# Ultrasensitive Assays Detect Different Conformations of Plasma β Amyloids

## TL;DR

This study shows that different blood tests detect different forms of a protein linked to Alzheimer's disease, which may explain conflicting results in measuring its levels.

## Contribution

The study reveals that IMR and SIMOA assays detect different Aβ1–42 conformations, explaining inconsistent plasma Aβ1–42 measurements in Alzheimer's.

## Key findings

- IMR detects both monomers and oligomers of Aβ1–42, while SIMOA primarily detects monomers.
- Differences in plasma Aβ1–42 levels between Alzheimer's and healthy individuals using IMR are due to oligomer formation.
- The choice of assay depends on whether monomers or all Aβ1–42 conformations are the target.

## Abstract

With the developments
of ultrasensitive technologies
such as immunomagnetic
reduction (IMR) assay, single molecule array (SIMOA) assay, electrochemiluminescence
immunoassay (ECLIA), the assay of blood-based amyloid 1–42
(Aβ1–42) becomes possible. However, the changes
in measured plasma Aβ1–42 concentrations in
Alzheimer’s disease (AD) compared to cognitively unimpaired
subjects (CU) are inconsistent. A possible reason for the inconsistency
regarding various conformations of Aβ1–42 in
plasma is explored in this study. Three samples with equal amounts
of Aβ1–42 but different proportions of monomers
and oligomers of Aβ1–42 were prepared. The
Aβ1–42 composition of monomers and oligomers
in samples was analyzed with Western blot. Identically diluted versions
of these three samples were assayed with IMR and SIMOA for Aβ1–42 concentrations. The three diluted samples showed
similar levels of Aβ1–42 assayed with IMR,
whereas much lower levels for samples with more oligomers assayed
with SIOMA. The results imply that IMR detects both monomers and oligomers
of Aβ1–42. The measured levels of Aβ1–42 are independent of the proportions of monomer or
oligomer Aβ1–42 but depend on the total amounts
of Aβ1–42. In the case of SIMOA, monomers
of Aβ1–42 are the primary target measured.
By comparing Aβ1–42 concentrations of the
plasma using IMR and SIMOA, the significant difference in plasma Aβ1–42 levels using IMR in AD compared to CU is mainly
due to the formations of oligomeric Aβ1–42. Therefore, if the target molecules are monomers of Aβ1–42, SIMOA is the method of choice. Still, if the target
molecules should include monomers, small and large oligomers, IMR
would be an optimal consideration. In the future, the clinical implications
of the proportion of oligomeric Aβ1–42 need
to be elucidated.