Katanin-mediated severing generates microtubules during neurite outgrowth

Neurons contain a polarized and staggered microtubule array that is important for supporting intracellular transport. How these microtubules are generated and oriented during development is not fully understood. Here we have taken advantage of the low microtubule density in the C. elegans PVD neuron to observe in-vivo microtubule dynamics during neurite outgrowth. We found that individual microtubules are added to outgrowing neurites via microtubule severing events, which generate new microtubules while maintaining existing polarity. We further show that the Katanin enzyme complex is specifically required for severing. Surprisingly, both PTRN-1/CAMSAP and NMTN-1/WDR47, two microtubule minus-end proteins, are also required for microtubule severing. These two proteins colocalize together, and confine Katanin-mediated severing activity towards the distal neurite growth cone. When microtubule severing is impaired, the resulting microtubule array contains fewer microtubules, impacting the trafficking of various endosomes and mitochondria. Together, our findings demonstrate that microtubule severing is an important mechanism to build the microtubule array in developing neurons.