Ultrasensitive Assays Detect Different Conformations of Plasma β Amyloids.

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 (Aβ) becomes possible. However, the changes in measured plasma Aβ concentrations in Alzheimer's disease (AD) compared to cognitively unimpaired subjects (CU) are inconsistent. A possible reason for the inconsistency regarding various conformations of Aβ in plasma is explored in this study. Three samples with equal amounts of Aβ but different proportions of monomers and oligomers of Aβ were prepared. The Aβ 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 Aβ concentrations. The three diluted samples showed similar levels of Aβ 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 Aβ. The measured levels of Aβ are independent of the proportions of monomer or oligomer Aβ but depend on the total amounts of Aβ. In the case of SIMOA, monomers of Aβ are the primary target measured. By comparing Aβ concentrations of the plasma using IMR and SIMOA, the significant difference in plasma Aβ levels using IMR in AD compared to CU is mainly due to the formations of oligomeric Aβ. Therefore, if the target molecules are monomers of Aβ, 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 Aβ need to be elucidated.