Low Micromolar Zinc Accelerates the Fibrillization of Human Tau via Bridging of Cys-291 and Cys-322

A hallmark of a group of neurodegenerative diseases such as Alzheimer disease is the formation of neurofibrillary tangles, which are principally composed of bundles of filaments formed by microtubule-associated protein Tau. Clarifying how natively unstructured Tau protein forms abnormal aggregates is of central importance for elucidating the etiology of these diseases. There is considerable evidence showing that zinc, as an essential element that is highly concentrated in brain, is linked to the development or progression of these diseases. Herein, by using recombinant human Tau fragment Tau_244–372 and its mutants, we have investigated the effect of zinc on the aggregation of Tau. Low micromolar concentrations of Zn²⁺ dramatically accelerate fibril formation of wild-type Tau_244–372 under reducing conditions, compared with no Zn²⁺. Higher concentrations of Zn²⁺, however, induce wild-type Tau_244–372 to form granular aggregates in reducing conditions. Moreover, these non-fibrillar aggregates assemble into mature Tau filaments when Zn²⁺ has been chelated by EDTA. Unlike wild-type Tau_244–372, low micromolar concentrations of Zn²⁺ have no obvious effects on fibrillization kinetics of single mutants C291A and C322A and double mutant C291A/C322A under reducing conditions. The results from isothermal titration calorimetry show that one Zn²⁺ binds to one Tau molecule via tetrahedral coordination to Cys-291 and Cys-322 as well as two histidines, with moderate, micromolar affinity. Our data demonstrate that low micromolar zinc accelerates the fibrillization of human Tau protein via bridging Cys-291 and Cys-322 in physiological reducing conditions, providing clues to understanding the relationship between zinc dyshomeostasis and the etiology of neurodegenerative diseases.