Cellular Uptake\nof Tau Aggregates Triggers Disulfide\nBond Formation in Four-Repeat Tau Monomers

Oxidative stress\nis an important driver of aging and has been linked\nto numerous neurodegenerative disorders, including Alzheimer’s\ndisease. A key pathological hallmark of Alzheimer’s are filamentous\ninclusions made of the microtubule associated protein Tau. Based on\nalternative splicing, Tau protein can feature either three or four\nmicrotubule binding repeats. Distinctively, three-repeat Tau contains\na single cysteine; four-repeat Tau contains two. Although there is\nevidence that the cysteines in pathological Tau filaments exist in\nthe reduced form, very little is known about the alternative disulfide-bonded\nstate. It is unclear whether it can exist nontransiently in the reducing\nenvironment of the cytosol. Such knowledge, however, is important\nas different redox states of Tau could modulate aggregation. To address\nthis question, we transfected HEK293 cells expressing the P301S variant\nof four-repeat Tau with fibril seeds composed of compact, disulfide-bonded\nTau monomers. In vitro, these fibrils are observed to recruit only\ncompact Tau, but not Tau in which the cysteines are reduced or replaced\nby alanines or serines. In line with this characteristic, the fibrils\ndissociate when treated with a reducing agent. When offered to HEK293\ncells, variant Tau protein is recruited to the seeds forming intracellular\nfibrils with the same seeding properties as the in vitro counterparts.\nMarkedly, the proteins in these fibrils have a compact, disulfide-bonded\nconfiguration and dissociate upon reduction. These findings reveal\nthat uptake of exogeneous fibril seeds triggers oxidation of Tau monomers,\nmodulating intracellular aggregation.