Axon Growth of CNS Neurons in Three Dimensions Is Amoeboid and Independent of Adhesions

During development of the central nervous system (CNS), neurons polarize and rapidly extend their axons to assemble neuronal circuits. The growth cone leads the axon to its target and drives axon growth. Here, we explored the mechanisms underlying axon growth in three dimensions. Live in situ imaging and super-resolution microscopy combined with pharmacological and molecular manipulations as well as biophysical force measurements revealed that growth cones extend CNS axons independent of pulling forces on their substrates and without the need for adhesions in three-dimensional (3D) environments. In 3D, microtubules grow unrestrained from the actomyosin cytoskeleton into the growth cone leading edge to enable rapid axon extension. Axons extend and polarize even in adhesion-inert matrices. Thus, CNS neurons use amoeboid mechanisms to drive axon growth. Together with our understanding that adult CNS axons regenerate by reactivating developmental processes, our findings illuminate how cytoskeletal manipulations enable axon regeneration in the adult CNS.