KIF1A-driven entry of degradative lysosomes into the axon is developmentally regulated by the microtubule environment at the axon initial segment

Neuronal function requires precise transport of lysosomal vesicles along highly polarized processes, yet how this trafficking is spatially and developmentally regulated remains poorly understood. Here, we show that the neuronally enriched kinesin-3 motor KIF1A is required for dendritic and axonal localization of lysosomal vesicles in both immature and mature rat hippocampal neurons. Remarkably, the specialized microtubule environment of the axon initial segment (AIS) restricts KIF1A-driven transport of degradative, cathepsin D-positive lysosomal vesicles while permitting less degradative lysosomal vesicles to enter axons in mature neurons. Disrupting the AIS microtubule environment or expressing a hereditary spastic paraplegia type 30-associated KIF1A variant with increased affinity for GTP-tubulin bypasses this restriction, enabling degradative lysosomal vesicles to enter mature axons. In human iPSC-derived neurons, KIF1A depletion similarly disrupts axonal lysosomal vesicle transport, causes autophagic vesicle accumulation, and triggers axonal degeneration. Together, our findings reveal a developmentally regulated AIS gate that selectively controls KIF1A-driven lysosomal vesicle transport into axons and link its subversion to KIF1A-associated neurological disease.