Defects in exosome biogenesis are associated with sensorimotor defects in zebrafishvps4amutants

Abstract

Mutations in humanVPS4Aare associated with neurodevelopmental defects, including motor delays and defective muscle tone.VPS4Aencodes a AAA-ATPase that is required for membrane scission, but how mutations inVPS4Alead to impaired control of motor function is not known. Here we identified a mutation in zebrafishvps4a, T248I, that affects sensorimotor transformation. In biochemical experiments we show that the T248I mutation reduces the ATPase activity of Vps4a and disassembly of its substrate, ESCRT filaments, which mediate membrane scission. Consistent with the established role for Vps4a in the endocytic pathway and exosome biogenesis,vps4aT248Imutants have enlarged endosomal compartments in the CNS and decreased numbers of circulating exosomes. Resembling the central form of hypotonia in humanVPS4Apatients, motor neurons and muscle cells are unaffected in mutant zebrafish as they react robustly to touch. Unlike somatosensory function, optomotor responses, vestibulospinal (VS), and acoustic startle reflexes are severely impaired invps4aT248Imutants, indicating a greater sensitivity of these circuits to the T248I mutation. ERG recordings indicate that visual ability is largely reduced in the mutants, however,in vivoimaging of tone-evoked responses in the inner ear and ascending auditory pathway show comparable activity. Further investigation of central pathways invps4aT248Imutants revealed that sensory cues failed to fully activate neurons in the VS and medial longitudinal fasciculus (MLF) nuclei that directly innervate motor neurons. Our results suggest that a defect in sensorimotor transformation underlies the profound yet selective effects on motor reflexes resulting from the loss of membrane scission mediated by Vps4a.