Efa6 regulates axon growth, branching and maintenance by eliminating off-track microtubules at the cortex

Axons are the enormously long, cable-like neuronal extensions that wire our nervous system. The formation and prolonged maintenance of these delicate structures requires parallel bundles of microtubules (MTs), which form the structural backbones and highways for life-sustaining transport in axons. In ageing and certain axonopathies, axonal MTs lose their bundled appearances forming areas of disorganisation. To understand this deteriorating condition, we use Drosophila and study the various mechanisms that promote axonal MT bundle organisation during development and maintenance. Here we report that the membrane-associated protein Efa6 acts as a typical cortical MT collapse factor eliminating MTs approaching the axonal surface. This action provides a quality control mechanism that prevents MT disorganisation during the formation and maintenance of axons by eliminating MTs accidentally leaving the axon bundle. Furthermore, it is important for the regulation of axonal growth and branch formation in cultured neurons and in vivo. Using structure-function analyses, we show that cytoplasmic localisation of a small N-terminal peptide motif alone causes the disappearance of entire MT networks in mouse fibroblasts and loss of whole axons in Drosophila primary neurons; its activity becomes meaningful when restricted to the cell plasma membrane via the plekstrin homology domain present in the Efa6 C-terminus. As explained in detail, our data provide a paradigm for the fundamental and relevant role of cortical collapse factors in neurons.