Tau hyperphosphorylation impairs cooperative binding to microtubules and perturbs organelle trafficking in neurons

Tau, a neuronal microtubule-associated protein (MAP), organizes the axonal cytoskeleton and regulates intracellular transport. Tau hyperphosphorylation is closely linked to neurodegeneration in tauopathies including Alzheimer’s disease. Tau binds microtubules cooperatively, forming envelopes that restrict access to the microtubule lattice and regulate the activity of motor proteins and other MAPs. However, the effect of disease-related perturbations on tau dynamics and function remains unclear. Using tau phosphomutants in vitro and in live neurons, we show that tau hyperphosphorylation disrupts cooperative binding and dysregulates lysosome transport. Hyperphosphorylated tau does not form envelopes, distributes more uniformly along the axon, and dissociates faster from microtubules. Tau mildly inhibits KIF5C motility, but strongly inhibits KIF1A. Hyperphosphorylation reduces KIF5C inhibition but increases KIF1A inhibition by decreasing processivity and accelerating detachment. Consistent with these effects, hyperphosphorylated tau alters lysosome transport in neurons. While phospho-resistant tau inhibits processive lysosome motility, hyperphosphorylated tau weakens tau-mediated regulation of lysosome transport– mimicking tau knockout neurons that exhibit enhanced processivity. Altogether, hyperphosphorylation disrupts tau envelopes and impairs lysosome trafficking, contributing to defects in degradative pathways that drive neurodegeneration.