Evaluation of SP3 for antibody-free quantification of tau in CSF mimic and brain by mass spectrometry

Tubulin-associated unit (tau) has an important role in the pathogenesis and the diagnosis of Alzheimer's disease (AD) and other tauopathies. In view of the diversity of tau proteoforms, antibody-free methods represent a good approach for unbiased quantification. We adapted and evaluated the single-pot, solid-phase-enhanced sample-preparation (SP3) protocol for antibody-free extraction of the tau protein in cerebro-spinal fluid (CSF) mimic and in human brain. A total of 13 non-modified peptides were quantified by high-resolution mass spectrometry (HRMS) after digestion of tau by trypsin. We significantly improved the basic SP3 protocol by carefully optimizing the organic solvents and incubation time for tau binding, as well as the digestion step for the release directly from the SP3 beads of the 13 tau peptides. These optimizations proved to be primarily beneficial for the most hydrophilic tau peptides, increasing the sequence coverage of recombinant tau. Mean recovery in CSF mimic of the 13 non-modified peptides was of 53%, with LODs ranging from 0.75 to 10 ng/mL. Next, we tested the optimized SP3 protocol on pathological tau extracted from the soluble fraction from an AD brain sample (middle frontal gyrus). We could successfully identify and quantify biologically relevant tau peptides including representative peptides of two isoforms and two phospho-peptides (pTau217 and pTau181).