Sensitive ELISA detection of amyloid-beta protofibrils in biological samples

Sensitive detection of Aβ protofibrils by proximity ligation--relevance for Alzheimer's disease

Background

Protein aggregation plays important roles in several neurodegenerative disorders. For instance, insoluble aggregates of phosphorylated tau and of Aβ peptides are cornerstones in the pathology of Alzheimer's disease. Soluble protein aggregates are therefore potential diagnostic and prognostic biomarkers for their cognate disorders. Detection of the aggregated species requires sensitive tools that efficiently discriminate them from monomers of the same proteins. Here we have established a proximity ligation assay (PLA) for specific and sensitive detection of Aβ protofibrils via simultaneous recognition of three identical determinants present in the aggregates. PLA is a versatile technology in which the requirement for multiple target recognitions is combined with the ability to translate signals from detected target molecules to amplifiable DNA strands, providing very high specificity and sensitivity.

Results

For specific detection of Aβ protofibrils we have used a monoclonal antibody, mAb158, selective for Aβ protofibrils in a modified PLA, where the same monoclonal antibody was used for the three classes of affinity reagents required in the assay. These reagents were used for detection of soluble Aβ aggregates in solid-phase reactions, allowing detection of just 0.1 pg/ml Aβ protofibrils, and with a dynamic range greater than six orders of magnitude. Compared to a sandwich ELISA setup of the same antibody the PLA increases the sensitivity of the Aβ protofibril detection by up to 25-fold. The assay was used to measure soluble Aβ aggregates in brain homogenates from mice transgenic for a human allele predisposing to Aβ aggregation.

Conclusions

The proximity ligation assay is a versatile analytical technology for proteins, which can provide highly sensitive and specific detection of Aβ aggregates - and by implication other protein aggregates of relevance in Alzheimer's disease and other neurodegenerative disorders.

Stabilization of neurotoxic Alzheimer amyloid-beta oligomers by protein engineering

Soluble oligomeric aggregates of the amyloid-beta peptide (Abeta) have been implicated in the pathogenesis of Alzheimer's disease (AD). Although the conformation adopted by Abeta within these aggregates is not known, a beta-hairpin conformation is known to be accessible to monomeric Abeta. Here we show that this beta-hairpin is a building block of toxic Abeta oligomers by engineering a double-cysteine mutant (called Abetacc) in which the beta-hairpin is stabilized by an intramolecular disulfide bond. Abeta(40)cc and Abeta(42)cc both spontaneously form stable oligomeric species with distinct molecular weights and secondary-structure content, but both are unable to convert into amyloid fibrils. Biochemical and biophysical experiments and assays with conformation-specific antibodies used to detect Abeta aggregates in vivo indicate that the wild-type oligomer structure is preserved and stabilized in Abetacc oligomers. Stable oligomers are expected to become highly toxic and, accordingly, we find that beta-sheet-containing Abeta(42)cc oligomers or protofibrillar species formed by these oligomers are 50 times more potent inducers of neuronal apoptosis than amyloid fibrils or samples of monomeric wild-type Abeta(42), in which toxic aggregates are only transiently formed. The possibility of obtaining completely stable and physiologically relevant neurotoxic Abeta oligomer preparations will facilitate studies of their structure and role in the pathogenesis of AD. For example, here we show how kinetic partitioning into different aggregation pathways can explain why Abeta(42) is more toxic than the shorter Abeta(40), and why certain inherited mutations are linked to protofibril formation and early-onset AD.

High-molecular-weight beta-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.

A specific enzyme-linked immunosorbent assay for measuring beta-amyloid protein oligomers in human plasma and brain tissue of patients with Alzheimer disease

OBJECTIVE

To examine in vivo levels of beta-amyloid (Abeta) oligomers (oAbeta) vs monomeric Abeta in plasma and brain tissue of patients with sporadic and familial Alzheimer disease (AD) using a new enzyme-linked immunosorbent assay (ELISA) specific for oAbeta.

DESIGN

To establish the oAbeta ELISA, the same N-terminal Abeta antibody was used for antigen capture and detection. Plasma and postmortem brain tissue from patients with AD and control subjects were systematically analyzed by conventional monomeric Abeta and new oAbeta ELISAs.

SUBJECTS

We measured oAbeta species in plasma samples from 36 patients with clinically well-characterized AD and 10 control subjects. In addition, postmortem samples were obtained from brain autopsies of 9 patients with verified AD and 7 control subjects.

MAIN OUTCOME MEASURES

Oligomeric Abeta and 4 monomeric Abeta species in plasma samples from patients with AD and control subjects were measured by ELISA.

RESULTS

The specificity of the oAbeta ELISA was validated with a disulfide-crossed-linked, synthetic Abeta(1-40)Ser26Cys dimer that was specifically detected before but not after the dissociation of the dimers in beta-mercaptoethanol. Plasma assays showed that relative oAbeta levels were closely associated with relative Abeta(42) monomer levels across all of the subjects. Analysis of sequential plasma samples from a subset of the patients with AD, including a patient with AD caused by a presenilin mutation, revealed decreases in both oAbeta and Abeta(42) monomer levels over a 1- to 2-year period. In brain tissue from 9 patients with AD and 7 control subjects, both oAbeta and monomeric Abeta(42) levels were consistently higher in the AD cases.

CONCLUSIONS

An oAbeta-specific ELISA reveals a tight link between oAbeta and Abeta(42) monomer levels in plasma and brain. Both forms can decline over time in plasma, presumably reflecting their increasing insolubility in the brain.

Amyloid-beta protofibril levels correlate with spatial learning in Arctic Alzheimer's disease transgenic mice

Oligomeric assemblies of amyloid-beta (Abeta) are suggested to be central in the pathogenesis of Alzheimer's disease because levels of soluble Abeta correlate much better with the extent of cognitive dysfunctions than do senile plaque counts. Moreover, such Abeta species have been shown to be neurotoxic, to interfere with learned behavior and to inhibit the maintenance of hippocampal long-term potentiation. The tg-ArcSwe model (i.e. transgenic mice with the Arctic and Swedish Alzheimer mutations) expresses elevated levels of Abeta protofibrils in the brain, making tg-ArcSwe a highly suitable model for investigating the pathogenic role of these Abeta assemblies. In the present study, we estimated Abeta protofibril levels in the brain and cerebrospinal fluid of tg-ArcSwe mice, and also assessed their role with respect to cognitive functions. Protofibril levels, specifically measured with a sandwich ELISA, were found to be elevated in young tg-ArcSwe mice compared to several transgenic models lacking the Arctic mutation. In aged tg-ArcSwe mice with considerable plaque deposition, Abeta protofibrils were approximately 50% higher than in younger mice, whereas levels of total Abeta were exponentially increased. Young tg-ArcSwe mice showed deficits in spatial learning, and individual performances in the Morris water maze were correlated inversely with levels of Abeta protofibrils, but not with total Abeta levels. We conclude that Abeta protofibrils accumulate in an age-dependent manner in tg-ArcSwe mice, although to a far lesser extent than total Abeta. Our findings suggest that increased levels of Abeta protofibrils could result in spatial learning impairment.

Maximizing the potential of plasma amyloid-beta as a diagnostic biomarker for Alzheimer's disease

Amyloid plaques are composed primarily of amyloid-beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) and are considered to play a pivotal role in Alzheimer's disease (AD) pathogenesis. Presently, AD is diagnosed after the onset of clinical manifestations. With the arrival of novel therapeutic agents for treatment of AD, there is an urgent need for biomarkers to detect early stages of AD. Measurement of plasma Abeta has been suggested as an inexpensive and non-invasive tool to diagnose AD and to monitor Abeta modifying therapies. However, the majority of cross-sectional studies on plasma Abeta levels in humans have not shown differences between individuals with AD compared to controls. Similarly, cross-sectional studies of mouse plasma Abeta have yielded inconsistent trends in different mouse models. However, longitudinal studies appear to be more promising in humans. Recently, efforts to modify plasma Abeta levels using modulators have shown some promise. In this review, we will summarize the present data on plasma Abeta in humans and mouse models of AD. We will discuss the potential of modulators of Abeta levels in plasma, including antibodies and insulin, and the challenges associated with measuring plasma Abeta. Modulators of plasma Abeta may provide an important tool to optimize plasma Abeta levels and may improve the diagnostic potential of this approach.