BDNF and hereditary spastic paraplegia: As such, HSP-related KIF1A mutants, whether loss-of-function or gain-of-function, may result in misdelivery of Syt-IV, thereby dysregulating synaptic vesicle fusion and release of a variety of neurotrophins, altering LTP, and leading to downstream effects such as learning and memory defects, problems with motor control, and other brain region specific effects.