L-type voltage-gated calcium channel agonists improve hearing loss and modify ribbon synapse morphology in the zebrafish model of Usher Syndrome Type 1

Usher Syndrome (USH) is the most common cause of human deaf/blindness. The zebrafish myo7aa-/- mutant, faithfully models USH1; homozygous zebrafish are deaf and exhibit circular swimming. We hypothesized that hair cell morphology would differ in myo7aa-/- mutants compared to wild type. We also tested the hypothesis that agonists of L-type voltage-gated calcium channels would alter ribbon synapse morphology and behavior of zebrafish myo7aa-/- mutants. We discovered that myo7aa-/- zebrafish have fewer glutamatergic vesicles tethered to hair cell ribbon synapses, yet maintain a comparable ribbon area. We identified that myo7aa-/- mutants have fewer total active hair cells, fewer total CTBP2 expressing puncta, and an altered distribution of CTBP2 puncta compared to wildtype. We also identified that myo7aa-/- mutants have fewer active post-synaptic cells and fewer total MAGUK puncta, compared to wildtype. Behaviorally, myo7aa-/- mutant fish have abnormal swimming as measured by larger absolute smooth orientations and have little to no acoustic startle. Treatment with L-type voltage-gated calcium channel agonists altered the abnormal cell and behavioral phenotypes toward wildtype. Our data supports that L-type voltage-gated calcium channel agonists induce morphological changes at the ribbon synapse--in both the number of tethered vesicles and the distribution of CTBP2 puncta, shifts swimming behavior towards wildtype swimming and improves acoustic startle response. Summary StatementWe identified that the hair cell biology and behavior of the myo7aa-/- mutant differs from wildtype and this difference can be rescued using L-type voltage-gated calcium channel agonists.