Single-molecule tracking of tau reveals fast kiss-and-hop interaction with microtubules in living neurons.
Bottom Line: Furthermore, we observed by quantitative imaging using fluorescence decay after photoactivation recordings of photoactivatable GFP-tagged tubulin that, despite this rapid dynamics, tau is capable of regulating the tubulin-microtubule balance.Our data imply a novel kiss-and-hop mechanism by which tau promotes neuronal microtubule assembly.The rapid kiss-and-hop interaction explains why tau, although binding to microtubules, does not interfere with axonal transport.
Affiliation: Department of Neurobiology, University of Osnabrück, D-49076 Osnabrück, Germany.Show MeSH
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Mentions: The data show that tau only briefly resides on one particular MT until it hops to a neighboring one. We asked whether this short-lived interaction is compatible with an effect of tau on microtubule polymerization in living cells. To address this question, we coexpressed mCherry-tagged tau or, as a control, 3×mCherry together with PAGFP-tagged α-tubulin in neuronally differentiated PC12 cells. Tau-induced changes in MT dynamics were then monitored by FDAP measurements from cells in which the fluorescence of PAGFP had been focally activated in the middle of a process (Gauthier-Kemper et al., 2012; Figure 5, A and B). The normalized intensity of the control (3×PAGFP) showed a considerably faster decay than that of α-tubulin (Figure 5C), indicating reaction-dependent diffusion of tubulin versus free diffusion of 3×PAGFP. The presence of mCherry-tau significantly slowed the decay of α-tubulin, whereas 3×mCherry did not have any effect (Figure 5C, left vs. right). To determine the fractions of free and bound tubulin, we fitted the FDAP curves to a reaction-diffusion model (see Materials and Methods). In the absence of exogenous tau, about two-thirds of PAGFP-tubulin was polymerized in the cell processes. The exogenous mCherry-tau increased polymerization by ∼10% (Figure 5C, bottom). This was consistent with previous results from in vitro experiments, in which tau was shown to induce microtubule polymerization in a concentration-dependent manner (Brandt and Lee, 1993a). Taken together, the data indicate that tau is capable of promoting MT assembly in neurites despite its rapid kiss-and-hop dynamics. However, it should be taken into account that the experiment uses an overexpression system in which tubulin and tau are expressed on top of the respective endogenous proteins. It remains to be shown whether it is also the physiological function of tau to promote microtubule assembly in neurites.
Affiliation: Department of Neurobiology, University of Osnabrück, D-49076 Osnabrück, Germany.