Loss and gain of motor protein function can cause ROS-dependent or -independent microtubule bundle damage in axons

Neurodegeneration often starts by atrophy of the cable-like nerve fibres (axons) that wire nervous systems. Maintaining axons requires supply via motor protein-driven transport along uninterrupted bundles of microtubules. Functional loss of motor proteins, but surprisingly also their hyperactivation, link to conditions of axonal atrophy; in both cases the underlying mechanisms are little understood. To bridge this important knowledge gap, we carried out systematic studies using 40 different genetic tools to manipulate 19 context-related genes in one standardised <i>Drosophila</i> primary neuron system. Starting with transport motors, we found that downregulation in at least three of them, Dynein heavy chain, the KIF5 orthologue Kinesin heavy chain (Khc) and KIF1A orthologue Unc-104, caused disintegration of axonal microtubule bundles which we refer to as ‘microtubule-curling’; this damages the essential highways for life-sustaining axonal transport. To understand this phenomenon, we focussed on Khc’s various subfunctions. We found that abolishing Khc-mediated mitochondrial and lysosomal transport affects the homeostasis of reactive oxygen species (ROS) which, in turn, triggers microtubule-curling in fly and mouse neurons alike. Taking the opposite approach by using conditions where Khc is hyperactive, comparable microtubule-curling is observed, triggered by a ROS-independent mechanism likely involving excessive mechanical force generation. To assess wider relevance of our findings, we studied Unc-104, its binding partner KIFBP and human KIF5A. These studies suggest that functional loss and hyperactivation also of other transport motors cause ROS-dependent and -independent microtubule-curling, which could therefore represent two fundamental pathways that link transport motors to microtubule bundle decay and neurodegeneration.