Fig. 4.

Roles of severing enzymes in neuronal morphogenesis. Severing enzymes (blue scissors) are highly enriched in developing and adult neurons in all three major compartments of the neuron: the cell body, the dendrites and the axon. In the dendrites (left-hand panel), spastin and a katanin-like protein sever microtubules at branch sites to promote arborization. In the cell body (middle panel), katanin is required to sever microtubules from the centrosome (dark green) to allow microtubule transport down the axon. In axons (right-hand panel), katanin is used to create short microtubules for transport up the axon during elongation. The activity of katanin is regulated by tau, whose dephosphorylated form (purple) protects microtubules from katanin. When phosphorylated (light green), tau dissociates from the microtubules and exposes them to severing. Spastin is used to control branching in the axon, and short microtubules can rotate to grow into forming branches.