Characterization of the Tau Interactome in Human Brain Reveals Isoform-Dependent Interaction with 14-3-3 Family Proteins.

Despite exhibiting tau phosphorylation similar to Alzheimer disease, the human fetal brain is remarkably resilient to tau aggregation and toxicity. To identify potential mechanisms for this resilience, we used co-immunoprecipitation with mass spectrometry to characterize the tau interactome in human fetal, adult, and Alzheimer disease brains. We found significant differences between the tau interactome in fetal and AD brain tissue, with little difference between adult and AD, although these findings are limited by the low throughput and small sample size of these experiments. Differentially interacting proteins were enriched for 14-3-3 domains, and we found that the 14-3-3-β, η, and γ isoforms interacted with phosphorylated tau in Alzheimer disease but not the fetal brain. Since long isoform (4R) tau is only seen in the adult brain and this is one of the major differences between fetal and AD tau, we tested the ability of our strongest hit (14-3-3-β) to interact with 3R and 4R tau using co-immunoprecipitation, mass photometry, and nuclear magnetic resonance (NMR). We found that 14-3-3-β interacts preferentially with phosphorylated 4R tau, forming a complex consisting of two 14-3-3-β molecules to one tau. By NMR, we mapped 14-3-3 binding regions on tau that span the second microtubule binding repeat, which is unique to 4R tau. Our findings suggest that there are isoform-driven differences between the phospho-tau interactome in fetal and Alzheimer disease brain, including differences in interaction with the critical 14-3-3 family of protein chaperones, which may explain, in part, the resilience of fetal brain to tau toxicity. Significance Statement Aggregation of phosphorylated tau is the final common pathway for neuronal death across multiple neurodegenerative diseases, but the mechanisms that trigger this process remain unclear. The fetal brain shows high levels of potentially toxic phosphorylated tau without apparent adverse effects, and therefore represents an unprecedented, and heretofore unexplored, opportunity to identify age-related mechanisms for tau toxicity. Our results use a novel data set mapping the tau interactome in human fetal, adult, and AD brain to identify splicing-dependent changes in tau interaction with 14-3-3 family chaperone proteins, which represent highly plausible protective mechanisms in the fetal brain.