Defects in exosome biogenesis are associated with sensorimotor defects in zebrafish vps4a mutants

Mutations in human VPS4A are associated with neurodevelopmental defects, including motor delays and defective muscle tone. VPS4A encodes a AAA-ATPase required for membrane scission, but how mutations in VPS4A lead to impaired control of motor function is not known. Here we identified a mutation in zebrafish vps4a, T248I, that affects sensorimotor transformation. In biochemical experiments we show 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, vps4aT248I mutants have enlarged endosomal compartments in the CNS and decreased numbers of circulating exosomes. Resembling the central form of hypotonia in human VPS4A patients, motor neurons and muscle cells are unaffected in mutant zebrafish as touch sensitivity is intact. Unlike somatosensory function, optomotor responses, vestibulospinal (VS), and acoustic startle reflexes are severely impaired in vps4aT248I mutants, 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 vivo imaging of tone-evoked responses in the inner ear and ascending auditory pathway show comparable activity. Further investigation of central pathways in vps4aT248I mutants 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. Significance StatementHere we present a T248I mutation in vps4a, which causes sensorimotor defects in zebrafish larvae. Spanning biochemical to systems level analyses, our study indicates that a reduction in Vps4a enzymatic activity leads to endosomal defects and reduced exosome biogenesis, resulting in pronounced deficits in vision and sensorimotor transformation of auditory and vestibular cues. We suggest that the mechanisms underlying this type of dysfunction in zebrafish may also contribute to the condition seen in human patients with de novo mutations in the human VPS4A orthologue.