Disease-associated mutations hyperactivate KIF1A motility and anterograde axonal transport of synaptic vesicle precursors

KIF1A is a kinesin-family motor involved in the axonal transport of synaptic vesicle precursors (SVPs) along microtubules. In humans, more than ten point mutations in KIF1A are associated with the motor neuron disease, hereditary spastic paraplegia (SPG). However, not all of these mutations appear to inhibit the motility of the KIF1A motor, and thus, a clear molecular explanation for how KIF1A mutations lead to neuropathy is not available. In this study, we established in vitro motility assays with purified full-length human KIF1A and found that KIF1A mutations associated with the pure form of spastic paraplegia hyperactivate motility of the KIF1A motor. Introduction of the corresponding mutations into Caenorhabditis elegans KIF1A homologue unc-104 revealed abnormal accumulation of SVPs at the tips of axons and increased anterograde axonal transport of SVPs. Our data reveal that hyper-activation of kinesin motor activity, rather than its loss-of-function, is a novel cause of motor neuron disease in humans.Significance StatementAnterograde axonal transport supplies organelles and protein complexes throughout axonal processes to support neuronal morphology and function. It has been observed that reduced anterograde axonal transport is associated with neuronal diseases. In contrast, here we show that particular disease-associated mutations in KIF1A, an anterograde axonal motor for synaptic vesicle precursors, induce hyperactivation of KIF1A motor activity and increased axonal transport of synaptic vesicle precursors. Our results advance the knowledge of the regulation of motor proteins and axonal transport and cell biology of motor neuron diseases.