Protocol for efficient purification of endotoxin-free human recombinant tau protein

Microglia, the brain-resident immune cells, can propagate tau protein in a prion-like manner in neurodegenerative diseases called tauopathies. Here, we present an efficient protocol for the purification of endotoxin-free human recombinant tau protein. We detail steps for tau expression in E. coli , followed by on-column Triton X-114-based endotoxin depletion during purification, concentration, residual endotoxin removal, and quality control of the final product. We then describe a workflow for heparin-induced in vitro aggregation of purified tau into preformed fibrils. This protocol enables an accurate investigation of the microglial response to tau without the confounding effect of residual endotoxin. For complete details on the use and execution of this protocol, please refer to Karabova et al. 1