Missense point mutations of tau to segregate with FTDP-17 exhibit site-specific effects on microtubule structure in COS cells: a novel action of R406W mutation.

Missense and splicing point mutations have been found in the tau gene in families with frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Of these mutations, we examined four exonic missense point mutations (G272V, P301L, V337M and R406W) in 3-repeat or 4-repeat tau isoform on the transfection experiment. The effects of two mutations (G272V or P301L) on microtubules were subtle whereas those of two other mutations (V337M or R406W) were dramatically significant when these two mutations were constructed into 3-repeat tau but not into 4-repeat tau. The R406W mutation induced an alternation of microtubules to form dotted or fragmented forms retaining colocalization of tau with tubulin whereas the V337M mutation predominantly disrupted microtubule networks and diminished colocalization of tau and tubulin. The effect of the mutations on microtubules were thus site-dependent and isoform-dependent. Tau with R406W mutation was found to be colocalized with tubulin without filamentous structures on confocal views, suggesting that the carboxyl region of tau played a different role from tubulin-binding domain on microtubule assemble. Another abnormal property was identified in tau with R406W mutation that failed to suffer phosphorylation. Thus, diverse effects of tau mutations on microtubules may explain the various clinicopathologies of FTDP-17 and related tauopathies.