Tau Mutants Alter the Rheological Properties of Phase‐Separated Tau Condensates

Tau has been implicated in many neurogenerative diseases called "tauopathies" where tau self-assembles into neurofibrillary tangles. Alzheimer's disease, the most common tauopathy, affects over 50 million people causing a dire need for therapeutics to be generated. However, the cellular mechanism of Alzheimer's disease and related tauopathies remains poorly understood. Tau has recently been discovered to phase separate into liquid-like droplets, or condensates, a phenomenon with growing significance in biology with the potential to inspire new therapeutic avenues. To gain insight into the putative role of phase separation in tau pathology, here we quantify the impact of pathological tau mutations on the phase behavior and material properties of tau condensates. We combine DIC and confocal microscopy with particle tracking microrheology measurements in order to characterize droplet morphology and quantify droplet viscoelasticity, respectively. We find that WT full-length tau readily phase separates with polyU RNA to form condensed viscous fluids. Interestingly, while some pathological mutations have a pronounced effect on droplet viscosity and droplet aging dynamics, others display no significant effect on droplet properties. These results demonstrate an important, albeit complex relationship between tau phase separation, aggregation and pathology. Continued parsing of the complex landscape of tau assembly states, coupled with genetic and cellular analysis has the potential to reveal the roots of tau pathology and its ultimate therapeutic targeting.