The Proteoform Landscape of Tau from the Human Brain.

Tau is a microtubule-associated protein (MAP) and is critical for maintaining the cytoskeleton of neurons. Tau and its post-translational modifications (PTMs) have been studied for decades, yet the exact composition of intact tau and its truncation products present in the human brain has evaded study at the proteoform level. Here, we show that tau proteoform profiling and exact characterization are possible using immunoprecipitation (IP) and the new approach of individual ion mass spectrometry (I2MS). We provide a first glimpse of the tau proteoform landscape present in the CHAPS-soluble extracts from the temporal cortex of a control subject and a donor with Alzheimer's disease (AD). Profiling and identification of four isoforms (0N3R, 1N3R, 0N4R, and 1N4R), truncated products (e.g., 72-172 derived from the 0N3R/0N4R isoforms), and intact tau proteoforms harboring PTMs include phosphorylation, methylation, and acetylation. The specific tau proteoform identification typically employs proton transfer charge reduction (PTCR) and electron transfer dissociation (ETD) with spectral readout by individual ion tandem mass spectrometry (I2MS2). A precise understanding of the tau proteoform landscape over the course of neurodegeneration is critical to understand AD pathology vs related dementias. The assay approach reported here will advance AD research, gives a sense of what is technologically possible for new biomarker discovery and will assist the development of therapeutics using the most exact kind of compositional information on tau.