A complex mechanism for inducer mediated tau polymerization

The accumulation of polymers of the microtubule associated protein tau is correlative with increased neurodegeneration in Alzheimer's disease and other related tauopathies. In vitro models have been developed in order to investigate molecular mechanisms that regulate the polymerization of tau. Arachidonic acid and heparin have been proposed to induce tau polymerization via a ligand dependent nucleation-elongation mechanism. However, certain aspects of these in vitro results are inconsistent with a classic nucleation-elongation mechanism. Using steady state and kinetic analyses of tau polymerization at a variety of protein and inducer concentrations, we have found that the thermodynamic barrier for nucleation in the presence of inducers is negligible, which was manifested by increases in protein polymerization at low tau concentrations and very rapid kinetics of polymerization. However, the mechanism of polymerization is complicated by the observation that high concentrations of inducer molecules result in the inhibition of tau fibril formation through different mechanisms for arachidonic acid and heparin. These observations indicate that the molar ratio of inducer to protein is a greater determinant of the rate and extent of tau polymerization than the concentration of tau itself. Our results are therefore not consistent with a canonical nucleation-elongation reaction but rather are more consistent with an allosteric regulation model in which the presence of small molecules induce a conformational change in the protein that decreases the thermodynamic barrier for polymerization essentially to zero.