Modulation of Microtubule Dynamics by Tau in Living Cells: Implications for Development and Neurodegeneration

Abstract

The neural microtubule-associated protein tau binds to and stabilizes microtubules. Because of alternative mRNA splicing, tau is expressed with either 3 or 4 C-terminal repeats. Two observations indicate that differences between these tau isoforms are functionally important. First, the pattern of tau isoform expression is tightly regulated during development. Second, mutation-induced changes in tau RNA splicing cause neuronal cell death and dementia simply by altering the isoform expression ratio. To investigate whether 3-and 4-repeat tau differentially regulate microtubule behavior in cells, we microinjected physiological levels of these two isoforms into EGFP-tubulin-expressing cultured MCF7 cells and measured the effects on the dynamic instability behavior of individual microtubules by time-lapse microscopy. Both isoforms suppressed microtubule dynamics, though to different extents. Specifically, 4-repeat tau reduced the rate and extent of both growing and shortening events. In contrast, 3-repeat tau stabilized most dynamic parameters about threefold less potently than 4-repeat tau and had only a minimal ability to suppress shortening events. These differences provide a mechanistic rationale for the developmental shift in tau isoform expression and are consistent with a loss-of-function model in which abnormal tau isoform expression results in the inability to properly regulate microtubule dynamics, leading to neuronal cell death and dementia. INTRODUCTIONThe microtubule-associated protein tau promotes neuronal cell polarization and axonal outgrowth; it is also necessary for maintaining axonal morphology and axonal transport (Caceres and Kosik, 1990;Esmaeli-Azad et al., 1994;Trinczek et al., 1999;Stamer et al., 2002). Alternatively, abnormal tau is the major component of neurofibrillary tangles, hallmark pathological features of Alzheimer's disease and related dementias.Mechanistically, tau acts by binding to microtubules directly and regulating their growing, shortening, and treadmilling dynamics (Drechsel et al., 1992;Panda et al., 1995Panda et al., , 1999Trinczek et al., 1995). Because tight regulation of microtubule dynamics and microtubule behavior can be critical to cell viability (Jordan et al., 1996;Goncalves et al., 2001), fine regulation of tau activity may be equally critical.Tau activity is regulated in part by alternative mRNA splicing, which leads to the expression of two families of tau isoforms, those containing four C-terminal repeats and those with three such repeats ( Figure 1A; Lee et al., 1988;Himmler, 1989). In vitro studies have shown that 4-repeat tau binds to, assembles, and stabilizes microtubules more effectively than 3-repeat tau (Butner and Kirschner, 1991;Gustke et al., 1994;Trinczek et al., 1995;Goode et al., 2000;Panda et al., 2003). However, there are limited data assessing the abilities of 3-or 4-repeat tau to modulate dynamics in a complex cellular environment.The importance of functional differences between 3-and 4-repeat tau is highlighted by two observations. First, altho...

Direct force measurements reveal that protein Tau confers short-range attractions and isoform-dependent steric stabilization to microtubules

“…Tau was expressed and purified as described (11,76). Briefly, the human cDNA for all six Tau isoforms was encoded in pRK expression vectors and expressed through BL21(DE3) cells.…”

βIII-Tubulin Induces Paclitaxel Resistance in Association with Reduced Effects on Microtubule Dynamic Instability

“…It is likely that many of the binding sites for drugs on microtubules are also binding sites for microtubule regulatory proteins. The microtubule-associated protein tau, for example, may bind to the same or an overlapping site on microtubules as paclitaxel (32), and, like paclitaxel, suppresses microtubule dynamic instability in cells (33). Thus it is plausible that the tubulin isotype composition of microtubules may function to determine which regulatory proteins bind to and act on microtubules.…”

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

“…Mechanistically, both 3-repeat and 4-repeat Tau bind directly to microtubules, stimulate microtubule polymerization, and regulate microtubule dynamics (17-23). Both quantitative and qualitative mechanistic differences exist between the two isoform classes, with 4-repeat Tau generally being more potent than 3-repeat Tau (17,20,22,24,25).A number of models have been suggested to explain the mechanistic significance of the repeat/interrepeat region of Tau. Initially, each 18-amino acid repeat was thought to serve as * This work was supported, in whole or in part, by National Institutes of Health Grants NS-35010 (to S. C. F.), NS-13560 (to L. W.), and CA-57291 (to M. A.…”

“…The exclusion of exon 10, which encodes the first interrepeat and second repeat, results in 3-repeat Tau. Mechanistically, both 3-repeat and 4-repeat Tau bind directly to microtubules, stimulate microtubule polymerization, and regulate microtubule dynamics (17)(18)(19)(20)(21)(22)(23). Both quantitative and qualitative mechanistic differences exist between the two isoform classes, with 4-repeat Tau generally being more potent than 3-repeat Tau (17,20,22,24,25).…”