Tau Axonal Sorting and Interaction With Synaptic Plasticity Modulators Is Domain‐ and Isoform‐Dependent in Human <scp>iPSC</scp> ‐Derived Neurons

Somatodendritic missorting of the axonal microtubule-associated protein Tau is an early hallmark of Alzheimer's disease (AD) and other tauopathies. Tau missorting causes synaptic loss and neuronal dysfunction, but the mechanisms underlying both normal axonal sorting and pathological missorting remain unclear. The six human brain Tau isoforms show different axodendritic distribution, but the Tau domains governing intracellular sorting and essential interactors are unknown. Here, we aimed to identify domains or motifs of human Tau and cellular binding partners required for efficient axonal Tau sorting and to unravel isoform-specific Tau interactors. Using human MAPT-KO induced pluripotent stem cell (iPSC)-derived glutamatergic neurons, we analyzed the sorting behavior of more than 20 truncation- or phosphorylation-mutant Tau constructs and used TurboID-based proximity labeling and proteomics to identify sorting- and isoform-specific Tau interactors. We found that efficient axonal Tau sorting was independent of the N-terminal tail, the C-terminal repeat domains, AD-associated phosphorylation, and the general microtubule affinity of Tau, but it requires the presence of the proline-rich region 2 (PRR2). Our interactome data revealed peroxisomal accumulation of the Tau N-terminal half, while axonal Tau interacted with the PP2A activator HSP110. Further, we found 0N4R-specific interactions of Tau with regulators of presynaptic exocytosis and postsynaptic plasticity, which are partially associated with AD pathogenesis, including members of the CDC42 pathway and the RAB11 proteins, while 0N3R-Tau bound to various cytoskeletal elements. In sum, our study i) postulates that axonal Tau sorting relies on the PRR2 domain but not on microtubule affinity and ii) unravels a potential isoform-specific role in synaptic function and AD-related dysfunction.