Microtubule retrograde flow retains neuronal polarization in a fluctuating state

In developing vertebrate neurons, a neurite is formed by more than a hundred microtubules. While individual microtubules are dynamic, the microtubule array itself has been regarded as stationary. Using live cell imaging in combination with photoconversion techniques and pharmacological manipulations, we uncovered that the microtubule array flows retrogradely within neurites to the soma. This microtubule retrograde flow drives cycles of microtubule density, a hallmark of the fluctuating state before axon formation. Shortly after axon formation, microtubule retrograde flow slows down in the axon, which stabilizes microtubule density cycles and thereby functions as a molecular wedge to enable axon extension. We propose microtubule retrograde flow and its specific slowdown in the axon to be the long-sought mechanism to single one neurite out to drive neuronal polarization.