Beyond quantification of very low Aβ concentrations, the assay provides additional hints about the size of Aβ oligomers, which could be important for diagnosis and a mechanistic understanding of disease progression. The work, published earlier this year (Haes et al., 2005), was described in a talk by Van Duyne’s graduate student Amanda Haes at the meeting of the American Chemical Society last week.

The method is based on localized surface plasmon resonance, where protein binding to triangular silver nanoparticles produces a shift in their fluorescence spectra that can be measured by UV-visible spectrometry. In this case, the silver particles are studded with anti-Aβ antibodies, and binding of Aβ produces a wavelength shift proportional to the mass bound. The signal is amplified by binding of a second Aβ-specific antibody in a sandwich assay configuration, resulting in a lower detection limit in the 10 picomolar range.

While Haes’s nanodetector is not quite as sensitive as the previously described bio-barcode assay for ADDLs (see ARF related news story), the sandwich assay provides new information about binding affinity and the size of the ADDLs that the bio-barcode does not. The dose curves constructed with synthetic ADDLs demonstrate several distinct binding affinities and differences in the mass of amyloid bound that reflect size heterogeneity of the ADDLs. The capability of detecting different ADDL oligomerization states at low concentrations of Aβ means that the nanodetector could be one of the best techniques for screening drugs that block oligomerization, the authors write.

Once the detector was optimized with synthetic ADDLs, Haes and colleagues moved on to test brain and CSF samples from AD patients or normal aged people. In both cases, ADDLs were picked up in samples from AD patients but not controls. Though preliminary (the experiments compared just one each of AD and normal CSF), the results raise hope that further development could eventually lead to a diagnostic test using serum, Haes said. In addition, the ADDLs detected in CSF seemed somewhat larger than synthetic ADDLs, suggesting that the testing of disease tissues with the nanosensor could answer outstanding questions about the role of amyloid oligomerization in producing AD symptoms.

This first demonstration that a nano-scale antibody-based sandwich assay can be applied successfully to biological samples opens up possibilities for diagnosis and mechanistic studies far beyond Aβ, Haes said. This versatile system can potentially be adapted to track any rare biomarker simply by changing the antibodies. With several companies demonstrating interest in the technology, Haes believes that within a few years researchers will know if the technique will be useful for experimental or clinical studies.—Pat McCaffrey.

Reference:
Haes AJ, Chang L, Klein WL, Van Duyne RP. Detection of a biomarker for Alzheimer's disease from synthetic and clinical samples using a nanoscale optical biosensor. J Am Chem Soc. 2005 Feb 23;127(7):2264-71. Abstract