FTDP-17 Mutations in Tau Alter the Regulation of Microtubule Dynamics

Abstract

Mutations affecting either the structure or regulation of the microtubule-associated protein Tau cause neuronal cell death and dementia. However, the molecular mechanisms mediating these deleterious effects remain unclear. Among the most characterized activities of Tau is the ability to regulate microtubule dynamics, known to be essential for proper cell function and viability. Here we have tested the hypothesis that Tau mutations causing neurodegeneration also alter the ability of Tau to regulate the dynamic instability behaviors of microtubules. Using in vitro microtubule dynamics assays to assess average microtubule growth rates, microtubule growth rate distributions, and catastrophe frequencies, we found that all tested mutants possessing amino acid substitutions or deletions mapping to either the repeat or interrepeat regions of Tau do indeed compromise its ability to regulate microtubule dynamics. Further mutational analyses suggest a novel mechanism of Tau regulatory action based on an "alternative core" of microtubule binding and regulatory activities composed of two repeats and the interrepeat between them. In this model, the interrepeat serves as the primary regulator of microtubule dynamics, whereas the flanking repeats serve as tethers to properly position the interrepeat on the microtubule. Importantly, since there are multiple interrepeats on each Tau molecule, there are also multiple cores on each Tau molecule, each with distinct mechanistic capabilities, thereby providing significant regulatory potential. Taken together, the data are consistent with a microtubule misregulation mechanism for Tau-mediated neuronal cell death and provide a novel mechanistic model for normal and pathological Tau action.The microtubule-associated protein Tau is necessary for the establishment of neuronal cell polarity, axonal outgrowth, and axonal transport and the maintenance of axonal morphology (1-5). Tau dysfunction has long been correlated with a variety of neurodegenerative diseases, including Alzheimer disease, fronto-temporal dementia, and Parkinsonism associated with chromosome 17 (FTDP-17), 3 Pick disease, and progressive supranuclear palsy. Each of these diseases is characterized by extensive neuronal cell death and the presence of abnormal pathological fibers composed primarily of hyperphosphorylated Tau (6 -8). In 1998, direct genetic linkages between mutations in the Tau gene and FTDP-17 were reported (9 -12). These mutations all exhibit dominant phenotypes and fall into two classes: structural mutations that alter the encoded sequence of the Tau protein and regulatory mutations that alter the pattern of Tau RNA alternative splicing (13). Mutations in the latter class do not affect the primary sequence of Tau but rather alter the expression ratios of the different wild type isoforms. The fact that such alterations cause neurodegeneration indicates that different Tau isoforms must exert at least some functionally distinct effects.Alternative splicing of Tau RNA produces six different Tau isoforms in the c...

FTDP-17 Tau Mutations Induce Distinct Effects on Aggregation and Microtubule Interactions

“…Only mutations that occur within one of the 18 amino acid repeats in the MTBR displayed effects on the ability of the protein to stabilize assembly of microtubules, confirming previous results with other FTDP-17 mutations (57). Of the mutants that fell outside of the MTBR or in one of the interrepeats (R5L, L315R, V337M, E342V, K369I, and G389R) only L315R displayed any decrease from WT tau in the total amount of microtubule polymerization.…”

Combinatorial Tau Pseudophosphorylation

“…Alternatively, integrating work in dividing cells with our recent in vitro (8,21,22,30,60) and cell culture work (10, 61), we and our collaborators have hypothesized that Tau-mediated neuronal cell death in the Tauopathies might be caused, at least in part, by aberrant Taumediated regulation of microtubule dynamic instability. In the original presentations of this hypothesis, the emphasis was upon errors in Tau action caused by mutations (61,62). Here, with the confirmation of the fact that Tau phosphorylation can alter the regulation of microtubule dynamic instability, we can expand the earlier hypothesis to propose that errors in Tau action, caused by either mutations or aberrant phosphorylation, can induce neuronal cell death and dementia.…”

Elucidating Tau function and dysfunction in the era of cryo-EM

“…1). Strictly speaking, each repetition is composed of an 18-amino acid imperfect repeat and a 13-14-residue interrepeat region (22), and we will make the distinction when necessary. N-terminal to the MTBR region is a proline-rich region (PRR), and these two regions are flanked by N-terminal and C-terminal extensions ( Fig.…”