Figure 6.

FRAP of CFP-tau on MTs in axons of RGC neurons. a, Photobleaching of CFP-tau8R in a 3 μm area of an axon. Because diffusion over this short distance is fast, the half-time of recovery represents the off-rate of tau from the MTs. b, Mean signal intensities along the axon shown in a before the bleach (black), directly after the bleach (red) and 15.4 s after the bleach (light blue). c, Mean recovery time ± SD of different tau mutants and of tau at different positions along the axon, showing only small variations (3–4 s). wt, Wild type. d, Comparison of tau distribution along the axon by diffusion and transport (computed on the basis of observed and published rates of transport and diffusion; details in supplemental material, available at www.jneurosci.org). The advance of the “front” of tau (defined as half of the initial tau concentration) is plotted versus time. Top, Long time axis (30 d). Bottom, Initial short time axis (16 h). The modeling shows that diffusion can move tau in 4 d by 1 mm along the axon (top, solid black line) before it becomes too slow to account for the published transport rate of ∼0.003 μm/s (solid blue line) (Mercken et al., 1995). At longer distances (beyond the intersection between the black and blue solid curves), diffusion is too inefficient to distribute tau, and active transport is needed for axons longer than ∼1 mm. The spreading of tau into the axon reported by Utton et al. (2002) (bottom, short pink line, marked by cross) could be fully explained by diffusion alone.