Figure 8.

Local MT stabilization promotes axon formation. (A and B) Rat hippocampal neuron (1 DIV) before (A) and after (B) UV-mediated photoactivation (circle) of caged taxol at the tip of one randomly chosen minor neurite. (C and D) Photoactivation did not interfere with the overall growth cone dynamics; most growth cones, including the pulsed one (arrow), are active. (E and F) 2 d after uncaging, the pulsed process had become the axon (arrow), which is Tau-1–positive (red) and MAP2-negative (green; F). (G) Probability of axon formation in the targeted area doubles after local activation of caged taxol compared with that expected by random chance (mean ± SEM; **, P < 0.01 by χ² test). Control treatment (DMSO and UV) does not influence randomized axon formation (P > 0.8 by χ² test). (H–L) Caged taxol was locally activated at the tip of a nongrowing neurite (circle) of an EB3-GFP–transfected neuron at 1 DIV. Before uncaging, the chosen neurite does not grow and shows little MT dynamics (H and I [arrow], and L, 1), whereas another neurite is rapidly growing (H and I, arrowhead; also see J and K). During uncaging (L, 2 and 3), the process becomes activated, visualized by enrichment of EB3-GFP at its tip (L, 3, arrow). After uncaging, the pulsed neurite shows increased thickness (J and K). Dynamic MTs keep protruding to the peripheral part of the process, promoting its outgrowth (K and L, 4 and 5, arrow). The asterisk in H and J indicates the initial position of the neurite tip in H. Bars: (A, B, and H–K) 20 μm; (C and D) 5 μm; (E and F) 50 μm; (L) 10 μm.