Despite intense interests in developing blood measurements of Alzheimer’s disease (AD), the progress has been confounded by limited sensitivity and poor correlation to brain pathology. Researchers from the National University of Singapore present a dedicated analytical platform for measuring different populations of circulating amyloid β (Aβ) proteins – exosome-bound vs. unbound – directly from blood. The technology, termed amplified plasmonic exosome (APEX), leverages in situ enzymatic conversion of localized optical deposits and double-layered plasmonic nanostructures to enable sensitive, multiplexed population analysis. It demonstrates superior sensitivity (~200 exosomes), and enables diverse target co-localization in exosomes. Employing the platform, the researchers find that prefibrillar Aβ aggregates preferentially bind with exosomes. They thus define a population of Aβ as exosome-bound (Aβ42+ CD63+) and measure its abundance directly from AD and control blood samples. As compared to the unbound or total circulating Aβ, the exosome-bound Aβ measurement could better reflect PET imaging of brain amyloid plaques and differentiate various clinical groups.
a Exosomes associate with Aβ proteins. Aβ proteins, the main component of amyloid plaques found in AD brain pathology, are released into the extracellular space. Exosomes are nanoscale extracellular membrane vesicles actively secreted by mammalian cells. Through their surface glycoproteins and glycolipids, exosomes can associate with the released Aβ proteins. b Transmission electron micrograph of exosome-bound Aβ. Exosomes derived from neuronal cells (SH-SY5Y) were treated with Aβ42 aggregates, and labeled with gold nanoparticles (10 nm) via a Aβ42-specific antibody. The nanoparticles appear as block dots (indicated by the red arrow). c APEX assay schematics. To enable sensitive profiling at the nanoscale, exosomes are first immuno-captured onto a plasmonic nanosensor (before amplification). Through an in situ enzymatic amplification, insoluble optical deposits are locally formed on the sensor-bound exosomes (after amplification). This deposition is spatially defined for molecular co-localization analysis, and changes the refractive index for SPR signal enhancement. To complement the enzymatic amplification, the nanosensor is back illuminated (away from the enzyme activity) to achieve analytical stability. The deposition causes a resultant red shift in the transmitted light through the nanosensor. d A representative schematic of changes in the transmission spectra with APEX amplification. Specific exosome binding (before) and subsequent amplification profiling (after) were monitored as transmission spectral shifts (Δλ) by the APEX platform. a.u., arbitrary unit. e Exosome-bound Aβ was measured using the APEX platform, in blood samples of patients with Alzheimer’s disease (AD), mild cognitive impairment (MCI), and controls with no cognitive impairment (NCI). The blood measurements were correlated to corresponding PET imaging of brain amyloid plaque deposition. f A photograph of the APEX microarray. Each sensor chip contains 6 × 10 sensing elements, made up of uniformly fabricated plasmonic lattice, for multiplexed measurements. See Supplementary Fig. 3–6 for sensor fabrication, characterization and design optimization, respectively