Huntingtin bundles and changes the local proteome of actin filaments in neurons

Huntingtin (HTT) is a large protein whose best-known function being the facilitation of intracellular dynamics along the microtubule network by scaffolding molecular motors complexes. Our recent finding that the defective axonal growth in HD was due to altered growth cone architecture led us to ask whether HTT also influences the cytoskeleton itself. In developing neurons, we found that a large proportion of HTT associates with F-actin in growth cones. Using cell free system and purified recombinant proteins, we observed that HTT binds directly filamentous actin (F-actin) and organizes filaments into bundles. Transmission electron microscopy shows that HTT dimers crosslink adjacent filaments 20 nm apart. We also provide evidence that HTT binding on F-actin modulates the association of other proteins to this cytoskeleton. Notably, HTT limits the association of the growth cone protein Drebrin1 with F-actin. HTT depletion leads to abnormal cytoskeletal organization, localization of Drebrin1 in growth cones, and axonal growth. HTT therefore serves a scaffolding function for the cytoskeleton itself, what might be relevant for HD pathophysiology.