In vitro assembly of Alzheimer-like filaments: How a small cluster of charged residues in Tau and MAP2 controls filament morphology

Although the microtubule-binding regions (MTBRs) of both Tau and MAP2 can undergo self-assembly into straight filaments (SFs) in vitro, only the Tau MTBR forms paired helical filaments (PHFs). Moreover, Tau appears to be the exclusive building block of the neuropathic filaments observed in Alzheimer's disease and certain frontotemporal dementias (FTDs). Despite significant conservation in the MTBR sequences, there are two persistently different stretches of amino acids (designated here as Module-A and Module-B) between Tau and MAP2 from a number of organisms. To evaluate the role of charged residues in these modules as potential morphology-specifying elements, we used site-directed mutagenesis to replace selected residues within the MAP2 MTBR by residues at corresponding positions in Tau. We then employed electron microscopy to determine the frequency of occurrence of SF and PHF morphology in filaments assembled from these mutant microtubule-binding regions. Our experimental results indicate that a very small number of residues are especially significant determinants of filament morphology; this inference is also supported by the observation that site-directed substitutions of individual Tau residues into MAP2 Module-B likewise result in the formation of PHF-like structures. Because the Module-B in Tau contains two naturally occurring FTD mutations, residues in this region may play a critical role in neuropathic filament assembly.