The establishment of neuronal circuits requires axonal growth cone pathfinding, which is mediated by an integrated network of microtubules (MTs) and actin filaments. Accordingly, aberrant axon pathfinding, for instance owing to mutations in cytoskeletal regulators, such as doublecortin (Dcx), cause neurodevelopmental disorders. However, the molecular basis of regulation and coordination of the growth cone cytoskeleton is not well understood. Here, Joseph Atherton, Carolyn Moores and colleagues (Atherton et al., 2022) use cryo-electron tomography to visualise the 3D nanostructure of mouse hippocampal neuron growth cones throughout their different subregions, the central (C-) domain, transition (T-) zone and peripheral (P-) domain. They observe hexagonal pseudo-crystalline actin bundles in the P-domain that form filopodia and extend into the T-zone, together with branched networks. In addition, this region contains subpopulations of more tightly twisted actin filaments that exhibit additional binding protein densities, suggestive of cofilin decoration. In contrast, MTs are most prevalent in the C-domain; they are mainly blunt-ended, and exhibit lumenal particles and lattice defects. Finally, Dcx knockout does not affect the overall growth cone organisation, but results in a higher proportion of MTs with defects, suggesting a crucial role for Dcx in maintaining MT integrity. This work thus provides important new insights into the cytoskeletal organisation of the growth cone and the molecular mechanisms underlying associated disorders.
