Abstract
Proteins exist as diverse proteoforms resulting from a combination of genetic variation, alternative splicing, and post-translational modifications. Current methods struggle to capture this complexity at the single-molecule level. Here we introduce Iterative Ma pping of P roteoforms (IMaP), a method that enables the massively-parallel interrogation of millions to billions of single-protein molecules through iterative probing with fluorescently labeled antibodies. Using 12 site-specific antibodies, the method is capable of measuring 2 (12) (4,096) potential proteoform groups. We used IMaP to measure proteoform group profiles of the tau protein, a key player in neurodegenerative diseases, using two pan anti-tau antibodies (Tau-13, Tau-216), three isoform-specific antibodies (Anti-0N, Anti-2N, Anti-4R), and seven phosphosite-specific antibodies (Anti-pT181, Anti-pS202+pT205, Anti-pT205, Anti-pS214, Anti-pT217, Anti-pT231, and Anti-pS396). The method demonstrates high sensitivity (detecting proteoforms at 0.1% abundance), high reproducibility (median CV <5.5%), and broad dynamic range (>3 orders of magnitude), outperforming conventional techniques in resolving closely related proteoform groups. We demonstrated that the method can be used on relevant biological samples by examining various neuronal models (iNeuron cells, organoids, MiBrains, and mouse brains) and human samples. This examination revealed 130 distinct tau proteoform groups with as many as six phosphorylation events. The non-random distribution of these phosphorylation events suggests ordered and site-specific modification processes rather than random, stochastic accumulation. Certain combinations of phosphorylation events were more abundant than others; for example, pT217 preferentially co-occurred with pT181. In validating the applicability of the assay to human disease samples, we noted a specific pattern of multiple phosphorylation events in an advanced Alzheimer's disease patient that suggests a sequential pathway of pathological tau modification. Iterative Mapping of Proteoforms provides insights into proteoform complexity at the single-molecule level, with significant implications for understanding protein regulation in neurodegenerative diseases and beyond.