Truncation of the otoferlin transmembrane domain alters the development of hair cells and reduces membrane docking

Release of neurotransmitter from sensory hair cells is regulated by otoferlin. Despite the importance of otoferlin in the auditory and vestibular pathways, the functional contributions of the domains of the protein have not been fully characterized. Using a zebrafish model, we investigated a mutant otoferlin with a stop codon at the start of the transmembrane domain. We found that both the phenotype severity and the expression level of mutant otoferlin changed with the age of the zebrafish. At the early developmental timepoint of 72 hours post-fertilization (hpf) low expression of the otoferlin mutant coincided with synaptic ribbon deficiencies, reduced endocytosis, and abnormal transcription of several hair cell genes. As development proceeded, expression of the mutant otoferlin increased, and both synaptic ribbons and hair cell transcript levels resembled wild type. However, hair cell endocytosis deficits and abnormalities in the expression of GABA receptors persisted even after upregulation of mutant otoferlin. Analysis of membrane-reconstituted otoferlin measurements suggest a function for the transmembrane domain in liposome docking. We conclude that that deletion of the transmembrane domain reduces membrane docking, attenuates endocytosis, and results in developmental delay of the hair cell.