of O-N intramolecular acyl migration in medicinal chemistry, this concept was recently used in peptide chemistry for the synthesis of "difficult sequence-containing peptides." The strategy was based on hydrophilic O-acyl isopeptide synthesis followed by the O-N intramolecular acyl migration reaction, leading to the desired peptide. In a model study with small, difficult sequence-containing peptides, synthesized "O-acyl isopeptides" not only improved the solubility in various media and efficiently performed the high performance liquid chromatography purification, but also altered the nature of the difficult sequence during SPPS, resulting in the efficient synthesis of O-acyl isopeptides with no complications. The subsequent O-N intramolecular acyl migration of purified O-acyl isopeptides afforded the desired peptides as precipitates with high yield and purity. Further study of the synthesis of a larger difficult sequence-containing peptide, Alzheimer's disease-related peptide (A beta 1-42), surprisingly showed that only one insertion of the O-acyl group drastically improved the unfavorable nature of the difficult sequence in A beta 1-42, and achieved efficient synthesis of 26-O-acyl isoA beta 1-42 and subsequent complete conversion to A beta 1-42 via the O-N intramolecular acyl migration reaction of 26-O-acyl isoA beta 1-42. This suggests that our new method based on O-N intramolecular acyl migration is an important method for the synthesis of difficult sequence-containing bioactive peptides.

Identification of naturally occurring spirostenols preventing beta-amyloid-induced neurotoxicity

22R-Hydroxycholesterol is an intermediate in the steroid biosynthesis pathway shown to exhibit a neuroprotective property against beta-amyloid (1-42) (Abeta) toxicity in rat PCl2 and human NT2N neuronal cells by binding and inactivating Abeta. In search of potent 22R-hydroxycholesterol derivatives, we assessed the ability of a series of naturally occurring entities containing the 22R-hydroxycholesterol structure to protect PC12 cells against Abeta-induced neurotoxicity, determined by measuring changes in membrane potential, mitochondrial diaphorase activity, ATP levels and trypan blue uptake. 22R-Hydroxycholesterol derivatives sharing a common spirost-5-en-3-ol or a furost-5-en-3-ol structure were tested. Although some of these compounds were neuroprotective against 0.1 microM Abeta, only three protected against the 1-10 microM Abeta-induced toxicity and, in contrast to 22R-hydroxycholesterol, all were devoid of steroidogenic activity. These entities shared a common structural feature, a long chain ester in position 3 and common stereochemistry. The neuroprotective property of these compounds was coupled to their ability to displace radiolabeled 22R-hydroxycholesterol from Abeta, suggesting that the Abeta-22R-hydroxycholesterol physicochemical interaction contributes to their beneficial effect. In addition, a 22R-hydroxycholesterol derivative inhibited the formation of neurotoxic amyloid-derived diffusible ligands. Computational docking simulations of 22R-hydroxycholesterol and its derivatives on Abeta identified two binding sites. Chemical entities, as 22R-hydroxycholesterol, seem to bind preferentially only to one site. In contrast, the presence of the ester chain seems to confer the ability to bind to both sites on Abeta, leading to neuroprotection against high concentrations of Abeta. In conclusion, these results suggest that spirost-5-en-3-ol naturally occurring derivatives of 22R-hydroxycholesterol might offer a new approach for Alzheimer's disease therapy.

Interactions of Alzheimer amyloid peptides with cultured cells and brain tissue, and their biological consequences

The Alzheimer amyloid peptides are the main constituent of the diagnostic hallmark of Alzheimer disease, the senile plaque. A halo of neurodegeneration surrounds the senile plaques observed in the brains of Alzheimer patients. Significant efforts are under way to determine whether the Alzheimer peptides are the causal agents of this neurodegeneration. We review the developments in identifying the putative interaction sites of Alzheimer amyloid peptides on cells and intact brain tissue. We focus on the specificity of this interaction and on the molecular nature of potential receptors. These studies form the bases for developing therapeutics that target potential interaction sites and inhibit Alzheimer amyloid peptide deposition.

Glutamate and amyloid beta-protein rapidly inhibit fast axonal transport in cultured rat hippocampal neurons by different mechanisms

Impairment of axonal transport leads to neurodegeneration and synapse loss. Glutamate and amyloid beta-protein (Abeta) have critical roles in the pathogenesis of Alzheimer's disease (AD). Here we show that both agents rapidly inhibit fast axonal transport in cultured rat hippocampal neurons. The effect of glutamate (100 microm), but not of Abeta25-35 (20 microm), was reversible, was mimicked by NMDA or AMPA, and was blocked by NMDA and AMPA antagonists and by removal of extracellular Ca2+. The effect of Abeta25-35 was progressive and irreversible, was prevented by the actin-depolymerizing agent latrunculin B, and was mimicked by the actin-polymerizing agent jasplakinolide. Abeta25-35 induced intracellular actin aggregation, which was prevented by latrunculin B. Abeta31-35 but not Abeta15-20 exerted effects similar to those of Abeta25-35. Full-length Abeta1-42 incubated for 7 d, which specifically contained 30-100 kDa molecular weight assemblies, also caused an inhibition of axonal transport associated with intracellular actin aggregation, whereas freshly dissolved Abeta1-40, incubated Abeta1-40, and fresh Abeta1-42 had no effect. These results suggest that glutamate inhibits axonal transport via activation of NMDA and AMPA receptors and Ca2+ influx, whereas Abeta exerts its inhibitory effect via actin polymerization and aggregation. The ability of Abeta to inhibit axonal transport seems to require active amino acid residues, which is probably present in the 31-35 sequence. Full-length Abeta may be effective when it represents a structure in which these active residues can access the cell membrane. Our results may provide insight into the early pathogenetic mechanisms of AD.

Protofibrils, pores, fibrils, and neurodegeneration: separating the responsible protein aggregates from the innocent bystanders

Many neurodegenerative diseases, including Alzheimer's and Parkinson's and the transmissible spongiform encephalopathies (prion diseases), are characterized at autopsy by neuronal loss and protein aggregates that are typically fibrillar. A convergence of evidence strongly suggests that protein aggregation is neurotoxic and not a product of cell death. However, the identity of the neurotoxic aggregate and the mechanism by which it disables and eventually kills a neuron are unknown. Both biophysical studies aimed at elucidating the precise mechanism of in vitro aggregation and animal modeling studies support the emerging notion that an ordered prefibrillar oligomer, or protofibril, may be responsible for cell death and that the fibrillar form that is typically observed at autopsy may actually be neuroprotective. A subpopulation of protofibrils may function as pathogenic amyloid pores. An analogous mechanism may explain the neurotoxicity of the prion protein; recent data demonstrates that the disease-associated, infectious form of the prion protein differs from the neurotoxic species. This review focuses on recent experimental studies aimed at identification and characterization of the neurotoxic protein aggregates.

The peptide KLVFF-K(6) promotes beta-amyloid(1-40) protofibril growth by association but does not alter protofibril effects on cellular reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)

The peptide KLVFF-K6 was observed by Lowe et al. to simultaneously enhance amyloid beta-protein (Abeta) fibrillogenesis and decrease cellular toxicity, as measured in a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay. It was postulated that accelerated Abeta aggregation and precipitation induced by KLVFF-K6 may lead to an increase in less toxic insoluble fibrils at the expense of more toxic soluble protofibrils. In a previous study, we distinguished between two modes of protofibril growth: elongation by monomer deposition and direct protofibril-protofibril association. These growth mechanisms could be resolved by varying Abeta monomer and NaCl concentrations. Using assays designed to isolate these distinct modes of protofibril growth, we report here that larger Abeta aggregates formed in the presence of KLVFF-K6 resulted from enhanced protofibril association. 3H-Radiomethylated KLVFF-K6 bound to associated protofibrils with an apparent Kd of 180 nM, and concentrations of free [3H]KLVFF-K6 in this range were sufficient to convert soluble protofibrils to sedimentable fibrils. However, promotion of Abeta protofibril association by KLVFF-K6 had no effect on Abeta-induced decreases in cellular MTT reduction. Therefore, our data do not support the proposal that insoluble fibrils formed with KLVFF-K6 are less toxic than soluble protofibrils. KLVFF-K6 did not alter rates of protofibril elongation by monomer deposition. In contrast, when added to Abeta monomers isolated with the use of size-exclusion chromatography, KLVFF-K6 inhibited fibrillogenesis, as measured by thioflavin T fluorescence, and this inhibition was paralleled by a failure to alter cellular MTT reduction.

Amyloid precursor protein (APP) and the biology of proteolytic processing: relevance to Alzheimer's disease

The processing of amyloid precursor protein (APP) generates amyloid-beta (Abeta) peptides 1-40 and 1-42. The latter is neurotoxic and its accumulation results in amyloid fibril formation and the generation of senile plaques, the hallmark of Alzheimer's disease (AD). Whilst there has been considerable progress made in understanding the generation of Abeta by alpha-, beta- and gamma-secretase activity on APP, recently enzymes involved in the degradation of Abeta have been identified including neprilysin and insulin-degrading enzyme (IDE). We review the pathways involved in proteolytic processing of APP and discuss the potential implications of aberrant proteolysis on neurodegeneration. It is conceivable that single nucleotide polymorphisms (SNPs) in the regulatory regions of genes in these proteolytic cascades, which alter their expression, could contribute to some of the age-related changes seen in AD.

Beta-amyloid peptide activates non-alpha7 nicotinic acetylcholine receptors in rat basal forebrain neurons

Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by profound deficits in memory and cognitive function. Neuropathological hallmarks of the disease include a loss of basal forebrain cholinergic neurons and the deposition of beta-amyloid peptide (Abeta) in neuritic plaques. At a cellular level, considerable attention has focused on a study of Abeta interactions with the neuronal nicotinic acetylcholine receptor (nAChR) subtypes. In this study, using cell-attached and outside-out single channel recordings from acutely dissociated rat basal forebrain neurons, we report that Abeta and nicotine activate nAChRs with two distinct levels of single-channel conductance. Whole cell recordings from these neurons reveal Abeta and nicotine, in a concentration-dependent and reversible manner, evoke brisk depolarizing responses and an inward current. The effects of Abeta on both single channel and whole cell are blocked by the noncompetitive nAChR antagonist mecamylamine and competitive nAChR antagonist dihydro-beta-erythroidine, but not the specific alpha7-selective nAChR antagonist methyllycaconitine, indicating that Abeta activated non-alpha7 nAChRs on basal forebrain neurons. In addition, the non-alpha7 nAChR agonists UB-165, epibatidine, and cytisine, but not the selective alpha7 agonist AR-R17779, induced similar responses as Abeta and nicotine. Thus non-alpha7 nAChRs may also represent a novel target in mediating the effects of Abeta in AD.

Human neprilysin is capable of degrading amyloid beta peptide not only in the monomeric form but also the pathological oligomeric form

Amyloid beta-peptide (Abeta) is widely believed to play a central role in Alzheimer's disease (AD). Coordinate regulation of cerebral Abeta level is important in the pathogenesis of AD since either increased production of Abeta from amyloid precursor protein or decreased degradation causes elevated levels of Abeta, leading to accumulation of cerebral plaque formation or amyloid angiopathy. Here we studied neprilysin, a putative proteolytic enzyme for Abeta, and found that it degraded not only monomeric but also oligomeric forms of Abeta1-40. Moreover, neprilysin was found to be capable of degradation of the oligomeric form of Abeta1-42, a significant Abeta species in early pathogenesis. Neprilysin to decrease cerebral Abeta is suggested to be inevitable factor as a vital therapeutic target.

β-Amyloid protein oligomers induced by metal ions and acid pH are distinct from those generated by slow spontaneous ageing at neutral pH

Amyloid protein (Abeta1-40) aggregation and conformation was examined using native and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and the results compared with those obtained by atomic force microscopy, and with Congo red binding, sedimentation and turbidity assays. The amount of Abeta aggregation measured was different, depending upon the method used. Incubation for 15 min at pH 5.0 or in the presence of Fe2+, Cu2+ or Zn2+ did not alter the level of Abeta oligomers observed on SDS and native gels. However, the slow aggregation of Abeta to form high molecular mass species over 5 days was inhibited. In contrast, when Abeta aggregation was monitored using a Congo red binding assay or sedimentation assay, a rapid increase in Abeta aggregation was observed after incubation for 15 min at pH 5.0, or in the presence of Fe2+, Cu2+ or Zn2+. The low pH-, Zn2+- or Cu2+-induced Abeta aggregation measured in a turbidity assay was reversible. In contrast, a considerable proportion of the Abeta aggregation measured by native and SDS/PAGE was stable. Atomic force microscopy studies showed that Abeta aged at pH 5.0 or in the presence of Zn2+ produced larger looser rod-shaped aggregates than at pH 7.4. Abeta that had been aged at pH 7.4 was more cytotoxic than Abeta aged at pH 5.0. Taken together, the results suggest that Abeta oligomerizes via two mutually exclusive mechanisms to form two different types of aggregates, which differ in their cytotoxic properties.

The intact human acetylcholinesterase C-terminal oligomerization domain is alpha-helical in situ and in isolation, but a shorter fragment forms beta-sheet-rich amyloid fibrils and protofibrillar oligomers

A 14-residue fragment of the C-terminal oligomerization domain, or T-peptide, of human acetylcholinesterase (AChE) shares sequence homology with the amyloid-beta peptide implicated in Alzheimer's disease and can spontaneously self-assemble into classical amyloid fibrils under physiological conditions [Greenfield, S. A., and Vaux, D. J. (2002) Neuroscience 113, 485-492; Cottingham, M. G., Hollinshead, M. S., and Vaux, D. J. (2002) Biochemistry 41, 13539-13547]. Here we demonstrate that the conformation of this AChE(586-599) peptide, both before and after fibril formation, is different from that of a longer peptide, T(40), corresponding to the entire 40-amino acid T-peptide (residues 575-614 of AChE). This peptide is prone to homomeric hydrophobic interactions, consistent with its role in AChE subunit assembly, and possesses an alpha-helical structure which protects against the development of the beta-sheet-rich amyloidogenic conformation favored by the shorter constituent AChE(586-599) fragment. Using a conformation-sensitive monoclonal antibody raised against the alpha-helical T(40) peptide, we demonstrate that the conformation of the T-peptide domain within intact AChE is antigenically indistinguishable from that of the synthetic T(40) peptide. A second monoclonal antibody raised against the fibrillogenic AChE(586-599) fragment recognizes not only beta-sheet amyloid aggregates but also SDS-resistant protofibrillar oligomers. A single-antibody sandwich ELISA confirms that such oligomers exist at micromolar peptide concentrations, well below that required for formation of classical amyloid fibrils. Epitope mapping with this monoclonal antibody identifies a region near the N-terminus of the peptide that remains accessible in oligomer and fibril alike, suggesting a model for the arrangement of subunits within AChE(586-599) protofibrils and fibrils.

Elucidation of primary structure elements controlling early amyloid beta-protein oligomerization

Assembly of monomeric amyloid beta-protein (A beta) into oligomeric structures is an important pathogenetic feature of Alzheimer's disease. The oligomer size distributions of aggregate-free, low molecular weight A beta 40 and A beta 42 can be assessed quantitatively using the technique of photo-induced cross-linking of unmodified proteins. This approach revealed that low molecular weight A beta 40 is a mixture of monomer, dimer, trimer, and tetramer, in rapid equilibrium, whereas low molecular weight A beta 42 preferentially exists as pentamer/hexamer units (paranuclei), which self-associate to form larger oligomers. Here, photo-induced cross-linking of unmodified proteins was used to evaluate systematically the oligomerization of 34 physiologically relevant A beta alloforms, including those containing familial Alzheimer's disease-linked amino acid substitutions, naturally occurring N-terminal truncations, and modifications altering the charge, the hydrophobicity, or the conformation of the peptide. The most important structural feature controlling early oligomerization was the length of the C terminus. Specifically, the side-chain of residue 41 in A beta 42 was important both for effective formation of paranuclei and for self-association of paranuclei into larger oligomers. The side-chain of residue 42, and the C-terminal carboxyl group, affected paranucleus self-association. A beta 40 oligomerization was particularly sensitive to substitutions of Glu22 or Asp23 and to truncation of the N terminus, but not to substitutions of Phe19 or Ala21. A beta 42 oligomerization, in contrast, was largely unaffected by substitutions at positions 22 or 23 or by N-terminal truncations, but was affected significantly by substitutions of Phe19 or Ala21. These results reveal how specific regions and residues control A beta oligomerization and show that these controlling elements differ between A beta 40 and A beta 42.

Identification of a mutant amyloid peptide that predominantly forms neurotoxic protofibrillar aggregates

The amyloid peptide (Abeta), derived from the proteolytic cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases, undergoes multistage assemblies to fibrillar depositions in the Alzheimer's brains. Abeta protofibrils were previously identified as an intermediate preceding insoluble fibrils. While characterizing a synthetic Abeta variant named EV40 that has mutations in the first two amino acids (D1E/A2V), we discerned unusual aggregation profiles of this variant. In comparison of the fibrillogenesis and cellular toxicity of EV40 to the wild-type Abeta peptide (Abeta40), we found that Abeta40 formed long fibrillar aggregates while EV40 formed only protofibrillar aggregates under the same in vitro incubation conditions. Cellular toxicity assays indicated that EV40 was slightly more toxic than Abeta40 to human neuroblastoma SHEP cells, rat primary cortical, and hippocampal neurons. Like Abeta40, the neurotoxicity of the protofibrillar EV40 could be partially attributed to apoptosis since multiple caspases such as caspase-9 were activated after SHEP cells were challenged with toxic concentrations of EV40. This suggested that apoptosis-induced neuronal loss might occur before extensive depositions of long amyloid fibrils in AD brains. This study has been the first to show that a mutated Abeta peptide formed only protofibrillar species and mutations of the amyloid peptide at the N-terminal side affect the dynamic amyloid fibrillogenesis. Thus, the identification of EV40 may lead to further understanding of the structural perturbation of Abeta to its fibrillation.

Human amylin actions on rat cholinergic basal forebrain neurons: antagonism of beta-amyloid effects

Human amylin (hAmylin), a 37-amino acid pancreatic peptide, and amyloid beta protein (A beta), a 39-43 amino acid peptide, abundantly deposited in the brains of Alzheimer's patients, induce neurotoxicity in hippocampal and cortical cultures. Although the mechanism of this neurotoxicity is unknown, both peptides are capable of modulating ion channel function that may result in a disruption of cellular homeostasis. In this study, we examined the effects of hAmylin on whole cell currents in chemically identified neurons from the rat basal forebrain and the interactions of hAmylin-induced responses with those of A beta. Whole cell patch-clamp recordings were performed on enzymatically dissociated neurons of the diagonal band of Broca (DBB), a cholinergic basal forebrain nucleus. Bath application of hAmylin (1 nM to 5 microM) resulted in a dose-dependent reduction in whole cell currents in a voltage range between -30 and +30 mV. Single-cell RT-PCR analysis reveal that all DBB neurons responding to hAmylin or A beta were cholinergic. Using specific ion channel blockers, we identified hAmylin and A beta effects on whole cell currents to be mediated, in part, by calcium-dependent conductances. Human amylin also depressed the transient outward (IA) and the delayed rectifier (IK) potassium currents. The hAmylin effects on whole cell currents could be occluded by A beta and vice versa. Human amylin and A beta responses could be blocked with AC187 (50 nM to 1 microM), a specific antagonist for the amylin receptor. The present study indicates that hAmylin, like A beta, is capable of modulating ion channel function in cholinergic basal forebrain neurons. Furthermore, the two peptides may share a common mechanism of action. The ability of an amylin antagonist to block the responses evoked by hAmylin and A beta may provide a novel therapeutic approach for Alzheimer's disease.

In vitro characterization of conditions for amyloid-beta peptide oligomerization and fibrillogenesis

Extensive research causally links amyloid-beta peptide (A beta) to Alzheimer's disease, although the pathologically relevant A beta conformation remains unclear. A beta spontaneously aggregates into the fibrils that deposit in senile plaques. However, recent in vivo and in vitro reports describe a potent biological activity for oligomeric assemblies of A beta. To consistently prepare in vitro oligomeric and fibrillar forms of A beta 1-42, a detailed knowledge of how solution parameters influence structure is required. This manuscript represents the first study using a single chemically and structurally homogeneous unaggregated starting material to demonstrate that the formation of oligomers, fibrils, and fibrillar aggregates is determined by time, concentration, temperature, pH, ionic strength, and A beta species. We recently reported that oligomers inhibit neuronal viability 10-fold more than fibrils and approximately 40-fold more than unaggregated peptide, with oligomeric A beta 1-42-induced neurotoxicity significant at 10 nm. In addition, we were able to differentiate by structure and neurotoxic activity wild-type A beta1-42 from isoforms containing familial mutations (Dahlgren, K. N., Manelli, A. M., Stine, W. B., Jr., Baker, L. K., Krafft, G. A., and LaDu, M. J. (2002) J. Biol. Chem. 277, 32046-32053). Understanding the biological role of specific A beta conformations may define the link between A beta and Alzheimer's disease, re-focusing therapeutic approaches by identifying the pernicious species of A beta ultimately responsible for the cognitive dysfunction that defines the disease.

Amyloid beta -protein (Abeta) assembly: Abeta 40 and Abeta 42 oligomerize through distinct pathways

Amyloid beta-protein (Abeta) is linked to neuronal injury and death in Alzheimer's disease (AD). Of particular relevance for elucidating the role of Abeta in AD is new evidence that oligomeric forms of Abeta are potent neurotoxins that play a major role in neurodegeneration and the strong association of the 42-residue form of Abeta, Abeta42, with the disease. Detailed knowledge of the structure and assembly dynamics of Abeta thus is important for the development of properly targeted AD therapeutics. Recently, we have shown that Abeta oligomers can be cross-linked efficiently, and their relative abundances quantified, by using the technique of photo-induced cross-linking of unmodified proteins (PICUP). Here, PICUP, size-exclusion chromatography, dynamic light scattering, circular dichroism spectroscopy, and electron microscopy have been combined to elucidate fundamental features of the early assembly of Abeta40 and Abeta42. Carefully prepared aggregate-free Abeta40 existed as monomers, dimers, trimers, and tetramers, in rapid equilibrium. In contrast, Abeta42 preferentially formed pentamerhexamer units (paranuclei) that assembled further to form beaded superstructures similar to early protofibrils. Addition of Ile-41 to Abeta40 was sufficient to induce formation of paranuclei, but the presence of Ala-42 was required for their further association. These data demonstrate that Abeta42 assembly involves formation of several distinct transient structures that gradually rearrange into protofibrils. The strong etiologic association of Abeta42 with AD may thus be a result of assemblies formed at the earliest stages of peptide oligomerization.