Soluble Aβ Oligomers Research Articles

High‐molecular‐weight β‐amyloid oligomers are elevated in cerebrospinal fluid of Alzheimer patients

There is accumulating evidence that soluble amyloid-beta (Abeta) oligomers, rather than amyloid fibrils, are the principal pathogenic species in Alzheimer disease (AD). Here, we have developed a novel enzyme-linked immunosorbent assay (ELISA) specific for high-molecular-weight (HMW) Abeta oligomers. Analysis of Abeta oligomers derived from synthetic Abeta 1-42, by size-exclusion chromatography (SEC), revealed that our ELISA specifically detected HMW Abeta oligomers of 40-200 kDa. Using this ELISA, we detected significantly higher (P<0.0001) signals in cerebrospinal fluid (CSF) samples from 25 patients with AD or mild cognitive impairment (MCI), compared to 25 age-matched controls. As a test for discriminating between the AD/MCI and control groups, the area under the curve in receiver operating characteristic analysis for the CSF HMW Abeta oligomers was greater than that for CSF Abeta x-42. Furthermore, the CSF levels of HMW Abeta oligomers showed a negative correlation with Mini-Mental State Examination scores in the AD/MCI group. We conclude that the CSF HMW Abeta oligomers detected by our ELISA could be useful as a diagnostic marker for AD, and also as a potential surrogate marker for disease severity. Our results support the idea that soluble HMW Abeta oligomers play a critical role in the pathogenesis and progression of AD.

Amyloid oligomers: formation and toxicity of Aβ oligomers

Alzheimer's disease (AD) is an age-related, progressive degenerative disorder that is characterized by synapse and neuron loss in the brain and the accumulation of protein-containing deposits (referred to as 'senile plaques') and neurofibrillary tangles. Insoluble amyloid beta-peptide (Abeta) fibrillar aggregates found in extracellular plaques have long been thought to cause the neurodegenerative cascades of AD. However, accumulating evidence suggests that prefibrillar soluble Abeta oligomers induce AD-related synaptic dysfunction. The size of Abeta oligomers is distributed over a wide molecular weight range (from < 10 kDa to > 100 kDa), with structural polymorphism in Abeta oligomers of similar sizes. Recent studies have demonstrated that Abeta can accumulate in living cells, as well as in extracellular spaces. This review summarizes current research on Abeta oligomers, focusing on their structures and toxicity mechanism. We also discuss possible formation mechanisms of intracellular and extracellular Abeta oligomers.

Amyloid β Induces the Morphological Neurodegenerative Triad of Spine Loss, Dendritic Simplification, and Neuritic Dystrophies through Calcineurin Activation

Amyloid beta (Abeta)-containing plaques are surrounded by dystrophic neurites in the Alzheimer's disease (AD) brain, but whether and how plaques induce these neuritic abnormalities remain unknown. We tested the hypothesis that soluble oligomeric assemblies of Abeta, which surround plaques, induce calcium-mediated secondary cascades that lead to dystrophic changes in local neurites. We show that soluble Abeta oligomers lead to activation of the calcium-dependent phosphatase calcineurin (CaN) (PP2B), which in turn activates the transcriptional factor nuclear factor of activated T cells (NFAT). Activation of these signaling pathways, even in the absence of Abeta, is sufficient to produce a virtual phenocopy of Abeta-induced dystrophic neurites, dendritic simplification, and dendritic spine loss in both neurons in culture and in the adult mouse brain. Importantly, the morphological deficits in the vicinity of Abeta deposits in a mouse model of AD are ameliorated by CaN inhibition, supporting the hypothesis that CaN-NFAT are aberrantly activated by Abeta and that CaN-NFAT activation is responsible for disruption of neuronal structure near plaques. In accord with this, we also detect increased levels of an active form of CaN and NFATc4 in the nuclear fraction from the cortex of patients with AD. Thus, Abeta appears to mediate the neurodegeneration of AD, at least in part, by activation of CaN and subsequent NFAT-mediated downstream cascades.

Loss of α7 Nicotinic Receptors Enhances β-Amyloid Oligomer Accumulation, Exacerbating Early-Stage Cognitive Decline and Septohippocampal Pathology in a Mouse Model of Alzheimer's Disease

Early Alzheimer's disease (AD) is marked by cholinergic hypofunction, neuronal marker loss, and decreased nicotinic acetylcholine receptor (nAChR) density from the cortex and hippocampus. alpha7 nAChRs expressed on cholinergic projection neurons and target regions have been implicated in neuroprotection against beta-amyloid (Abeta) toxicity and maintenance of the septohippocampal phenotype. We tested the role that alpha7 nAChRs perform in the etiology of early AD by genetically deleting the alpha7 nAChR subunit from the Tg2576 mouse model for AD and assessing animals for cognitive function and septohippocampal integrity. Thus, Tg2576 mice transgenic for mutant human amyloid precursor protein (APP) were crossed with alpha7 nAChR knock-out mice (A7KO) to render an animal with elevated Abeta in the absence of alpha7 nAChRs (A7KO-APP). We found that learning and memory deficits seen in 5-month-old APP mice are more severe in the A7KO-APP animals. Analyses of animals in early-stage preplaque cognitive decline revealed signs of neurodegeneration in A7KO-APP hippocampus as well as loss of cholinergic functionality in the basal forebrain and hippocampus. These changes occurred concomitant with the appearance of a dodecameric oligomer of Abeta that was absent from all other genotypic groups, generating the hypothesis that increased soluble oligomeric Abeta may underlie additional impairment of A7KO-APP cognitive function. Thus, alpha7 nAChRs in a mouse model for early-stage AD appear to serve a neuroprotective role through maintenance of the septohippocampal cholinergic phenotype and preservation of hippocampal integrity possibly through influences on Abeta accumulation and oligomerization.

RS-0406 Arrests Amyloid-β Oligomer-Induced Behavioural Deterioration In Vivo

Clinically accessible compounds that arrest or reverse the effects of amyloid-β (Aβ) on progressively developing behavioural symptomatology and neuropathology in Alzheimer's disease (AD) have yet to become available. However, a viable strategy may be to target and neutralise soluble Aβ oligomers, which have been shown to mediate synaptic dysfunction and to produce cognitive deficits in the intact organism. Inhibiting the aggregation of Aβ is therapeutically attractive, as Aβ aggregation is a pathological event and pharmacological interventions targeting this are likely to have a non-toxic profile. A behavioural assay, the alternating-lever cyclic-ratio schedule, was used to assess the effect of Aβ oligomers and the non-peptide small molecule RS-0406 in male Sprague-Dawley rats. RS-0406 has been shown to inhibit Aβ1-42 fibrillogenesis and protect against Aβ1-42–induced cytotoxicity in primary hippocampal neurons. In the current study, RS-0406 ameliorated the adverse effects of secreted oligomers of human Aβ on behaviour and dose dependently reduced the behavioural effects of Aβ oligomers, with the highest dose, 10μM, maintaining behaviour approximately at control levels. This effect appeared to be central; peripheral confounds having been extensively investigated. This is the first published report on the effects of RS-0406 in vivo and indicates that RS-0406 has potential as a pharmacotherapeutic intervention for behavioural deficits seen in the early stages of AD, and possibly as an intervention in the development of AD neuropathology. Indeed, an analogue of RS-0406 that could be administered peripherally might be a realistic candidate for the clinical treatment of AD.

How do soluble oligomers of amyloid β-protein impair hippocampal synaptic plasticity?

How do soluble oligomers of amyloid β-protein impair hippocampal synaptic plasticity?

Structural diversity of dimers of the Alzheimer amyloid-β(25–35) peptide and polymorphism of the resulting fibrils

The 25-35 fragment of the Alzheimer amyloid beta (Abeta) peptide is a naturally occurring proteolytic by-product that retains the toxicity of its larger, better-known counterpart, Abeta (1-40). Soluble oligomers of the amyloid-beta peptide have been implicated in the pathogenesis of Alzheimer's disease as a primary source of neurotoxicity. These oligomers are difficult to characterize experimentally due to their transient nature. As a result, a detailed knowledge of oligomeric structures at the atomic level is lacking. Using replica exchange molecular dynamics simulations, we investigated the conformations adopted by dimers, the smallest soluble oligomers of Abeta(25-35). Our simulations, which total 4 mus in length, reveal a diverse ensemble of well-organized dimers with high beta-sheet content coexisting with unstructured dimer complexes. The structured dimers comprise parallel and antiparallel extended beta-strand, beta-hairpin, and V-shaped beta-strand conformations. Protofibril models constructed from the extended and V-shaped dimers lead to stable structures consistent with experimentally available data from H/D exchange NMR and AFM spectroscopy. Our simulations suggest that fibril polymorphism may be encoded in the early stages of aggregation for the Abeta(25-35) peptide.

The Interaction with β-Amyloid Impairs the Mechanical Stability of Polymer Cushioned Membrane

The mechanism of neurodegeneration caused by β-amyloid (Aβ) in Alzheimer's disease is still controversial. Neuronal toxicity is exerted mostly by various species of soluble Aβ oligomers. Recent data depict membranes as the main sites where proteins/peptides are recruited and concentrated, misfold, and nucleate amyloids; at the same time, membranes are considered key triggers of amyloid toxicity.We demonstrated the capability of Aβ to penetrate and destabilize stacked lipid bilayers in a previous work. In this study, in order to maintain the natural fluidity of the membrane, polymer cushioned lipid bilayers have been used as a model for neuronal membrane. Layer-by-layer technique was used for the fabrication of the polymer cushion of charged poly-electrolytes, the lipid membrane is built on the polymer film by unilamellar vesicle fusion. Neutron reflectivity was used to monitor the kinetics of adsorption of the lipid bilayer onto the polymer surface; the conditions for the best surface coverage have been determined. The structure of the lipid bilayers is modified by the interaction with Aβ1-42; Neutron reflectivity showed a change of the scattering density profile in the direction perpendicular to the membrane plane, suggesting penetration of Aβ in the double layer. Atomic force microscope (AFM) has been used to test the lipid packing of the membrane through film rupture experiments and to compare the bilayer morphology in the presence or in the absence of Aβ. We demonstrated that the presence of Aβ weakens the lipid packing in the model membranes.We compared the results obtained on polymer cushioned lipid bilayers with those obtained using a rigid substrate (freshly cleaved mica) for membrane preparation.

Detecting and Characterizing Amyloid-β1-40 Oligomers using Single Molecule Fluorescence

The devastating symptoms of Alzheimer's Disease (AD) have been attributed to the behavior of aggregated Amyloid-β (Aβ) peptides. Cleaved from the extra-cellular portion of the transmembrane receptor, amyloid precursor protein (APP), Aβ has a very high propensity to aggregate into fibrils with a cross-β-sheet structure. Though fibrillar plaques have been seen as a hallmark of AD, there has recently been increasing evidence suggesting that smaller soluble Aβ oligomers are the agents of neuronal toxicity. Molecular level characterization of these oligomers using conventional biochemical techniques has been very difficult as they are both short-lived and heterogeneous.In this work we have used a single molecule fluorescence microscopy technique to follow the formation and resolve distributions of these oligomers. Two-color coincidence detection (TCCD) has the ability to detect such transient complexes even when they comprise only 0.1% of the population (1, 2). In this work, we have detected and characterized the species ranging from dimers to 50-mers formed during the oligomerization of monomeric Aβ1-40 (the 40-amino acid portion of Aβ) as well as those resulting from breakage of Aβ1-40 fibrils. Through experiments tracking the formation of oligomers throughout the Aβ aggregation process in real-time, we have gained novel insights into the mechanisms of oligomer formation and fibril breakage.1. A. Orte, R. Clarke, S. Balasubramanian, D. Klenerman, Anal.Chem.78, 7707 (2006).2. A. Orte et al., Proc. Natl. Acad. Sci. U. S. A.105, 14424 (2008).

Inherent Anti-amyloidogenic Activity of Human Immunoglobulin γ Heavy Chains

We have previously shown that a subpopulation of naturally occurring human IgGs were cross-reactive against conformational epitopes on pathologic aggregates of Abeta, a peptide that forms amyloid fibrils in the brains of patients with Alzheimer disease, inhibited amyloid fibril growth, and dissociated amyloid in vivo. Here, we describe similar anti-amyloidogenic activity that is a general property of free human Ig gamma heavy chains. A gamma(1) heavy chain, F1, had nanomolar binding to an amyloid fibril-related conformational epitope on synthetic oligomers and fibrils as well as on amyloid-laden tissue sections. F1 did not bind to native Abeta monomers, further indicating the conformational nature of its binding site. The inherent anti-amyloidogenic activity of Ig gamma heavy chains was demonstrated by nanomolar amyloid fibril and oligomer binding by polyclonal and monoclonal human heavy chains that were isolated from inert or weakly reactive antibodies. Most importantly, the F1 heavy chain prevented in vitro fibril growth and reduced in vivo soluble Abeta oligomer-induced impairment of rodent hippocampal long term potentiation, a cellular mechanism of learning and memory. These findings demonstrate that free human Ig gamma heavy chains comprise a novel class of molecules for developing potential therapeutics for Alzheimer disease and other amyloid disorders. Moreover, establishing the molecular basis for heavy chain-amyloidogenic conformer interactions should advance understanding on the types of interactions that these pathologic assemblies have with biological molecules.

Microglia Activated with the Toll-Like Receptor 9 Ligand CpG Attenuate Oligomeric Amyloid β Neurotoxicity in in Vitro and in Vivo Models of Alzheimer’s Disease

Soluble oligomeric amyloid beta (oAbeta) 1-42 causes synaptic dysfunction and neuronal injury in Alzheimer's disease (AD). Although accumulation of microglia around senile plaques is a hallmark of AD pathology, the role of microglia in oAbeta1-42 neurotoxicity is not fully understood. Here, we showed that oAbeta but not fibrillar Abeta was neurotoxic, and microglia activated with unmethylated DNA CpG motif (CpG), a ligand for Toll-like receptor 9, attenuated oAbeta1-42 neurotoxicity in primary neuron-microglia co-cultures. CpG enhanced microglial clearance of oAbeta1-42 and induced higher levels of the antioxidant enzyme heme oxygenase-1 in microglia without producing neurotoxic molecules such as nitric oxide and glutamate. Among subclasses of CpGs, class B and class C activated microglia to promote neuroprotection. Moreover, intracerebroventricular administration of CpG ameliorated both the cognitive impairments induced by oAbeta1-42 and the impairment of associative learning in Tg2576 mouse model of AD. We propose that CpG may be an effective therapeutic strategy for limiting oAbeta1-42 neurotoxicity in AD.

Preparation and Characterization of a Monoclonal Antibody with High Affinity for Soluble Aβ Oligomers

Amyloid beta-protein (Abeta) has been causally implicated in the neurodegenerative processes that accompany Alzheimer's disease. Soluble oligomers of the Abeta(1-42) fragment are thought to be significantly more neurotoxic than higher molecular weight aggregates. We report the isolation and characterization of a mouse monoclonal antibody (MAb) directed against soluble Abeta(1-42) oligomers. Synthetic Abeta(1-42) oligomers were assembled in vitro; these were used to immunize mice, and hybridomas were isolated following myeloma fusion of splenocytes from immunized animals. Screening for reactivity against Abeta(1-42) resulted in the identification of MAb A8 with high affinity for soluble oligomers. The isotype of A8 was found to be IgG(2b). Experiments using sub-peptides of Abeta(1-42) revealed that the epitope identified by A8 lies within the 1-6 region of Abeta. The antibody displays high affinity for soluble Abeta(1-42) oligomers in the molecular weight range of 16.5-25 kDa, and detected target antigen in brain sections from senescence-accelerated SAMP 8 mice. The sensitivity and optimal titers for the detection of soluble Abeta(1-42) oligomers were determined to be 0.625 microg/mL in indirect ELISA, and 1:10(6), 1:4000, and 1:150 for ELISA, Western blotting, and immunohistochemistry, respectively. The A8 antibody specific for soluble Abeta(1-42) oligomers will provide a valuable tool for Alzheimer's disease research.

Soluble fibrillar oligomer levels are elevated in Alzheimer's disease brain and correlate with cognitive dysfunction

Recent evidence has suggested a role for soluble oligomeric Aβ species in the pathology of Alzheimer's disease (AD). Fibrillar plaque deposits are present in non-demented individuals and levels of soluble Aβ correlate better with cognitive dysfunction in AD and transgenic mouse models. We have previously reported that there are at least two conformationally distinct types of Aβ oligomers: prefibrillar oligomers that are kinetic intermediates in fibril assembly reactions and are specifically recognized by A11 antibody and fibrillar oligomers that may represent fibril seeds or small pieces of fibrils and are recognized by a fibril specific antibody, OC. We have examined the levels of these two types of oligomers in the PBS soluble fraction of brain tissue from control cases, cases with senile degenerative changes (SDC) and AD patients. We found that the levels of soluble fibrillar oligomers detected by OC antibody are significantly elevated in multiple brain regions of AD patients. The elevated fibrillar oligomer levels were found not to be an artifact of tissue homogenization, nor a result of increased Aβ or APP levels. The concentration of fibrillar oligomers in adjacent brain regions of the same patient can vary widely and were not detected in post-mortem cerebrospinal fluid. In contrast, the level of prefibrillar oligomers are variable in both AD and age matched controls, indicating that they are not correlated with cognitive dysfunction and suggesting that they precede dementia in AD. Significant correlations were found between the levels of fibrillar oligomers and cognitive decline (MMSE scores) as well as the neuropathological hallmarks of AD. These results indicate that fibrillar oligomers may play a key role in the pathology of AD and may be a new target for diagnostic and therapeutic development.

Disruption of fast axonal transport is a pathogenic mechanism for intraneuronal amyloid beta

The pathological mechanism by which Abeta causes neuronal dysfunction and death remains largely unknown. Deficiencies in fast axonal transport (FAT) were suggested to play a crucial role in neuronal dysfunction and loss for a diverse set of dying back neuropathologies including Alzheimer's disease (AD), but the molecular basis for pathological changes in FAT were undetermined. Recent findings indicate that soluble intracellular oligomeric Abeta (oAbeta) species may play a critical role in AD pathology. Real-time analysis of vesicle mobility in isolated axoplasms perfused with oAbeta showed bidirectional axonal transport inhibition as a consequence of endogenous casein kinase 2 (CK2) activation. Conversely, neither unaggregated amyloid beta nor fibrillar amyloid beta affected FAT. Inhibition of FAT by oAbeta was prevented by two specific pharmacological inhibitors of CK2, as well as by competition with a CK2 substrate peptide. Furthermore, perfusion of axoplasms with active CK2 mimics the inhibitory effects of oAbeta on FAT. Both oAbeta and CK2 treatment of axoplasm led to increased phosphorylation of kinesin-1 light chains and subsequent release of kinesin from its cargoes. Therefore pharmacological modulation of CK2 activity may represent a promising target for therapeutic intervention in AD.

Aβ Oligomers Induce Neurotoxicity: Effects of HDACs and Nrf2 Modulation

The 40‐42 amino acid amyloid βpeptide (Aβ) contributes to one of the central neuropathologic features of Alzheimer's disease (AD). Recent evidence suggests that the oligomeric forms of Aβ may play a role in AD. The objective of the present study was to evaluate the morphological and neurochemical effects of oligomers in primary rat neuronal cells. Neuronal cells were exposed to different doses of soluble Aβ oligomers (0‐90 nM) and analyzed for multiple indices of neuronal homeostasis. Our results indicate that Aβ oligomers promote morphological alterations, neurotoxicity and oxidative stress. The effects of histone deacetylase inhibitors and modulators of the Nrf2 pathway were analyzed for their ability to inhibit the toxicity of Aβ oligomers. Our studies indicate a role for Aβ oligomers as mediators multiple aspects of AD pathogenesis and outline the effects of potentially novel therapeutic strategies as mediators of neuroprotection against Aβ oligomer toxicity.This work was supported by a grant from the Alzheimer's Association and NIA (P01AG005119) to Dr. Jeffrey Keller.

Stimulus pattern dependence of the Alzheimer's disease amyloid-β 42 peptide's inhibition of long term potentiation in mouse hippocampal slices

Increasing evidence has pointed to inhibition of Long Term Potentiation (LTP) by soluble Aβ42 oligomers as central in the etiology of the learning and memory deficits that are hallmarks of Alzheimer Disease. These effects are thought to occur by an interaction between Aβ42 and certain cellular effectors that induce LTP, however, the precise identity of the Aβ42-interactive signaling molecules is unknown. Identification of such effectors is made more difficult because LTP induced by different stimulation protocols can be expressed through heterogeneous signaling pathways. The aim of this study was to compare differences in the Aβ42-dependent levels of inhibition of LTPs that were induced using high frequency stimulation (HFS), versus theta burst stimulation (TBS). Our results show that untreated control brain slices tetanized with either HFS or TBS gave similar levels of LTP and post tetanic stimulation (PTP), suggesting that the response induced by either protocol was comparable. However, Aβ42 peptide significantly blocked LTP and PTP induced by HFS, but not when TBS was used. NMDA receptor antagonists, D-AP5 and ifenprodil, both blocked LTPs that were induced by HFS or TBS. We propose that unknown signaling effectors, other than the NMDA receptor, which are differentially involved in the induction of LTP by TBS, as compared to HFS, may be responsible for this resistance of TBS-induced LTP to Aβ42 dependent inhibition.

Protection of synapses against Alzheimer's-linked toxins: Insulin signaling prevents the pathogenic binding of Aβ oligomers

Synapse deterioration underlying severe memory loss in early Alzheimer's disease (AD) is thought to be caused by soluble amyloid beta (Abeta) oligomers. Mechanistically, soluble Abeta oligomers, also referred to as Abeta-derived diffusible ligands (ADDLs), act as highly specific pathogenic ligands, binding to sites localized at particular synapses. This binding triggers oxidative stress, loss of synaptic spines, and ectopic redistribution of receptors critical to plasticity and memory. We report here the existence of a protective mechanism that naturally shields synapses against ADDL-induced deterioration. Synapse pathology was investigated in mature cultures of hippocampal neurons. Before spine loss, ADDLs caused major downregulation of plasma membrane insulin receptors (IRs), via a mechanism sensitive to calcium calmodulin-dependent kinase II (CaMKII) and casein kinase II (CK2) inhibition. Most significantly, this loss of surface IRs, and ADDL-induced oxidative stress and synaptic spine deterioration, could be completely prevented by insulin. At submaximal insulin doses, protection was potentiated by rosiglitazone, an insulin-sensitizing drug used to treat type 2 diabetes. The mechanism of insulin protection entailed a marked reduction in pathogenic ADDL binding. Surprisingly, insulin failed to block ADDL binding when IR tyrosine kinase activity was inhibited; in fact, a significant increase in binding was caused by IR inhibition. The protective role of insulin thus derives from IR signaling-dependent downregulation of ADDL binding sites rather than ligand competition. The finding that synapse vulnerability to ADDLs can be mitigated by insulin suggests that bolstering brain insulin signaling, which can decline with aging and diabetes, could have significant potential to slow or deter AD pathogenesis.

Investigating the Efficacy of Peptide-based Inhibitors Against the Earliest Oligomers of Amyloid-β Peptide

Alzheimer's disease (AD) is linked to the self-association of amyloid-β peptide (Aβ), a protein of 39-43 amino acids that is normally soluble in the plasma and cerebrospinal fluid. Although large, fibrillar aggregates were long thought to be the pathogenic agents, recent evidence indicates that soluble Aβ oligomers are more closely linked to disease progression. In fact, negative effects have been observed from oligomers as small as dimers and trimers. A number of compounds have been found to inhibit the large-scale aggregation of Aβ in bulk solution, typically by manipulating the β-sheet structure characteristic of these assemblies, but little is known regarding inhibition of the earliest association steps.

Fibril formation of Aβ (10-35) studied by UV resonance Raman Spectroscopy

In Alzheimer's disease the major pathological feature observed is the progressive deposition of insoluble senile amyloid plaques within the cerebral cortex. The major component of these plaques is amyloid beta (Aβ), a 39-43 residue peptide. This peptide can be present as monomers with soluble disordered structure, which change conformation into partially folded intermediates and further assemble to form dimers, trimers, oligomers, protofibrils and larger fibrils. Earlier studies have suggested that neurotoxicity of the disease lies not in the insoluble fibrils but in the formation of soluble oligomers, which impair and alter neurotransmision. Thus, the solution conformation of Aβ is of significant interest for understanding the molecular mechanism of Aβ fibrillogenesis. In the present study we have investigated the process of fibril formation in the 25 residue Aβ(10-35) which has a similar fibril structure as Aβ40 and Aβ42. In addition, Aβ(10-35) contains the central hydrophobic cluster (residues 17-21) suspected to initiate folding.We have employed fluorescence, circular dichroism (CD) and time-dependent UVRR spectroscopic methods to explore the fibril formation process in vitro. Thioflavin-T induced fluorescence and CD studies indicate a conformational transition from an unfolded to β-sheet structure. UVRR studies conducted with 215 nm and 230 nm excitation facilitate characterization of the two phenylalanine and one tyrosine residues in the peptide, respectively. It is observed that formation of soluble Aβ oligomers is accompanied by a decrease in the intensities of the Phe vibrational mode, v8a (1606 cm-1) and the Tyr vibrational mode, v8a (1617 cm-1) over a 150 min incubation period. These results indicate that fibril formation proceeds via phenylalanine and tyrosine π-stacking interactions, which stabilize parallel β-sheets and reduce solvent exposure. Based on these results we have proposed a probable mechanism of fibril formation in Aβ(10-35).

Is Covalently Crosslinked A&amp;#946; Responsible for Synaptotoxicity in Alzheimers Disease?

Alzheimer's disease (AD) is the most common form of dementia in the elderly, and is characterized by the deposition of extracellular amyloid plaques primarily composed of the beta-amyloid peptide (Abeta). While these plaques define the pathology of AD, disease progression has been shown to correlate more closely with the level of synaptotoxicity induced by soluble Abeta oligomers. Recent evidence suggests that these oligomers are covalently crosslinked, possibly due to the interaction of Abeta with redox-active metal ions. These findings offer new avenues for the treatment and prevention of disease, by modulating metal binding or preventing the formation of neurotoxic Abeta oligomers. An understanding of the chemical nature of Abeta is also required to elucidate the synaptotoxic process or processes in AD, which have so far resisted explanation.