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.