Super-Resolution Microscopy of Cerebrospinal Fluid Biomarkers as a Tool for Alzheimer's Disease Diagnostics

Alzheimer's disease (AD) is neuropathologically characterized by aggregates of amyloid-beta peptides (A beta) and tau proteins. The consensus in the AD field is that A beta and tau should serve as diagnostic biomarkers for AD. However, their aggregates have been difficult to investigate by conventional fluorescence microscopy, since their size is below the diffraction limit (similar to 200 nm). To solve this, we turned to a super-resolution imaging technique, stimulated emission depletion (STED) microscopy, which has a high enough precision to allow the discrimination of low-and high-molecular weight aggregates prepared in vitro. We used STED to analyze the structural organization of A beta and tau in cerebrospinal fluid (CSF) from 36 AD patients, 11 patients with mild cognitive impairment (MCI), and 21 controls. We measured the numbers of aggregates in the CSF samples, and the aggregate sizes and intensities. These parameters enabled us to distinguish AD patients from controls with a specificity of similar to 87% and a sensitivity of similar to 79%. In addition, the aggregate parameters determined with STED microscopy correlated with the severity of cognitive impairment in AD patients. Finally, these parameters may be useful as predictive tools for MCI cases. The STED parameters of two MCI patients who developed AD during the course of the study, as well as of MCI patients whose A beta ELISA values fall within the accepted range for AD, placed them close to the AD averages. We suggest that super-resolution imaging is a promising tool for AD diagnostics.