Phosphorylation of tau at Specific Sites Regulates Axonal Transport and Neurite Outgrowth

Despite the importance of fast axonal transport (FAT) for neuron development and maintenance, mechanisms for physiological regulation of FAT and cargo delivery at the correct location remain an unresolved issue. Achieving the high specificity of cargo delivery necessary for correct neuronal function is likely to involve many intersecting and counteracting mechanisms. Tau is a neuronal microtubule-associated protein which aggregates in Alzheimer’s disease and other tauopathies. Tau is often described as a microtubule-stabilizing protein, but multiple lines of evidence suggest that this is not its primary function. In evaluating tau function, studies in isolated squid axoplasm found that aggregates of tau, or truncated tau monomers that expose the N-terminal, selectively inhibit anterograde fast axonal transport (FAT) through a mechanism dependent on amino acids 2-18 which comprise a phosphatase activating domain (PAD). When exposed, this domain activates a PP1/GSK3β signaling cascade causing release of vesicles from kinesin motor protein. While tau has a dynamic secondary structure, unphosphorylated monomers adopt a “paperclip” conformation concealing the N-terminal, but tau is subject to post-translational modifications such as phosphorylation that can stabilize more extended structures. Here we provide the first evidence that defined phosphorylation of tau at specific residues is sufficient to activate one of two distinct signaling pathways. Pseudophosphorylation at Thr205 caused exposure of the PAD and activation of the previously identified PP1/GSK3β signaling cascade, resulting in inhibition of anterograde FAT. Pseudophosphorylation at Ser199, as well as select dual phosphorylation epitopes, caused inhibition of anterograde and retrograde FAT through activation of pJNK. Using primary hippocampal neurons, we showed that the N-terminal tau PAD is exposed during development, co-localized with pT205 tau and active GSK3β at sites of cargo delivery. Moreover presence of the PAD and T205 phosphorylation were essential for normal outgrowth of hippocampal neurons. These results indicate that tau can act as a scaffolding protein for signaling pathways in the axon. This function is regulated by phosphorylation of defined residues, leading to downstream regulation of FAT. Furthermore, physiological regulation of tau phosphorylation and PAD exposure are important for neuronal development.